Node.js v6.13.0-rc.1 Documentation

Table of Contents

About this Documentation#

The goal of this documentation is to comprehensively explain the Node.js API, both from a reference as well as a conceptual point of view. Each section describes a built-in module or high-level concept.

Where appropriate, property types, method arguments, and the arguments provided to event handlers are detailed in a list underneath the topic heading.

Contributing#

If you find an error in this documentation, please submit an issue or see the contributing guide for directions on how to submit a patch.

Every file is generated based on the corresponding .md file in the doc/api/ folder in Node.js's source tree. The documentation is generated using the tools/doc/generate.js program. An HTML template is located at doc/template.html.

Stability Index#

Throughout the documentation, you will see indications of a section's stability. The Node.js API is still somewhat changing, and as it matures, certain parts are more reliable than others. Some are so proven, and so relied upon, that they are unlikely to ever change at all. Others are brand new and experimental, or known to be hazardous and in the process of being redesigned.

The stability indices are as follows:

Stability: 0 - Deprecated This feature is known to be problematic, and changes may be planned. Do not rely on it. Use of the feature may cause warnings to be emitted. Backwards compatibility across major versions should not be expected.
Stability: 1 - Experimental This feature is still under active development and subject to non-backwards compatible changes, or even removal, in any future version. Use of the feature is not recommended in production environments. Experimental features are not subject to the Node.js Semantic Versioning model.
Stability: 2 - Stable The API has proven satisfactory. Compatibility with the npm ecosystem is a high priority, and will not be broken unless absolutely necessary.

Note: Caution must be used when making use of Experimental features, particularly within modules that may be used as dependencies (or dependencies of dependencies) within a Node.js application. End users may not be aware that experimental features are being used, and therefore may experience unexpected failures or behavior changes when API modifications occur. To help avoid such surprises, Experimental features may require a command-line flag to explicitly enable them, or may cause a process warning to be emitted. By default, such warnings are printed to stderr and may be handled by attaching a listener to the process.on('warning') event.

JSON Output#

Added in: v0.6.12
Stability: 1 - Experimental

Every .html document has a corresponding .json document presenting the same information in a structured manner. This feature is experimental, and added for the benefit of IDEs and other utilities that wish to do programmatic things with the documentation.

Syscalls and man pages#

System calls like open(2) and read(2) define the interface between user programs and the underlying operating system. Node functions which simply wrap a syscall, like fs.open(), will document that. The docs link to the corresponding man pages (short for manual pages) which describe how the syscalls work.

Some syscalls, like lchown(2), are BSD-specific. That means, for example, that fs.lchown() only works on macOS and other BSD-derived systems, and is not available on Linux.

Most Unix syscalls have Windows equivalents, but behavior may differ on Windows relative to Linux and macOS. For an example of the subtle ways in which it's sometimes impossible to replace Unix syscall semantics on Windows, see Node issue 4760.

Usage#

node [options] [v8 options] [script.js | -e "script"] [arguments]

Please see the Command Line Options document for information about different options and ways to run scripts with Node.js.

Example#

An example of a web server written with Node.js which responds with 'Hello World':

const http = require('http');

const hostname = '127.0.0.1';
const port = 3000;

const server = http.createServer((req, res) => {
  res.statusCode = 200;
  res.setHeader('Content-Type', 'text/plain');
  res.end('Hello World\n');
});

server.listen(port, hostname, () => {
  console.log(`Server running at http://${hostname}:${port}/`);
});

To run the server, put the code into a file called example.js and execute it with Node.js:

$ node example.js
Server running at http://127.0.0.1:3000/

Many of the examples in the documentation can be run similarly.

C/C++ Addons#

Node.js Addons are dynamically-linked shared objects, written in C or C++, that can be loaded into Node.js using the require() function, and used just as if they were an ordinary Node.js module. They are used primarily to provide an interface between JavaScript running in Node.js and C/C++ libraries.

At the moment, the method for implementing Addons is rather complicated, involving knowledge of several components and APIs :

  • V8: the C++ library Node.js currently uses to provide the JavaScript implementation. V8 provides the mechanisms for creating objects, calling functions, etc. V8's API is documented mostly in the v8.h header file (deps/v8/include/v8.h in the Node.js source tree), which is also available online.

  • libuv: The C library that implements the Node.js event loop, its worker threads and all of the asynchronous behaviors of the platform. It also serves as a cross-platform abstraction library, giving easy, POSIX-like access across all major operating systems to many common system tasks, such as interacting with the filesystem, sockets, timers and system events. libuv also provides a pthreads-like threading abstraction that may be used to power more sophisticated asynchronous Addons that need to move beyond the standard event loop. Addon authors are encouraged to think about how to avoid blocking the event loop with I/O or other time-intensive tasks by off-loading work via libuv to non-blocking system operations, worker threads or a custom use of libuv's threads.

  • Internal Node.js libraries. Node.js itself exports a number of C/C++ APIs that Addons can use — the most important of which is the node::ObjectWrap class.

  • Node.js includes a number of other statically linked libraries including OpenSSL. These other libraries are located in the deps/ directory in the Node.js source tree. Only the V8 and OpenSSL symbols are purposefully re-exported by Node.js and may be used to various extents by Addons. See Linking to Node.js' own dependencies for additional information.

All of the following examples are available for download and may be used as a starting-point for your own Addon.

Hello world#

This "Hello world" example is a simple Addon, written in C++, that is the equivalent of the following JavaScript code:

module.exports.hello = () => 'world';

First, create the file hello.cc:

// hello.cc
#include <node.h>

namespace demo {

using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;

void Method(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  args.GetReturnValue().Set(String::NewFromUtf8(isolate, "world"));
}

void init(Local<Object> exports) {
  NODE_SET_METHOD(exports, "hello", Method);
}

NODE_MODULE(addon, init)

}  // namespace demo

Note that all Node.js Addons must export an initialization function following the pattern:

void Initialize(Local<Object> exports);
NODE_MODULE(module_name, Initialize)

There is no semi-colon after NODE_MODULE as it's not a function (see node.h).

The module_name must match the filename of the final binary (excluding the .node suffix).

In the hello.cc example, then, the initialization function is init and the Addon module name is addon.

Building#

Once the source code has been written, it must be compiled into the binary addon.node file. To do so, create a file called binding.gyp in the top-level of the project describing the build configuration of your module using a JSON-like format. This file is used by node-gyp -- a tool written specifically to compile Node.js Addons.

{
  "targets": [
    {
      "target_name": "addon",
      "sources": [ "hello.cc" ]
    }
  ]
}

Note: A version of the node-gyp utility is bundled and distributed with Node.js as part of npm. This version is not made directly available for developers to use and is intended only to support the ability to use the npm install command to compile and install Addons. Developers who wish to use node-gyp directly can install it using the command npm install -g node-gyp. See the node-gyp installation instructions for more information, including platform-specific requirements.

Once the binding.gyp file has been created, use node-gyp configure to generate the appropriate project build files for the current platform. This will generate either a Makefile (on Unix platforms) or a vcxproj file (on Windows) in the build/ directory.

Next, invoke the node-gyp build command to generate the compiled addon.node file. This will be put into the build/Release/ directory.

When using npm install to install a Node.js Addon, npm uses its own bundled version of node-gyp to perform this same set of actions, generating a compiled version of the Addon for the user's platform on demand.

Once built, the binary Addon can be used from within Node.js by pointing require() to the built addon.node module:

// hello.js
const addon = require('./build/Release/addon');

console.log(addon.hello());
// Prints: 'world'

Please see the examples below for further information or https://github.com/arturadib/node-qt for an example in production.

Because the exact path to the compiled Addon binary can vary depending on how it is compiled (i.e. sometimes it may be in ./build/Debug/), Addons can use the bindings package to load the compiled module.

Note that while the bindings package implementation is more sophisticated in how it locates Addon modules, it is essentially using a try-catch pattern similar to:

try {
  return require('./build/Release/addon.node');
} catch (err) {
  return require('./build/Debug/addon.node');
}

Linking to Node.js' own dependencies#

Node.js uses a number of statically linked libraries such as V8, libuv and OpenSSL. All Addons are required to link to V8 and may link to any of the other dependencies as well. Typically, this is as simple as including the appropriate #include <...> statements (e.g. #include <v8.h>) and node-gyp will locate the appropriate headers automatically. However, there are a few caveats to be aware of:

  • When node-gyp runs, it will detect the specific release version of Node.js and download either the full source tarball or just the headers. If the full source is downloaded, Addons will have complete access to the full set of Node.js dependencies. However, if only the Node.js headers are downloaded, then only the symbols exported by Node.js will be available.

  • node-gyp can be run using the --nodedir flag pointing at a local Node.js source image. Using this option, the Addon will have access to the full set of dependencies.

Loading Addons using require()#

The filename extension of the compiled Addon binary is .node (as opposed to .dll or .so). The require() function is written to look for files with the .node file extension and initialize those as dynamically-linked libraries.

When calling require(), the .node extension can usually be omitted and Node.js will still find and initialize the Addon. One caveat, however, is that Node.js will first attempt to locate and load modules or JavaScript files that happen to share the same base name. For instance, if there is a file addon.js in the same directory as the binary addon.node, then require('addon') will give precedence to the addon.js file and load it instead.

Native Abstractions for Node.js#

Each of the examples illustrated in this document make direct use of the Node.js and V8 APIs for implementing Addons. It is important to understand that the V8 API can, and has, changed dramatically from one V8 release to the next (and one major Node.js release to the next). With each change, Addons may need to be updated and recompiled in order to continue functioning. The Node.js release schedule is designed to minimize the frequency and impact of such changes but there is little that Node.js can do currently to ensure stability of the V8 APIs.

The Native Abstractions for Node.js (or nan) provide a set of tools that Addon developers are recommended to use to keep compatibility between past and future releases of V8 and Node.js. See the nan examples for an illustration of how it can be used.

Addon examples#

Following are some example Addons intended to help developers get started. The examples make use of the V8 APIs. Refer to the online V8 reference for help with the various V8 calls, and V8's Embedder's Guide for an explanation of several concepts used such as handles, scopes, function templates, etc.

Each of these examples using the following binding.gyp file:

{
  "targets": [
    {
      "target_name": "addon",
      "sources": [ "addon.cc" ]
    }
  ]
}

In cases where there is more than one .cc file, simply add the additional filename to the sources array. For example:

"sources": ["addon.cc", "myexample.cc"]

Once the binding.gyp file is ready, the example Addons can be configured and built using node-gyp:

$ node-gyp configure build

Function arguments#

Addons will typically expose objects and functions that can be accessed from JavaScript running within Node.js. When functions are invoked from JavaScript, the input arguments and return value must be mapped to and from the C/C++ code.

The following example illustrates how to read function arguments passed from JavaScript and how to return a result:

// addon.cc
#include <node.h>

namespace demo {

using v8::Exception;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::String;
using v8::Value;

// This is the implementation of the "add" method
// Input arguments are passed using the
// const FunctionCallbackInfo<Value>& args struct
void Add(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  // Check the number of arguments passed.
  if (args.Length() < 2) {
    // Throw an Error that is passed back to JavaScript
    isolate->ThrowException(Exception::TypeError(
        String::NewFromUtf8(isolate, "Wrong number of arguments")));
    return;
  }

  // Check the argument types
  if (!args[0]->IsNumber() || !args[1]->IsNumber()) {
    isolate->ThrowException(Exception::TypeError(
        String::NewFromUtf8(isolate, "Wrong arguments")));
    return;
  }

  // Perform the operation
  double value = args[0]->NumberValue() + args[1]->NumberValue();
  Local<Number> num = Number::New(isolate, value);

  // Set the return value (using the passed in
  // FunctionCallbackInfo<Value>&)
  args.GetReturnValue().Set(num);
}

void Init(Local<Object> exports) {
  NODE_SET_METHOD(exports, "add", Add);
}

NODE_MODULE(addon, Init)

}  // namespace demo

Once compiled, the example Addon can be required and used from within Node.js:

// test.js
const addon = require('./build/Release/addon');

console.log('This should be eight:', addon.add(3, 5));

Callbacks#

It is common practice within Addons to pass JavaScript functions to a C++ function and execute them from there. The following example illustrates how to invoke such callbacks:

// addon.cc
#include <node.h>

namespace demo {

using v8::Function;
using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Null;
using v8::Object;
using v8::String;
using v8::Value;

void RunCallback(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  Local<Function> cb = Local<Function>::Cast(args[0]);
  const unsigned argc = 1;
  Local<Value> argv[argc] = { String::NewFromUtf8(isolate, "hello world") };
  cb->Call(Null(isolate), argc, argv);
}

void Init(Local<Object> exports, Local<Object> module) {
  NODE_SET_METHOD(module, "exports", RunCallback);
}

NODE_MODULE(addon, Init)

}  // namespace demo

Note that this example uses a two-argument form of Init() that receives the full module object as the second argument. This allows the Addon to completely overwrite exports with a single function instead of adding the function as a property of exports.

To test it, run the following JavaScript:

// test.js
const addon = require('./build/Release/addon');

addon((msg) => {
  console.log(msg);
// Prints: 'hello world'
});

Note that, in this example, the callback function is invoked synchronously.

Object factory#

Addons can create and return new objects from within a C++ function as illustrated in the following example. An object is created and returned with a property msg that echoes the string passed to createObject():

// addon.cc
#include <node.h>

namespace demo {

using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;

void CreateObject(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  Local<Object> obj = Object::New(isolate);
  obj->Set(String::NewFromUtf8(isolate, "msg"), args[0]->ToString());

  args.GetReturnValue().Set(obj);
}

void Init(Local<Object> exports, Local<Object> module) {
  NODE_SET_METHOD(module, "exports", CreateObject);
}

NODE_MODULE(addon, Init)

}  // namespace demo

To test it in JavaScript:

// test.js
const addon = require('./build/Release/addon');

const obj1 = addon('hello');
const obj2 = addon('world');
console.log(obj1.msg, obj2.msg);
// Prints: 'hello world'

Function factory#

Another common scenario is creating JavaScript functions that wrap C++ functions and returning those back to JavaScript:

// addon.cc
#include <node.h>

namespace demo {

using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;

void MyFunction(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  args.GetReturnValue().Set(String::NewFromUtf8(isolate, "hello world"));
}

void CreateFunction(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, MyFunction);
  Local<Function> fn = tpl->GetFunction();

  // omit this to make it anonymous
  fn->SetName(String::NewFromUtf8(isolate, "theFunction"));

  args.GetReturnValue().Set(fn);
}

void Init(Local<Object> exports, Local<Object> module) {
  NODE_SET_METHOD(module, "exports", CreateFunction);
}

NODE_MODULE(addon, Init)

}  // namespace demo

To test:

// test.js
const addon = require('./build/Release/addon');

const fn = addon();
console.log(fn());
// Prints: 'hello world'

Wrapping C++ objects#

It is also possible to wrap C++ objects/classes in a way that allows new instances to be created using the JavaScript new operator:

// addon.cc
#include <node.h>
#include "myobject.h"

namespace demo {

using v8::Local;
using v8::Object;

void InitAll(Local<Object> exports) {
  MyObject::Init(exports);
}

NODE_MODULE(addon, InitAll)

}  // namespace demo

Then, in myobject.h, the wrapper class inherits from node::ObjectWrap:

// myobject.h
#ifndef MYOBJECT_H
#define MYOBJECT_H

#include <node.h>
#include <node_object_wrap.h>

namespace demo {

class MyObject : public node::ObjectWrap {
 public:
  static void Init(v8::Local<v8::Object> exports);

 private:
  explicit MyObject(double value = 0);
  ~MyObject();

  static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
  static void PlusOne(const v8::FunctionCallbackInfo<v8::Value>& args);
  static v8::Persistent<v8::Function> constructor;
  double value_;
};

}  // namespace demo

#endif

In myobject.cc, implement the various methods that are to be exposed. Below, the method plusOne() is exposed by adding it to the constructor's prototype:

// myobject.cc
#include "myobject.h"

namespace demo {

using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::Persistent;
using v8::String;
using v8::Value;

Persistent<Function> MyObject::constructor;

MyObject::MyObject(double value) : value_(value) {
}

MyObject::~MyObject() {
}

void MyObject::Init(Local<Object> exports) {
  Isolate* isolate = exports->GetIsolate();

  // Prepare constructor template
  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
  tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject"));
  tpl->InstanceTemplate()->SetInternalFieldCount(1);

  // Prototype
  NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne);

  constructor.Reset(isolate, tpl->GetFunction());
  exports->Set(String::NewFromUtf8(isolate, "MyObject"),
               tpl->GetFunction());
}

void MyObject::New(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  if (args.IsConstructCall()) {
    // Invoked as constructor: `new MyObject(...)`
    double value = args[0]->IsUndefined() ? 0 : args[0]->NumberValue();
    MyObject* obj = new MyObject(value);
    obj->Wrap(args.This());
    args.GetReturnValue().Set(args.This());
  } else {
    // Invoked as plain function `MyObject(...)`, turn into construct call.
    const int argc = 1;
    Local<Value> argv[argc] = { args[0] };
    Local<Context> context = isolate->GetCurrentContext();
    Local<Function> cons = Local<Function>::New(isolate, constructor);
    Local<Object> result =
        cons->NewInstance(context, argc, argv).ToLocalChecked();
    args.GetReturnValue().Set(result);
  }
}

void MyObject::PlusOne(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  MyObject* obj = ObjectWrap::Unwrap<MyObject>(args.Holder());
  obj->value_ += 1;

  args.GetReturnValue().Set(Number::New(isolate, obj->value_));
}

}  // namespace demo

To build this example, the myobject.cc file must be added to the binding.gyp:

{
  "targets": [
    {
      "target_name": "addon",
      "sources": [
        "addon.cc",
        "myobject.cc"
      ]
    }
  ]
}

Test it with:

// test.js
const addon = require('./build/Release/addon');

const obj = new addon.MyObject(10);
console.log(obj.plusOne());
// Prints: 11
console.log(obj.plusOne());
// Prints: 12
console.log(obj.plusOne());
// Prints: 13

Factory of wrapped objects#

Alternatively, it is possible to use a factory pattern to avoid explicitly creating object instances using the JavaScript new operator:

const obj = addon.createObject();
// instead of:
// const obj = new addon.Object();

First, the createObject() method is implemented in addon.cc:

// addon.cc
#include <node.h>
#include "myobject.h"

namespace demo {

using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::String;
using v8::Value;

void CreateObject(const FunctionCallbackInfo<Value>& args) {
  MyObject::NewInstance(args);
}

void InitAll(Local<Object> exports, Local<Object> module) {
  MyObject::Init(exports->GetIsolate());

  NODE_SET_METHOD(module, "exports", CreateObject);
}

NODE_MODULE(addon, InitAll)

}  // namespace demo

In myobject.h, the static method NewInstance() is added to handle instantiating the object. This method takes the place of using new in JavaScript:

// myobject.h
#ifndef MYOBJECT_H
#define MYOBJECT_H

#include <node.h>
#include <node_object_wrap.h>

namespace demo {

class MyObject : public node::ObjectWrap {
 public:
  static void Init(v8::Isolate* isolate);
  static void NewInstance(const v8::FunctionCallbackInfo<v8::Value>& args);

 private:
  explicit MyObject(double value = 0);
  ~MyObject();

  static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
  static void PlusOne(const v8::FunctionCallbackInfo<v8::Value>& args);
  static v8::Persistent<v8::Function> constructor;
  double value_;
};

}  // namespace demo

#endif

The implementation in myobject.cc is similar to the previous example:

// myobject.cc
#include <node.h>
#include "myobject.h"

namespace demo {

using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::Persistent;
using v8::String;
using v8::Value;

Persistent<Function> MyObject::constructor;

MyObject::MyObject(double value) : value_(value) {
}

MyObject::~MyObject() {
}

void MyObject::Init(Isolate* isolate) {
  // Prepare constructor template
  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
  tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject"));
  tpl->InstanceTemplate()->SetInternalFieldCount(1);

  // Prototype
  NODE_SET_PROTOTYPE_METHOD(tpl, "plusOne", PlusOne);

  constructor.Reset(isolate, tpl->GetFunction());
}

void MyObject::New(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  if (args.IsConstructCall()) {
    // Invoked as constructor: `new MyObject(...)`
    double value = args[0]->IsUndefined() ? 0 : args[0]->NumberValue();
    MyObject* obj = new MyObject(value);
    obj->Wrap(args.This());
    args.GetReturnValue().Set(args.This());
  } else {
    // Invoked as plain function `MyObject(...)`, turn into construct call.
    const int argc = 1;
    Local<Value> argv[argc] = { args[0] };
    Local<Function> cons = Local<Function>::New(isolate, constructor);
    Local<Context> context = isolate->GetCurrentContext();
    Local<Object> instance =
        cons->NewInstance(context, argc, argv).ToLocalChecked();
    args.GetReturnValue().Set(instance);
  }
}

void MyObject::NewInstance(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  const unsigned argc = 1;
  Local<Value> argv[argc] = { args[0] };
  Local<Function> cons = Local<Function>::New(isolate, constructor);
  Local<Context> context = isolate->GetCurrentContext();
  Local<Object> instance =
      cons->NewInstance(context, argc, argv).ToLocalChecked();

  args.GetReturnValue().Set(instance);
}

void MyObject::PlusOne(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  MyObject* obj = ObjectWrap::Unwrap<MyObject>(args.Holder());
  obj->value_ += 1;

  args.GetReturnValue().Set(Number::New(isolate, obj->value_));
}

}  // namespace demo

Once again, to build this example, the myobject.cc file must be added to the binding.gyp:

{
  "targets": [
    {
      "target_name": "addon",
      "sources": [
        "addon.cc",
        "myobject.cc"
      ]
    }
  ]
}

Test it with:

// test.js
const createObject = require('./build/Release/addon');

const obj = createObject(10);
console.log(obj.plusOne());
// Prints: 11
console.log(obj.plusOne());
// Prints: 12
console.log(obj.plusOne());
// Prints: 13

const obj2 = createObject(20);
console.log(obj2.plusOne());
// Prints: 21
console.log(obj2.plusOne());
// Prints: 22
console.log(obj2.plusOne());
// Prints: 23

Passing wrapped objects around#

In addition to wrapping and returning C++ objects, it is possible to pass wrapped objects around by unwrapping them with the Node.js helper function node::ObjectWrap::Unwrap. The following examples shows a function add() that can take two MyObject objects as input arguments:

// addon.cc
#include <node.h>
#include <node_object_wrap.h>
#include "myobject.h"

namespace demo {

using v8::FunctionCallbackInfo;
using v8::Isolate;
using v8::Local;
using v8::Number;
using v8::Object;
using v8::String;
using v8::Value;

void CreateObject(const FunctionCallbackInfo<Value>& args) {
  MyObject::NewInstance(args);
}

void Add(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  MyObject* obj1 = node::ObjectWrap::Unwrap<MyObject>(
      args[0]->ToObject());
  MyObject* obj2 = node::ObjectWrap::Unwrap<MyObject>(
      args[1]->ToObject());

  double sum = obj1->value() + obj2->value();
  args.GetReturnValue().Set(Number::New(isolate, sum));
}

void InitAll(Local<Object> exports) {
  MyObject::Init(exports->GetIsolate());

  NODE_SET_METHOD(exports, "createObject", CreateObject);
  NODE_SET_METHOD(exports, "add", Add);
}

NODE_MODULE(addon, InitAll)

}  // namespace demo

In myobject.h, a new public method is added to allow access to private values after unwrapping the object.

// myobject.h
#ifndef MYOBJECT_H
#define MYOBJECT_H

#include <node.h>
#include <node_object_wrap.h>

namespace demo {

class MyObject : public node::ObjectWrap {
 public:
  static void Init(v8::Isolate* isolate);
  static void NewInstance(const v8::FunctionCallbackInfo<v8::Value>& args);
  inline double value() const { return value_; }

 private:
  explicit MyObject(double value = 0);
  ~MyObject();

  static void New(const v8::FunctionCallbackInfo<v8::Value>& args);
  static v8::Persistent<v8::Function> constructor;
  double value_;
};

}  // namespace demo

#endif

The implementation of myobject.cc is similar to before:

// myobject.cc
#include <node.h>
#include "myobject.h"

namespace demo {

using v8::Context;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Isolate;
using v8::Local;
using v8::Object;
using v8::Persistent;
using v8::String;
using v8::Value;

Persistent<Function> MyObject::constructor;

MyObject::MyObject(double value) : value_(value) {
}

MyObject::~MyObject() {
}

void MyObject::Init(Isolate* isolate) {
  // Prepare constructor template
  Local<FunctionTemplate> tpl = FunctionTemplate::New(isolate, New);
  tpl->SetClassName(String::NewFromUtf8(isolate, "MyObject"));
  tpl->InstanceTemplate()->SetInternalFieldCount(1);

  constructor.Reset(isolate, tpl->GetFunction());
}

void MyObject::New(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  if (args.IsConstructCall()) {
    // Invoked as constructor: `new MyObject(...)`
    double value = args[0]->IsUndefined() ? 0 : args[0]->NumberValue();
    MyObject* obj = new MyObject(value);
    obj->Wrap(args.This());
    args.GetReturnValue().Set(args.This());
  } else {
    // Invoked as plain function `MyObject(...)`, turn into construct call.
    const int argc = 1;
    Local<Value> argv[argc] = { args[0] };
    Local<Context> context = isolate->GetCurrentContext();
    Local<Function> cons = Local<Function>::New(isolate, constructor);
    Local<Object> instance =
        cons->NewInstance(context, argc, argv).ToLocalChecked();
    args.GetReturnValue().Set(instance);
  }
}

void MyObject::NewInstance(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();

  const unsigned argc = 1;
  Local<Value> argv[argc] = { args[0] };
  Local<Function> cons = Local<Function>::New(isolate, constructor);
  Local<Context> context = isolate->GetCurrentContext();
  Local<Object> instance =
      cons->NewInstance(context, argc, argv).ToLocalChecked();

  args.GetReturnValue().Set(instance);
}

}  // namespace demo

Test it with:

// test.js
const addon = require('./build/Release/addon');

const obj1 = addon.createObject(10);
const obj2 = addon.createObject(20);
const result = addon.add(obj1, obj2);

console.log(result);
// Prints: 30

AtExit hooks#

An "AtExit" hook is a function that is invoked after the Node.js event loop has ended but before the JavaScript VM is terminated and Node.js shuts down. "AtExit" hooks are registered using the node::AtExit API.

void AtExit(callback, args)#

  • callback <void (*)(void*)> A pointer to the function to call at exit.
  • args <void*> A pointer to pass to the callback at exit.

Registers exit hooks that run after the event loop has ended but before the VM is killed.

AtExit takes two parameters: a pointer to a callback function to run at exit, and a pointer to untyped context data to be passed to that callback.

Callbacks are run in last-in first-out order.

The following addon.cc implements AtExit:

// addon.cc
#include <assert.h>
#include <stdlib.h>
#include <node.h>

namespace demo {

using node::AtExit;
using v8::HandleScope;
using v8::Isolate;
using v8::Local;
using v8::Object;

static char cookie[] = "yum yum";
static int at_exit_cb1_called = 0;
static int at_exit_cb2_called = 0;

static void at_exit_cb1(void* arg) {
  Isolate* isolate = static_cast<Isolate*>(arg);
  HandleScope scope(isolate);
  Local<Object> obj = Object::New(isolate);
  assert(!obj.IsEmpty()); // assert VM is still alive
  assert(obj->IsObject());
  at_exit_cb1_called++;
}

static void at_exit_cb2(void* arg) {
  assert(arg == static_cast<void*>(cookie));
  at_exit_cb2_called++;
}

static void sanity_check(void*) {
  assert(at_exit_cb1_called == 1);
  assert(at_exit_cb2_called == 2);
}

void init(Local<Object> exports) {
  AtExit(at_exit_cb2, cookie);
  AtExit(at_exit_cb2, cookie);
  AtExit(at_exit_cb1, exports->GetIsolate());
  AtExit(sanity_check);
}

NODE_MODULE(addon, init)

}  // namespace demo

Test in JavaScript by running:

// test.js
require('./build/Release/addon');

Assert#

Stability: 2 - Stable

The assert module provides a simple set of assertion tests that can be used to test invariants.

assert(value[, message])#

Added in: v0.5.9
  • value <any>
  • message <any>

An alias of assert.ok().

assert.deepEqual(actual, expected[, message])#

Added in: v0.1.21
  • actual <any>
  • expected <any>
  • message <any>

Tests for deep equality between the actual and expected parameters. Primitive values are compared with the equal comparison operator ( == ).

Only enumerable "own" properties are considered. The deepEqual() implementation does not test object prototypes, attached symbols, or non-enumerable properties. This can lead to some potentially surprising results. For example, the following example does not throw an AssertionError because the properties on the Error object are non-enumerable:

// WARNING: This does not throw an AssertionError!
assert.deepEqual(Error('a'), Error('b'));

"Deep" equality means that the enumerable "own" properties of child objects are evaluated also:

const assert = require('assert');

const obj1 = {
  a: {
    b: 1
  }
};
const obj2 = {
  a: {
    b: 2
  }
};
const obj3 = {
  a: {
    b: 1
  }
};
const obj4 = Object.create(obj1);

assert.deepEqual(obj1, obj1);
// OK, object is equal to itself

assert.deepEqual(obj1, obj2);
// AssertionError: { a: { b: 1 } } deepEqual { a: { b: 2 } }
// values of b are different

assert.deepEqual(obj1, obj3);
// OK, objects are equal

assert.deepEqual(obj1, obj4);
// AssertionError: { a: { b: 1 } } deepEqual {}
// Prototypes are ignored

If the values are not equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned.

assert.deepStrictEqual(actual, expected[, message])#

Added in: v1.2.0
  • actual <any>
  • expected <any>
  • message <any>

Generally identical to assert.deepEqual() with two exceptions. First, primitive values are compared using the strict equality operator ( === ). Second, object comparisons include a strict equality check of their prototypes.

const assert = require('assert');

assert.deepEqual({ a: 1 }, { a: '1' });
// OK, because 1 == '1'

assert.deepStrictEqual({ a: 1 }, { a: '1' });
// AssertionError: { a: 1 } deepStrictEqual { a: '1' }
// because 1 !== '1' using strict equality

If the values are not equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned.

assert.doesNotThrow(block[, error][, message])#

Added in: v0.1.21

Asserts that the function block does not throw an error. See assert.throws() for more details.

When assert.doesNotThrow() is called, it will immediately call the block function.

If an error is thrown and it is the same type as that specified by the error parameter, then an AssertionError is thrown. If the error is of a different type, or if the error parameter is undefined, the error is propagated back to the caller.

The following, for instance, will throw the TypeError because there is no matching error type in the assertion:

assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  SyntaxError
);

However, the following will result in an AssertionError with the message 'Got unwanted exception (TypeError)..':

assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  TypeError
);

If an AssertionError is thrown and a value is provided for the message parameter, the value of message will be appended to the AssertionError message:

assert.doesNotThrow(
  () => {
    throw new TypeError('Wrong value');
  },
  TypeError,
  'Whoops'
);
// Throws: AssertionError: Got unwanted exception (TypeError). Whoops

assert.equal(actual, expected[, message])#

Added in: v0.1.21
  • actual <any>
  • expected <any>
  • message <any>

Tests shallow, coercive equality between the actual and expected parameters using the equal comparison operator ( == ).

const assert = require('assert');

assert.equal(1, 1);
// OK, 1 == 1
assert.equal(1, '1');
// OK, 1 == '1'

assert.equal(1, 2);
// AssertionError: 1 == 2
assert.equal({a: {b: 1}}, {a: {b: 1}});
//AssertionError: { a: { b: 1 } } == { a: { b: 1 } }

If the values are not equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned.

assert.fail(message)#

assert.fail(actual, expected[, message[, operator[, stackStartFunction]]])#

Added in: v0.1.21
  • actual <any>
  • expected <any>
  • message <any>
  • operator <string> Default: '!='
  • stackStartFunction <function> Default: assert.fail

Throws an AssertionError. If message is falsy, the error message is set as the values of actual and expected separated by the provided operator. Otherwise, the error message is the value of message. If stackStartFunction is provided, all stack frames above that function will be removed from stacktrace (see Error.captureStackTrace).

const assert = require('assert');

assert.fail(1, 2, undefined, '>');
// AssertionError: 1 > 2

assert.fail(1, 2, 'fail');
// AssertionError: fail

assert.fail(1, 2, 'whoops', '>');
// AssertionError: whoops

assert.fail('boom');
// AssertionError: boom

assert.fail('a', 'b');
// AssertionError: 'a' != 'b'

Example use of stackStartFunction for truncating the exception's stacktrace:

function suppressFrame() {
  assert.fail('a', 'b', undefined, '!==', suppressFrame);
}
suppressFrame();
// AssertionError: 'a' !== 'b'
//     at repl:1:1
//     at ContextifyScript.Script.runInThisContext (vm.js:44:33)
//     ...

assert.ifError(value)#

Added in: v0.1.97
  • value <any>

Throws value if value is truthy. This is useful when testing the error argument in callbacks.

const assert = require('assert');

assert.ifError(0);
// OK
assert.ifError(1);
// Throws 1
assert.ifError('error');
// Throws 'error'
assert.ifError(new Error());
// Throws Error

assert.notDeepEqual(actual, expected[, message])#

Added in: v0.1.21
  • actual <any>
  • expected <any>
  • message <any>

Tests for any deep inequality. Opposite of assert.deepEqual().

const assert = require('assert');

const obj1 = {
  a: {
    b: 1
  }
};
const obj2 = {
  a: {
    b: 2
  }
};
const obj3 = {
  a: {
    b: 1
  }
};
const obj4 = Object.create(obj1);

assert.notDeepEqual(obj1, obj1);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

assert.notDeepEqual(obj1, obj2);
// OK: obj1 and obj2 are not deeply equal

assert.notDeepEqual(obj1, obj3);
// AssertionError: { a: { b: 1 } } notDeepEqual { a: { b: 1 } }

assert.notDeepEqual(obj1, obj4);
// OK: obj1 and obj4 are not deeply equal

If the values are deeply equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned.

assert.notDeepStrictEqual(actual, expected[, message])#

Added in: v1.2.0
  • actual <any>
  • expected <any>
  • message <any>

Tests for deep strict inequality. Opposite of assert.deepStrictEqual().

const assert = require('assert');

assert.notDeepEqual({a: 1}, {a: '1'});
// AssertionError: { a: 1 } notDeepEqual { a: '1' }

assert.notDeepStrictEqual({a: 1}, {a: '1'});
// OK

If the values are deeply and strictly equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned.

assert.notEqual(actual, expected[, message])#

Added in: v0.1.21
  • actual <any>
  • expected <any>
  • message <any>

Tests shallow, coercive inequality with the not equal comparison operator ( != ).

const assert = require('assert');

assert.notEqual(1, 2);
// OK

assert.notEqual(1, 1);
// AssertionError: 1 != 1

assert.notEqual(1, '1');
// AssertionError: 1 != '1'

If the values are equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned.

assert.notStrictEqual(actual, expected[, message])#

Added in: v0.1.21
  • actual <any>
  • expected <any>
  • message <any>

Tests strict inequality as determined by the strict not equal operator ( !== ).

const assert = require('assert');

assert.notStrictEqual(1, 2);
// OK

assert.notStrictEqual(1, 1);
// AssertionError: 1 !== 1

assert.notStrictEqual(1, '1');
// OK

If the values are strictly equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned.

assert.ok(value[, message])#

Added in: v0.1.21
  • value <any>
  • message <any>

Tests if value is truthy. It is equivalent to assert.equal(!!value, true, message).

If value is not truthy, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned.

const assert = require('assert');

assert.ok(true);
// OK
assert.ok(1);
// OK
assert.ok(false);
// throws "AssertionError: false == true"
assert.ok(0);
// throws "AssertionError: 0 == true"
assert.ok(false, 'it\'s false');
// throws "AssertionError: it's false"

assert.strictEqual(actual, expected[, message])#

Added in: v0.1.21
  • actual <any>
  • expected <any>
  • message <any>

Tests strict equality as determined by the strict equality operator ( === ).

const assert = require('assert');

assert.strictEqual(1, 2);
// AssertionError: 1 === 2

assert.strictEqual(1, 1);
// OK

assert.strictEqual(1, '1');
// AssertionError: 1 === '1'

If the values are not strictly equal, an AssertionError is thrown with a message property set equal to the value of the message parameter. If the message parameter is undefined, a default error message is assigned.

assert.throws(block[, error][, message])#

Added in: v0.1.21

Expects the function block to throw an error.

If specified, error can be a constructor, RegExp, or validation function.

If specified, message will be the message provided by the AssertionError if the block fails to throw.

Validate instanceof using constructor:

assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  Error
);

Validate error message using RegExp:

assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  /value/
);

Custom error validation:

assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  function(err) {
    if ((err instanceof Error) && /value/.test(err)) {
      return true;
    }
  },
  'unexpected error'
);

Note that error can not be a string. If a string is provided as the second argument, then error is assumed to be omitted and the string will be used for message instead. This can lead to easy-to-miss mistakes:

// THIS IS A MISTAKE! DO NOT DO THIS!
assert.throws(myFunction, 'missing foo', 'did not throw with expected message');

// Do this instead.
assert.throws(myFunction, /missing foo/, 'did not throw with expected message');

Buffer#

Stability: 2 - Stable

Prior to the introduction of TypedArray in ECMAScript 2015 (ES6), the JavaScript language had no mechanism for reading or manipulating streams of binary data. The Buffer class was introduced as part of the Node.js API to make it possible to interact with octet streams in the context of things like TCP streams and file system operations.

Now that TypedArray has been added in ES6, the Buffer class implements the Uint8Array API in a manner that is more optimized and suitable for Node.js' use cases.

Instances of the Buffer class are similar to arrays of integers but correspond to fixed-sized, raw memory allocations outside the V8 heap. The size of the Buffer is established when it is created and cannot be resized.

The Buffer class is a global within Node.js, making it unlikely that one would need to ever use require('buffer').Buffer.

Examples:

// Creates a zero-filled Buffer of length 10.
const buf1 = Buffer.alloc(10);

// Creates a Buffer of length 10, filled with 0x1.
const buf2 = Buffer.alloc(10, 1);

// Creates an uninitialized buffer of length 10.
// This is faster than calling Buffer.alloc() but the returned
// Buffer instance might contain old data that needs to be
// overwritten using either fill() or write().
const buf3 = Buffer.allocUnsafe(10);

// Creates a Buffer containing [0x1, 0x2, 0x3].
const buf4 = Buffer.from([1, 2, 3]);

// Creates a Buffer containing UTF-8 bytes [0x74, 0xc3, 0xa9, 0x73, 0x74].
const buf5 = Buffer.from('tést');

// Creates a Buffer containing Latin-1 bytes [0x74, 0xe9, 0x73, 0x74].
const buf6 = Buffer.from('tést', 'latin1');

Buffer.from(), Buffer.alloc(), and Buffer.allocUnsafe()#

In versions of Node.js prior to v6, Buffer instances were created using the Buffer constructor function, which allocates the returned Buffer differently based on what arguments are provided:

  • Passing a number as the first argument to Buffer() (e.g. new Buffer(10)), allocates a new Buffer object of the specified size. The memory allocated for such Buffer instances is not initialized and can contain sensitive data. Such Buffer instances must be initialized manually by using either buf.fill(0) or by writing to the Buffer completely. While this behavior is intentional to improve performance, development experience has demonstrated that a more explicit distinction is required between creating a fast-but-uninitialized Buffer versus creating a slower-but-safer Buffer.
  • Passing a string, array, or Buffer as the first argument copies the passed object's data into the Buffer.
  • Passing an ArrayBuffer or a SharedArrayBuffer returns a Buffer that shares allocated memory with the given array buffer.

Because the behavior of new Buffer() changes significantly based on the type of value passed as the first argument, applications that do not properly validate the input arguments passed to new Buffer(), or that fail to appropriately initialize newly allocated Buffer content, can inadvertently introduce security and reliability issues into their code.

To make the creation of Buffer instances more reliable and less error prone, the various forms of the new Buffer() constructor have been deprecated and replaced by separate Buffer.from(), Buffer.alloc(), and Buffer.allocUnsafe() methods.

Developers should migrate all existing uses of the new Buffer() constructors to one of these new APIs.

Buffer instances returned by Buffer.allocUnsafe() may be allocated off a shared internal memory pool if size is less than or equal to half Buffer.poolSize. Instances returned by Buffer.allocUnsafeSlow() never use the shared internal memory pool.

The --zero-fill-buffers command line option#

Added in: v5.10.0

Node.js can be started using the --zero-fill-buffers command line option to force all newly allocated Buffer instances created using either new Buffer(size), Buffer.allocUnsafe(), Buffer.allocUnsafeSlow() or new SlowBuffer(size) to be automatically zero-filled upon creation. Use of this flag changes the default behavior of these methods and can have a significant impact on performance. Use of the --zero-fill-buffers option is recommended only when necessary to enforce that newly allocated Buffer instances cannot contain potentially sensitive data.

Example:

$ node --zero-fill-buffers
> Buffer.allocUnsafe(5);
<Buffer 00 00 00 00 00>

What makes Buffer.allocUnsafe() and Buffer.allocUnsafeSlow() "unsafe"?#

When calling Buffer.allocUnsafe() and Buffer.allocUnsafeSlow(), the segment of allocated memory is uninitialized (it is not zeroed-out). While this design makes the allocation of memory quite fast, the allocated segment of memory might contain old data that is potentially sensitive. Using a Buffer created by Buffer.allocUnsafe() without completely overwriting the memory can allow this old data to be leaked when the Buffer memory is read.

While there are clear performance advantages to using Buffer.allocUnsafe(), extra care must be taken in order to avoid introducing security vulnerabilities into an application.

Buffers and Character Encodings#

Buffer instances are commonly used to represent sequences of encoded characters such as UTF-8, UCS2, Base64 or even Hex-encoded data. It is possible to convert back and forth between Buffer instances and ordinary JavaScript strings by using an explicit character encoding.

Example:

const buf = Buffer.from('hello world', 'ascii');

// Prints: 68656c6c6f20776f726c64
console.log(buf.toString('hex'));

// Prints: aGVsbG8gd29ybGQ=
console.log(buf.toString('base64'));

The character encodings currently supported by Node.js include:

  • 'ascii' - For 7-bit ASCII data only. This encoding is fast and will strip the high bit if set.

  • 'utf8' - Multibyte encoded Unicode characters. Many web pages and other document formats use UTF-8.

  • 'utf16le' - 2 or 4 bytes, little-endian encoded Unicode characters. Surrogate pairs (U+10000 to U+10FFFF) are supported.

  • 'ucs2' - Alias of 'utf16le'.

  • 'base64' - Base64 encoding. When creating a Buffer from a string, this encoding will also correctly accept "URL and Filename Safe Alphabet" as specified in RFC4648, Section 5.

  • 'latin1' - A way of encoding the Buffer into a one-byte encoded string (as defined by the IANA in RFC1345, page 63, to be the Latin-1 supplement block and C0/C1 control codes).

  • 'binary' - Alias for 'latin1'.

  • 'hex' - Encode each byte as two hexadecimal characters.

Note: Today's browsers follow the WHATWG spec which aliases both 'latin1' and ISO-8859-1 to win-1252. This means that while doing something like http.get(), if the returned charset is one of those listed in the WHATWG spec it's possible that the server actually returned win-1252-encoded data, and using 'latin1' encoding may incorrectly decode the characters.

Buffers and TypedArray#

Buffer instances are also Uint8Array instances. However, there are subtle incompatibilities with the TypedArray specification in ECMAScript 2015. For example, while ArrayBuffer#slice() creates a copy of the slice, the implementation of Buffer#slice() creates a view over the existing Buffer without copying, making Buffer#slice() far more efficient.

It is also possible to create new TypedArray instances from a Buffer with the following caveats:

  1. The Buffer object's memory is copied to the TypedArray, not shared.

  2. The Buffer object's memory is interpreted as an array of distinct elements, and not as a byte array of the target type. That is, new Uint32Array(Buffer.from([1, 2, 3, 4])) creates a 4-element Uint32Array with elements [1, 2, 3, 4], not a Uint32Array with a single element [0x1020304] or [0x4030201].

It is possible to create a new Buffer that shares the same allocated memory as a TypedArray instance by using the TypeArray object's .buffer property.

Example:

const arr = new Uint16Array(2);

arr[0] = 5000;
arr[1] = 4000;

// Copies the contents of `arr`
const buf1 = Buffer.from(arr);

// Shares memory with `arr`
const buf2 = Buffer.from(arr.buffer);

// Prints: <Buffer 88 a0>
console.log(buf1);

// Prints: <Buffer 88 13 a0 0f>
console.log(buf2);

arr[1] = 6000;

// Prints: <Buffer 88 a0>
console.log(buf1);

// Prints: <Buffer 88 13 70 17>
console.log(buf2);

Note that when creating a Buffer using a TypedArray's .buffer, it is possible to use only a portion of the underlying ArrayBuffer by passing in byteOffset and length parameters.

Example:

const arr = new Uint16Array(20);
const buf = Buffer.from(arr.buffer, 0, 16);

// Prints: 16
console.log(buf.length);

The Buffer.from() and TypedArray.from() have different signatures and implementations. Specifically, the TypedArray variants accept a second argument that is a mapping function that is invoked on every element of the typed array:

  • TypedArray.from(source[, mapFn[, thisArg]])

The Buffer.from() method, however, does not support the use of a mapping function:

Buffers and ES6 iteration#

Buffer instances can be iterated over using the ECMAScript 2015 (ES6) for..of syntax.

Example:

const buf = Buffer.from([1, 2, 3]);

// Prints:
//   1
//   2
//   3
for (const b of buf) {
  console.log(b);
}

Additionally, the buf.values(), buf.keys(), and buf.entries() methods can be used to create iterators.

Class: Buffer#

The Buffer class is a global type for dealing with binary data directly. It can be constructed in a variety of ways.

new Buffer(array)#

Deprecated since: v6.0.0
Stability: 0 - Deprecated: Use Buffer.from(array) instead.

Allocates a new Buffer using an array of octets.

Example:

// Creates a new Buffer containing the UTF-8 bytes of the string 'buffer'
const buf = new Buffer([0x62, 0x75, 0x66, 0x66, 0x65, 0x72]);

new Buffer(buffer)#

Deprecated since: v6.0.0
Stability: 0 - Deprecated: Use Buffer.from(buffer) instead.
  • buffer <Buffer> An existing Buffer to copy data from.

Copies the passed buffer data onto a new Buffer instance.

Example:

const buf1 = new Buffer('buffer');
const buf2 = new Buffer(buf1);

buf1[0] = 0x61;

// Prints: auffer
console.log(buf1.toString());

// Prints: buffer
console.log(buf2.toString());

new Buffer(arrayBuffer[, byteOffset [, length]])#

Deprecated since: v6.0.0
Stability: 0 - Deprecated: Use Buffer.from(arrayBuffer[, byteOffset [, length]]) instead.

This creates a view of the ArrayBuffer or SharedArrayBuffer without copying the underlying memory. For example, when passed a reference to the .buffer property of a TypedArray instance, the newly created Buffer will share the same allocated memory as the TypedArray.

The optional byteOffset and length arguments specify a memory range within the arrayBuffer that will be shared by the Buffer.

Example:

const arr = new Uint16Array(2);

arr[0] = 5000;
arr[1] = 4000;

// Shares memory with `arr`
const buf = new Buffer(arr.buffer);

// Prints: <Buffer 88 13 a0 0f>
console.log(buf);

// Changing the original Uint16Array changes the Buffer also
arr[1] = 6000;

// Prints: <Buffer 88 13 70 17>
console.log(buf);

new Buffer(size)#

Deprecated since: v6.0.0
Stability: 0 - Deprecated: Use Buffer.alloc() instead (also see Buffer.allocUnsafe()).
  • size <integer> The desired length of the new Buffer.

Allocates a new Buffer of size bytes. The size must be less than or equal to the value of buffer.kMaxLength. Otherwise, a RangeError is thrown. A zero-length Buffer will be created if size <= 0.

Unlike ArrayBuffers, the underlying memory for Buffer instances created in this way is not initialized. The contents of a newly created Buffer are unknown and could contain sensitive data. Use Buffer.alloc(size) instead to initialize a Buffer to zeroes.

Example:

const buf = new Buffer(10);

// Prints: (contents may vary): <Buffer 48 21 4b 00 00 00 00 00 30 dd>
console.log(buf);

buf.fill(0);

// Prints: <Buffer 00 00 00 00 00 00 00 00 00 00>
console.log(buf);

new Buffer(string[, encoding])#

Deprecated since: v6.0.0
Stability: 0 - Deprecated: Use Buffer.from(string[, encoding]) instead.
  • string <string> String to encode.
  • encoding <string> The encoding of string. Default: 'utf8'

Creates a new Buffer containing the given JavaScript string string. If provided, the encoding parameter identifies the character encoding of string.

Examples:

const buf1 = new Buffer('this is a tést');

// Prints: this is a tést
console.log(buf1.toString());

// Prints: this is a tC)st
console.log(buf1.toString('ascii'));


const buf2 = new Buffer('7468697320697320612074c3a97374', 'hex');

// Prints: this is a tést
console.log(buf2.toString());

Class Method: Buffer.alloc(size[, fill[, encoding]])#

Added in: v5.10.0
  • size <integer> The desired length of the new Buffer.
  • fill <string> | <Buffer> | <integer> A value to pre-fill the new Buffer with. Default: 0
  • encoding <string> If fill is a string, this is its encoding. Default: 'utf8'

Allocates a new Buffer of size bytes. If fill is undefined, the Buffer will be zero-filled.

Example:

const buf = Buffer.alloc(5);

// Prints: <Buffer 00 00 00 00 00>
console.log(buf);

The size must be less than or equal to the value of buffer.kMaxLength. Otherwise, a RangeError is thrown. A zero-length Buffer will be created if size <= 0.

If fill is specified, the allocated Buffer will be initialized by calling buf.fill(fill).

Example:

const buf = Buffer.alloc(5, 'a');

// Prints: <Buffer 61 61 61 61 61>
console.log(buf);

If both fill and encoding are specified, the allocated Buffer will be initialized by calling buf.fill(fill, encoding).

Example:

const buf = Buffer.alloc(11, 'aGVsbG8gd29ybGQ=', 'base64');

// Prints: <Buffer 68 65 6c 6c 6f 20 77 6f 72 6c 64>
console.log(buf);

Calling Buffer.alloc() can be significantly slower than the alternative Buffer.allocUnsafe() but ensures that the newly created Buffer instance contents will never contain sensitive data.

A TypeError will be thrown if size is not a number.

Class Method: Buffer.allocUnsafe(size)#

Added in: v5.10.0
  • size <integer> The desired length of the new Buffer.

Allocates a new non-zero-filled Buffer of size bytes. The size must be less than or equal to the value of buffer.kMaxLength. Otherwise, a RangeError is thrown. A zero-length Buffer will be created if size <= 0.

The underlying memory for Buffer instances created in this way is not initialized. The contents of the newly created Buffer are unknown and may contain sensitive data. Use Buffer.alloc() instead to initialize Buffer instances to zeroes.

Example:

const buf = Buffer.allocUnsafe(10);

// Prints: (contents may vary): <Buffer a0 8b 28 3f 01 00 00 00 50 32>
console.log(buf);

buf.fill(0);

// Prints: <Buffer 00 00 00 00 00 00 00 00 00 00>
console.log(buf);

A TypeError will be thrown if size is not a number.

Note that the Buffer module pre-allocates an internal Buffer instance of size Buffer.poolSize that is used as a pool for the fast allocation of new Buffer instances created using Buffer.allocUnsafe() and the deprecated new Buffer(size) constructor only when size is less than or equal to Buffer.poolSize >> 1 (floor of Buffer.poolSize divided by two).

Use of this pre-allocated internal memory pool is a key difference between calling Buffer.alloc(size, fill) vs. Buffer.allocUnsafe(size).fill(fill). Specifically, Buffer.alloc(size, fill) will never use the internal Buffer pool, while Buffer.allocUnsafe(size).fill(fill) will use the internal Buffer pool if size is less than or equal to half Buffer.poolSize. The difference is subtle but can be important when an application requires the additional performance that Buffer.allocUnsafe() provides.

Class Method: Buffer.allocUnsafeSlow(size)#

Added in: v5.12.0
  • size <integer> The desired length of the new Buffer.

Allocates a new non-zero-filled and non-pooled Buffer of size bytes. The size must be less than or equal to the value of buffer.kMaxLength. Otherwise, a RangeError is thrown. A zero-length Buffer will be created if size <= 0.

The underlying memory for Buffer instances created in this way is not initialized. The contents of the newly created Buffer are unknown and may contain sensitive data. Use buf.fill(0) to initialize such Buffer instances to zeroes.

When using Buffer.allocUnsafe() to allocate new Buffer instances, allocations under 4KB are, by default, sliced from a single pre-allocated Buffer. This allows applications to avoid the garbage collection overhead of creating many individually allocated Buffer instances. This approach improves both performance and memory usage by eliminating the need to track and cleanup as many Persistent objects.

However, in the case where a developer may need to retain a small chunk of memory from a pool for an indeterminate amount of time, it may be appropriate to create an un-pooled Buffer instance using Buffer.allocUnsafeSlow() then copy out the relevant bits.

Example:

// Need to keep around a few small chunks of memory
const store = [];

socket.on('readable', () => {
  const data = socket.read();

  // Allocate for retained data
  const sb = Buffer.allocUnsafeSlow(10);

  // Copy the data into the new allocation
  data.copy(sb, 0, 0, 10);

  store.push(sb);
});

Use of Buffer.allocUnsafeSlow() should be used only as a last resort after a developer has observed undue memory retention in their applications.

A TypeError will be thrown if size is not a number.

Class Method: Buffer.byteLength(string[, encoding])#

Added in: v0.1.90

Returns the actual byte length of a string. This is not the same as String.prototype.length since that returns the number of characters in a string.

Note that for 'base64' and 'hex', this function assumes valid input. For strings that contain non-Base64/Hex-encoded data (e.g. whitespace), the return value might be greater than the length of a Buffer created from the string.

Example:

const str = '\u00bd + \u00bc = \u00be';

// Prints: ½ + ¼ = ¾: 9 characters, 12 bytes
console.log(`${str}: ${str.length} characters, ` +
            `${Buffer.byteLength(str, 'utf8')} bytes`);

When string is a Buffer/DataView/TypedArray/ArrayBuffer/ SharedArrayBuffer, the actual byte length is returned.

Otherwise, converts to String and returns the byte length of string.

Class Method: Buffer.compare(buf1, buf2)#

Added in: v0.11.13

Compares buf1 to buf2 typically for the purpose of sorting arrays of Buffer instances. This is equivalent to calling buf1.compare(buf2).

Example:

const buf1 = Buffer.from('1234');
const buf2 = Buffer.from('0123');
const arr = [buf1, buf2];

// Prints: [ <Buffer 30 31 32 33>, <Buffer 31 32 33 34> ]
// (This result is equal to: [buf2, buf1])
console.log(arr.sort(Buffer.compare));

Class Method: Buffer.concat(list[, totalLength])#

Added in: v0.7.11
  • list <Array> List of Buffer instances to concat.
  • totalLength <integer> Total length of the Buffer instances in list when concatenated.
  • Returns: <Buffer>

Returns a new Buffer which is the result of concatenating all the Buffer instances in the list together.

If the list has no items, or if the totalLength is 0, then a new zero-length Buffer is returned.

If totalLength is not provided, it is calculated from the Buffer instances in list. This however causes an additional loop to be executed in order to calculate the totalLength, so it is faster to provide the length explicitly if it is already known.

If totalLength is provided, it is coerced to an unsigned integer. If the combined length of the Buffers in list exceeds totalLength, the result is truncated to totalLength.

Example: Create a single Buffer from a list of three Buffer instances

const buf1 = Buffer.alloc(10);
const buf2 = Buffer.alloc(14);
const buf3 = Buffer.alloc(18);
const totalLength = buf1.length + buf2.length + buf3.length;

// Prints: 42
console.log(totalLength);

const bufA = Buffer.concat([buf1, buf2, buf3], totalLength);

// Prints: <Buffer 00 00 00 00 ...>
console.log(bufA);

// Prints: 42
console.log(bufA.length);

Class Method: Buffer.from(array)#

Added in: v5.10.0

Allocates a new Buffer using an array of octets.

Example:

// Creates a new Buffer containing UTF-8 bytes of the string 'buffer'
const buf = Buffer.from([0x62, 0x75, 0x66, 0x66, 0x65, 0x72]);

A TypeError will be thrown if array is not an Array.

Class Method: Buffer.from(arrayBuffer[, byteOffset[, length]])#

Added in: v5.10.0

This creates a view of the ArrayBuffer without copying the underlying memory. For example, when passed a reference to the .buffer property of a TypedArray instance, the newly created Buffer will share the same allocated memory as the TypedArray.

Example:

const arr = new Uint16Array(2);

arr[0] = 5000;
arr[1] = 4000;

// Shares memory with `arr`
const buf = Buffer.from(arr.buffer);

// Prints: <Buffer 88 13 a0 0f>
console.log(buf);

// Changing the original Uint16Array changes the Buffer also
arr[1] = 6000;

// Prints: <Buffer 88 13 70 17>
console.log(buf);

The optional byteOffset and length arguments specify a memory range within the arrayBuffer that will be shared by the Buffer.

Example:

const ab = new ArrayBuffer(10);
const buf = Buffer.from(ab, 0, 2);

// Prints: 2
console.log(buf.length);

A TypeError will be thrown if arrayBuffer is not an ArrayBuffer or a SharedArrayBuffer.

Class Method: Buffer.from(buffer)#

Added in: v5.10.0
  • buffer <Buffer> An existing Buffer to copy data from.

Copies the passed buffer data onto a new Buffer instance.

Example:

const buf1 = Buffer.from('buffer');
const buf2 = Buffer.from(buf1);

buf1[0] = 0x61;

// Prints: auffer
console.log(buf1.toString());

// Prints: buffer
console.log(buf2.toString());

A TypeError will be thrown if buffer is not a Buffer.

Class Method: Buffer.from(string[, encoding])#

Added in: v5.10.0
  • string <string> A string to encode.
  • encoding <string> The encoding of string. Default: 'utf8'

Creates a new Buffer containing the given JavaScript string string. If provided, the encoding parameter identifies the character encoding of string.

Examples:

const buf1 = Buffer.from('this is a tést');

// Prints: this is a tést
console.log(buf1.toString());

// Prints: this is a tC)st
console.log(buf1.toString('ascii'));


const buf2 = Buffer.from('7468697320697320612074c3a97374', 'hex');

// Prints: this is a tést
console.log(buf2.toString());

A TypeError will be thrown if string is not a string.

Class Method: Buffer.isBuffer(obj)#

Added in: v0.1.101

Returns true if obj is a Buffer, false otherwise.

Class Method: Buffer.isEncoding(encoding)#

Added in: v0.9.1

Returns true if encoding contains a supported character encoding, or false otherwise.

Class Property: Buffer.poolSize#

Added in: v0.11.3

This is the number of bytes used to determine the size of pre-allocated, internal Buffer instances used for pooling. This value may be modified.

buf[index]#

The index operator [index] can be used to get and set the octet at position index in buf. The values refer to individual bytes, so the legal value range is between 0x00 and 0xFF (hex) or 0 and 255 (decimal).

This operator is inherited from Uint8Array, so its behavior on out-of-bounds access is the same as UInt8Array - that is, getting returns undefined and setting does nothing.

Example: Copy an ASCII string into a Buffer, one byte at a time

const str = 'Node.js';
const buf = Buffer.allocUnsafe(str.length);

for (let i = 0; i < str.length; i++) {
  buf[i] = str.charCodeAt(i);
}

// Prints: Node.js
console.log(buf.toString('ascii'));

buf.compare(target[, targetStart[, targetEnd[, sourceStart[, sourceEnd]]]])#

Added in: v0.11.13
  • target <Buffer> A Buffer to compare to.
  • targetStart <integer> The offset within target at which to begin comparison. Default: 0
  • targetEnd <integer> The offset with target at which to end comparison (not inclusive). Default: target.length
  • sourceStart <integer> The offset within buf at which to begin comparison. Default: 0
  • sourceEnd <integer> The offset within buf at which to end comparison (not inclusive). Default: buf.length
  • Returns: <integer>

Compares buf with target and returns a number indicating whether buf comes before, after, or is the same as target in sort order. Comparison is based on the actual sequence of bytes in each Buffer.

  • 0 is returned if target is the same as buf
  • 1 is returned if target should come before buf when sorted.
  • -1 is returned if target should come after buf when sorted.

Examples:

const buf1 = Buffer.from('ABC');
const buf2 = Buffer.from('BCD');
const buf3 = Buffer.from('ABCD');

// Prints: 0
console.log(buf1.compare(buf1));

// Prints: -1
console.log(buf1.compare(buf2));

// Prints: -1
console.log(buf1.compare(buf3));

// Prints: 1
console.log(buf2.compare(buf1));

// Prints: 1
console.log(buf2.compare(buf3));

// Prints: [ <Buffer 41 42 43>, <Buffer 41 42 43 44>, <Buffer 42 43 44> ]
// (This result is equal to: [buf1, buf3, buf2])
console.log([buf1, buf2, buf3].sort(Buffer.compare));

The optional targetStart, targetEnd, sourceStart, and sourceEnd arguments can be used to limit the comparison to specific ranges within target and buf respectively.

Examples:

const buf1 = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8, 9]);
const buf2 = Buffer.from([5, 6, 7, 8, 9, 1, 2, 3, 4]);

// Prints: 0
console.log(buf1.compare(buf2, 5, 9, 0, 4));

// Prints: -1
console.log(buf1.compare(buf2, 0, 6, 4));

// Prints: 1
console.log(buf1.compare(buf2, 5, 6, 5));

A RangeError will be thrown if: targetStart < 0, sourceStart < 0, targetEnd > target.byteLength or sourceEnd > source.byteLength.

buf.copy(target[, targetStart[, sourceStart[, sourceEnd]]])#

Added in: v0.1.90
  • target <Buffer> | <Uint8Array> A Buffer or Uint8Array to copy into.
  • targetStart <integer> The offset within target at which to begin copying to. Default: 0
  • sourceStart <integer> The offset within buf at which to begin copying from. Default: 0
  • sourceEnd <integer> The offset within buf at which to stop copying (not inclusive). Default: buf.length
  • Returns: <integer> The number of bytes copied.

Copies data from a region of buf to a region in target even if the target memory region overlaps with buf.

Example: Create two Buffer instances, buf1 and buf2, and copy buf1 from byte 16 through byte 19 into buf2, starting at the 8th byte in buf2

const buf1 = Buffer.allocUnsafe(26);
const buf2 = Buffer.allocUnsafe(26).fill('!');

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'
  buf1[i] = i + 97;
}

buf1.copy(buf2, 8, 16, 20);

// Prints: !!!!!!!!qrst!!!!!!!!!!!!!
console.log(buf2.toString('ascii', 0, 25));

Example: Create a single Buffer and copy data from one region to an overlapping region within the same Buffer

const buf = Buffer.allocUnsafe(26);

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'
  buf[i] = i + 97;
}

buf.copy(buf, 0, 4, 10);

// Prints: efghijghijklmnopqrstuvwxyz
console.log(buf.toString());

buf.entries()#

Added in: v1.1.0
  • Returns: <Iterator>

Creates and returns an iterator of [index, byte] pairs from the contents of buf.

Example: Log the entire contents of a Buffer

const buf = Buffer.from('buffer');

// Prints:
//   [0, 98]
//   [1, 117]
//   [2, 102]
//   [3, 102]
//   [4, 101]
//   [5, 114]
for (const pair of buf.entries()) {
  console.log(pair);
}

buf.equals(otherBuffer)#

Added in: v0.11.13

Returns true if both buf and otherBuffer have exactly the same bytes, false otherwise.

Examples:

const buf1 = Buffer.from('ABC');
const buf2 = Buffer.from('414243', 'hex');
const buf3 = Buffer.from('ABCD');

// Prints: true
console.log(buf1.equals(buf2));

// Prints: false
console.log(buf1.equals(buf3));

buf.fill(value[, offset[, end]][, encoding])#

Added in: v0.5.0

Fills buf with the specified value. If the offset and end are not given, the entire buf will be filled. This is meant to be a small simplification to allow the creation and filling of a Buffer to be done on a single line.

Example: Fill a Buffer with the ASCII character 'h'

const b = Buffer.allocUnsafe(50).fill('h');

// Prints: hhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh
console.log(b.toString());

value is coerced to a uint32 value if it is not a String or Integer.

If the final write of a fill() operation falls on a multi-byte character, then only the first bytes of that character that fit into buf are written.

Example: Fill a Buffer with a two-byte character

// Prints: <Buffer c8 a2 c8>
console.log(Buffer.allocUnsafe(3).fill('\u0222'));

buf.includes(value[, byteOffset][, encoding])#

Added in: v5.3.0
  • value <string> | <Buffer> | <integer> What to search for.
  • byteOffset <integer> Where to begin searching in buf. Default: 0
  • encoding <string> If value is a string, this is its encoding. Default: 'utf8'
  • Returns: <boolean> true if value was found in buf, false otherwise.

Equivalent to buf.indexOf() !== -1.

Examples:

const buf = Buffer.from('this is a buffer');

// Prints: true
console.log(buf.includes('this'));

// Prints: true
console.log(buf.includes('is'));

// Prints: true
console.log(buf.includes(Buffer.from('a buffer')));

// Prints: true
// (97 is the decimal ASCII value for 'a')
console.log(buf.includes(97));

// Prints: false
console.log(buf.includes(Buffer.from('a buffer example')));

// Prints: true
console.log(buf.includes(Buffer.from('a buffer example').slice(0, 8)));

// Prints: false
console.log(buf.includes('this', 4));

buf.indexOf(value[, byteOffset][, encoding])#

Added in: v1.5.0
  • value <string> | <Buffer> | <integer> What to search for.
  • byteOffset <integer> Where to begin searching in buf. Default: 0
  • encoding <string> If value is a string, this is its encoding. Default: 'utf8'
  • Returns: <integer> The index of the first occurrence of value in buf or -1 if buf does not contain value.

If value is:

  • a string, value is interpreted according to the character encoding in encoding.
  • a Buffer, value will be used in its entirety. To compare a partial Buffer use buf.slice().
  • a number, value will be interpreted as an unsigned 8-bit integer value between 0 and 255.

Examples:

const buf = Buffer.from('this is a buffer');

// Prints: 0
console.log(buf.indexOf('this'));

// Prints: 2
console.log(buf.indexOf('is'));

// Prints: 8
console.log(buf.indexOf(Buffer.from('a buffer')));

// Prints: 8
// (97 is the decimal ASCII value for 'a')
console.log(buf.indexOf(97));

// Prints: -1
console.log(buf.indexOf(Buffer.from('a buffer example')));

// Prints: 8
console.log(buf.indexOf(Buffer.from('a buffer example').slice(0, 8)));


const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'ucs2');

// Prints: 4
console.log(utf16Buffer.indexOf('\u03a3', 0, 'ucs2'));

// Prints: 6
console.log(utf16Buffer.indexOf('\u03a3', -4, 'ucs2'));

If value is not a string, number, or Buffer, this method will throw a TypeError. If value is a number, it will be coerced to a valid byte value, an integer between 0 and 255.

If byteOffset is not a number, it will be coerced to a number. Any arguments that coerce to NaN or 0, like {}, [], null or undefined, will search the whole buffer. This behavior matches String#indexOf().

const b = Buffer.from('abcdef');

// Passing a value that's a number, but not a valid byte
// Prints: 2, equivalent to searching for 99 or 'c'
console.log(b.indexOf(99.9));
console.log(b.indexOf(256 + 99));

// Passing a byteOffset that coerces to NaN or 0
// Prints: 1, searching the whole buffer
console.log(b.indexOf('b', undefined));
console.log(b.indexOf('b', {}));
console.log(b.indexOf('b', null));
console.log(b.indexOf('b', []));

buf.includes(value[, byteOffset][, encoding])#

Added in: v5.3.0
  • value <String> | <Buffer> | <Integer> What to search for.
  • byteOffset <Integer> Where to begin searching in buf. Default: 0
  • encoding <String> If value is a string, this is its encoding. Default: 'utf8'
  • Returns: <Boolean> true if value was found in buf, false otherwise

Equivalent to buf.indexOf() !== -1.

Examples:

const buf = Buffer.from('this is a buffer');

// Prints: true
console.log(buf.includes('this'));

// Prints: true
console.log(buf.includes('is'));

// Prints: true
console.log(buf.includes(Buffer.from('a buffer')));

// Prints: true
// (97 is the decimal ASCII value for 'a')
console.log(buf.includes(97));

// Prints: false
console.log(buf.includes(Buffer.from('a buffer example')));

// Prints: true
console.log(buf.includes(Buffer.from('a buffer example').slice(0, 8)));

// Prints: false
console.log(buf.includes('this', 4));

buf.keys()#

Added in: v1.1.0
  • Returns: <Iterator>

Creates and returns an iterator of buf keys (indices).

Example:

const buf = Buffer.from('buffer');

// Prints:
//   0
//   1
//   2
//   3
//   4
//   5
for (const key of buf.keys()) {
  console.log(key);
}

buf.lastIndexOf(value[, byteOffset][, encoding])#

Added in: v6.0.0
  • value <string> | <Buffer> | <integer> What to search for.
  • byteOffset <integer> Where to begin searching in buf. Default: buf.length- 1
  • encoding <string> If value is a string, this is its encoding. Default: 'utf8'
  • Returns: <integer> The index of the last occurrence of value in buf or -1 if buf does not contain value.

Identical to buf.indexOf(), except buf is searched from back to front instead of front to back.

Examples:

const buf = Buffer.from('this buffer is a buffer');

// Prints: 0
console.log(buf.lastIndexOf('this'));

// Prints: 17
console.log(buf.lastIndexOf('buffer'));

// Prints: 17
console.log(buf.lastIndexOf(Buffer.from('buffer')));

// Prints: 15
// (97 is the decimal ASCII value for 'a')
console.log(buf.lastIndexOf(97));

// Prints: -1
console.log(buf.lastIndexOf(Buffer.from('yolo')));

// Prints: 5
console.log(buf.lastIndexOf('buffer', 5));

// Prints: -1
console.log(buf.lastIndexOf('buffer', 4));


const utf16Buffer = Buffer.from('\u039a\u0391\u03a3\u03a3\u0395', 'ucs2');

// Prints: 6
console.log(utf16Buffer.lastIndexOf('\u03a3', undefined, 'ucs2'));

// Prints: 4
console.log(utf16Buffer.lastIndexOf('\u03a3', -5, 'ucs2'));

If value is not a string, number, or Buffer, this method will throw a TypeError. If value is a number, it will be coerced to a valid byte value, an integer between 0 and 255.

If byteOffset is not a number, it will be coerced to a number. Any arguments that coerce to NaN, like {} or undefined, will search the whole buffer. This behavior matches String#lastIndexOf().

const b = Buffer.from('abcdef');

// Passing a value that's a number, but not a valid byte
// Prints: 2, equivalent to searching for 99 or 'c'
console.log(b.lastIndexOf(99.9));
console.log(b.lastIndexOf(256 + 99));

// Passing a byteOffset that coerces to NaN
// Prints: 1, searching the whole buffer
console.log(b.lastIndexOf('b', undefined));
console.log(b.lastIndexOf('b', {}));

// Passing a byteOffset that coerces to 0
// Prints: -1, equivalent to passing 0
console.log(b.lastIndexOf('b', null));
console.log(b.lastIndexOf('b', []));

buf.length#

Added in: v0.1.90

Returns the amount of memory allocated for buf in bytes. Note that this does not necessarily reflect the amount of "usable" data within buf.

Example: Create a Buffer and write a shorter ASCII string to it

const buf = Buffer.alloc(1234);

// Prints: 1234
console.log(buf.length);

buf.write('some string', 0, 'ascii');

// Prints: 1234
console.log(buf.length);

While the length property is not immutable, changing the value of length can result in undefined and inconsistent behavior. Applications that wish to modify the length of a Buffer should therefore treat length as read-only and use buf.slice() to create a new Buffer.

Examples:

let buf = Buffer.allocUnsafe(10);

buf.write('abcdefghj', 0, 'ascii');

// Prints: 10
console.log(buf.length);

buf = buf.slice(0, 5);

// Prints: 5
console.log(buf.length);

buf.readDoubleBE(offset[, noAssert])#

buf.readDoubleLE(offset[, noAssert])#

Added in: v0.11.15
  • offset <integer> Number of bytes to skip before starting to read. Must satisfy: 0 <= offset <= buf.length - 8.
  • noAssert <boolean> Skip offset validation? Default: false
  • Returns: <number>

Reads a 64-bit double from buf at the specified offset with specified endian format (readDoubleBE() returns big endian, readDoubleLE() returns little endian).

Setting noAssert to true allows offset to be beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.from([1, 2, 3, 4, 5, 6, 7, 8]);

// Prints: 8.20788039913184e-304
console.log(buf.readDoubleBE());

// Prints: 5.447603722011605e-270
console.log(buf.readDoubleLE());

// Throws an exception: RangeError: Index out of range
console.log(buf.readDoubleLE(1));

// Warning: reads passed end of buffer!
// This will result in a segmentation fault! Don't do this!
console.log(buf.readDoubleLE(1, true));

buf.readFloatBE(offset[, noAssert])#

buf.readFloatLE(offset[, noAssert])#

Added in: v0.11.15
  • offset <integer> Number of bytes to skip before starting to read. Must satisfy: 0 <= offset <= buf.length - 4.
  • noAssert <boolean> Skip offset validation? Default: false
  • Returns: <number>

Reads a 32-bit float from buf at the specified offset with specified endian format (readFloatBE() returns big endian, readFloatLE() returns little endian).

Setting noAssert to true allows offset to be beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.from([1, 2, 3, 4]);

// Prints: 2.387939260590663e-38
console.log(buf.readFloatBE());

// Prints: 1.539989614439558e-36
console.log(buf.readFloatLE());

// Throws an exception: RangeError: Index out of range
console.log(buf.readFloatLE(1));

// Warning: reads passed end of buffer!
// This will result in a segmentation fault! Don't do this!
console.log(buf.readFloatLE(1, true));

buf.readInt8(offset[, noAssert])#

Added in: v0.5.0
  • offset <integer> Number of bytes to skip before starting to read. Must satisfy: 0 <= offset <= buf.length - 1.
  • noAssert <boolean> Skip offset validation? Default: false
  • Returns: <integer>

Reads a signed 8-bit integer from buf at the specified offset.

Setting noAssert to true allows offset to be beyond the end of buf, but the result should be considered undefined behavior.

Integers read from a Buffer are interpreted as two's complement signed values.

Examples:

const buf = Buffer.from([-1, 5]);

// Prints: -1
console.log(buf.readInt8(0));

// Prints: 5
console.log(buf.readInt8(1));

// Throws an exception: RangeError: Index out of range
console.log(buf.readInt8(2));

buf.readInt16BE(offset[, noAssert])#

buf.readInt16LE(offset[, noAssert])#

Added in: v0.5.5
  • offset <integer> Number of bytes to skip before starting to read. Must satisfy: 0 <= offset <= buf.length - 2.
  • noAssert <boolean> Skip offset validation? Default: false
  • Returns: <integer>

Reads a signed 16-bit integer from buf at the specified offset with the specified endian format (readInt16BE() returns big endian, readInt16LE() returns little endian).

Setting noAssert to true allows offset to be beyond the end of buf, but the result should be considered undefined behavior.

Integers read from a Buffer are interpreted as two's complement signed values.

Examples:

const buf = Buffer.from([0, 5]);

// Prints: 5
console.log(buf.readInt16BE());

// Prints: 1280
console.log(buf.readInt16LE());

// Throws an exception: RangeError: Index out of range
console.log(buf.readInt16LE(1));

buf.readInt32BE(offset[, noAssert])#

buf.readInt32LE(offset[, noAssert])#

Added in: v0.5.5
  • offset <integer> Number of bytes to skip before starting to read. Must satisfy: 0 <= offset <= buf.length - 4.
  • noAssert <boolean> Skip offset validation? Default: false
  • Returns: <integer>

Reads a signed 32-bit integer from buf at the specified offset with the specified endian format (readInt32BE() returns big endian, readInt32LE() returns little endian).

Setting noAssert to true allows offset to be beyond the end of buf, but the result should be considered undefined behavior.

Integers read from a Buffer are interpreted as two's complement signed values.

Examples:

const buf = Buffer.from([0, 0, 0, 5]);

// Prints: 5
console.log(buf.readInt32BE());

// Prints: 83886080
console.log(buf.readInt32LE());

// Throws an exception: RangeError: Index out of range
console.log(buf.readInt32LE(1));

buf.readIntBE(offset, byteLength[, noAssert])#

buf.readIntLE(offset, byteLength[, noAssert])#

Added in: v0.11.15
  • offset <integer> Number of bytes to skip before starting to read. Must satisfy: 0 <= offset <= buf.length - byteLength.
  • byteLength <integer> Number of bytes to read. Must satisfy: 0 < byteLength <= 6.
  • noAssert <boolean> Skip offset and byteLength validation? Default: false.
  • Returns: <integer>

Reads byteLength number of bytes from buf at the specified offset and interprets the result as a two's complement signed value. Supports up to 48 bits of accuracy.

Setting noAssert to true allows offset to be beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

// Prints: -546f87a9cbee
console.log(buf.readIntLE(0, 6).toString(16));

// Prints: 1234567890ab
console.log(buf.readIntBE(0, 6).toString(16));

// Throws an exception: RangeError: Index out of range
console.log(buf.readIntBE(1, 6).toString(16));

buf.readUInt8(offset[, noAssert])#

Added in: v0.5.0
  • offset <integer> Number of bytes to skip before starting to read. Must satisfy: 0 <= offset <= buf.length - 1.
  • noAssert <boolean> Skip offset validation? Default: false
  • Returns: <integer>

Reads an unsigned 8-bit integer from buf at the specified offset.

Setting noAssert to true allows offset to be beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.from([1, -2]);

// Prints: 1
console.log(buf.readUInt8(0));

// Prints: 254
console.log(buf.readUInt8(1));

// Throws an exception: RangeError: Index out of range
console.log(buf.readUInt8(2));

buf.readUInt16BE(offset[, noAssert])#

buf.readUInt16LE(offset[, noAssert])#

Added in: v0.5.5
  • offset <integer> Number of bytes to skip before starting to read. Must satisfy: 0 <= offset <= buf.length - 2.
  • noAssert <boolean> Skip offset validation? Default: false
  • Returns: <integer>

Reads an unsigned 16-bit integer from buf at the specified offset with specified endian format (readUInt16BE() returns big endian, readUInt16LE() returns little endian).

Setting noAssert to true allows offset to be beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.from([0x12, 0x34, 0x56]);

// Prints: 1234
console.log(buf.readUInt16BE(0).toString(16));

// Prints: 3412
console.log(buf.readUInt16LE(0).toString(16));

// Prints: 3456
console.log(buf.readUInt16BE(1).toString(16));

// Prints: 5634
console.log(buf.readUInt16LE(1).toString(16));

// Throws an exception: RangeError: Index out of range
console.log(buf.readUInt16LE(2).toString(16));

buf.readUInt32BE(offset[, noAssert])#

buf.readUInt32LE(offset[, noAssert])#

Added in: v0.5.5
  • offset <integer> Number of bytes to skip before starting to read. Must satisfy: 0 <= offset <= buf.length - 4.
  • noAssert <boolean> Skip offset validation? Default: false
  • Returns: <integer>

Reads an unsigned 32-bit integer from buf at the specified offset with specified endian format (readUInt32BE() returns big endian, readUInt32LE() returns little endian).

Setting noAssert to true allows offset to be beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78]);

// Prints: 12345678
console.log(buf.readUInt32BE(0).toString(16));

// Prints: 78563412
console.log(buf.readUInt32LE(0).toString(16));

// Throws an exception: RangeError: Index out of range
console.log(buf.readUInt32LE(1).toString(16));

buf.readUIntBE(offset, byteLength[, noAssert])#

buf.readUIntLE(offset, byteLength[, noAssert])#

Added in: v0.11.15
  • offset <integer> Number of bytes to skip before starting to read. Must satisfy: 0 <= offset <= buf.length - byteLength.
  • byteLength <integer> Number of bytes to read. Must satisfy: 0 < byteLength <= 6.
  • noAssert <boolean> Skip offset and byteLength validation? Default: false
  • Returns: <integer>

Reads byteLength number of bytes from buf at the specified offset and interprets the result as an unsigned integer. Supports up to 48 bits of accuracy.

Setting noAssert to true allows offset to be beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.from([0x12, 0x34, 0x56, 0x78, 0x90, 0xab]);

// Prints: 1234567890ab
console.log(buf.readUIntBE(0, 6).toString(16));

// Prints: ab9078563412
console.log(buf.readUIntLE(0, 6).toString(16));

// Throws an exception: RangeError: Index out of range
console.log(buf.readUIntBE(1, 6).toString(16));

buf.slice([start[, end]])#

Added in: v0.3.0

Returns a new Buffer that references the same memory as the original, but offset and cropped by the start and end indices.

Note that modifying the new Buffer slice will modify the memory in the original Buffer because the allocated memory of the two objects overlap.

Example: Create a Buffer with the ASCII alphabet, take a slice, and then modify one byte from the original Buffer

const buf1 = Buffer.allocUnsafe(26);

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'
  buf1[i] = i + 97;
}

const buf2 = buf1.slice(0, 3);

// Prints: abc
console.log(buf2.toString('ascii', 0, buf2.length));

buf1[0] = 33;

// Prints: !bc
console.log(buf2.toString('ascii', 0, buf2.length));

Specifying negative indexes causes the slice to be generated relative to the end of buf rather than the beginning.

Examples:

const buf = Buffer.from('buffer');

// Prints: buffe
// (Equivalent to buf.slice(0, 5))
console.log(buf.slice(-6, -1).toString());

// Prints: buff
// (Equivalent to buf.slice(0, 4))
console.log(buf.slice(-6, -2).toString());

// Prints: uff
// (Equivalent to buf.slice(1, 4))
console.log(buf.slice(-5, -2).toString());

buf.swap16()#

Added in: v5.10.0

Interprets buf as an array of unsigned 16-bit integers and swaps the byte-order in-place. Throws a RangeError if buf.length is not a multiple of 2.

Examples:

const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

// Prints: <Buffer 01 02 03 04 05 06 07 08>
console.log(buf1);

buf1.swap16();

// Prints: <Buffer 02 01 04 03 06 05 08 07>
console.log(buf1);


const buf2 = Buffer.from([0x1, 0x2, 0x3]);

// Throws an exception: RangeError: Buffer size must be a multiple of 16-bits
buf2.swap16();

buf.swap32()#

Added in: v5.10.0

Interprets buf as an array of unsigned 32-bit integers and swaps the byte-order in-place. Throws a RangeError if buf.length is not a multiple of 4.

Examples:

const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

// Prints: <Buffer 01 02 03 04 05 06 07 08>
console.log(buf1);

buf1.swap32();

// Prints: <Buffer 04 03 02 01 08 07 06 05>
console.log(buf1);


const buf2 = Buffer.from([0x1, 0x2, 0x3]);

// Throws an exception: RangeError: Buffer size must be a multiple of 32-bits
buf2.swap32();

buf.swap64()#

Added in: v6.3.0

Interprets buf as an array of 64-bit numbers and swaps the byte-order in-place. Throws a RangeError if buf.length is not a multiple of 8.

Examples:

const buf1 = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8]);

// Prints: <Buffer 01 02 03 04 05 06 07 08>
console.log(buf1);

buf1.swap64();

// Prints: <Buffer 08 07 06 05 04 03 02 01>
console.log(buf1);


const buf2 = Buffer.from([0x1, 0x2, 0x3]);

// Throws an exception: RangeError: Buffer size must be a multiple of 64-bits
buf2.swap64();

Note that JavaScript cannot encode 64-bit integers. This method is intended for working with 64-bit floats.

buf.toString([encoding[, start[, end]]])#

Added in: v0.1.90
  • encoding <string> The character encoding to decode to. Default: 'utf8'
  • start <integer> The byte offset to start decoding at. Default: 0
  • end <integer> The byte offset to stop decoding at (not inclusive). Default: buf.length
  • Returns: <string>

Decodes buf to a string according to the specified character encoding in encoding. start and end may be passed to decode only a subset of buf.

Examples:

const buf1 = Buffer.allocUnsafe(26);

for (let i = 0; i < 26; i++) {
  // 97 is the decimal ASCII value for 'a'
  buf1[i] = i + 97;
}

// Prints: abcdefghijklmnopqrstuvwxyz
console.log(buf1.toString('ascii'));

// Prints: abcde
console.log(buf1.toString('ascii', 0, 5));


const buf2 = Buffer.from('tést');

// Prints: 74c3a97374
console.log(buf2.toString('hex'));

// Prints: té
console.log(buf2.toString('utf8', 0, 3));

// Prints: té
console.log(buf2.toString(undefined, 0, 3));

buf.toJSON()#

Added in: v0.9.2

Returns a JSON representation of buf. JSON.stringify() implicitly calls this function when stringifying a Buffer instance.

Example:

const buf = Buffer.from([0x1, 0x2, 0x3, 0x4, 0x5]);
const json = JSON.stringify(buf);

// Prints: {"type":"Buffer","data":[1,2,3,4,5]}
console.log(json);

const copy = JSON.parse(json, (key, value) => {
  return value && value.type === 'Buffer' ?
    Buffer.from(value.data) :
    value;
});

// Prints: <Buffer 01 02 03 04 05>
console.log(copy);

buf.values()#

Added in: v1.1.0
  • Returns: <Iterator>

Creates and returns an iterator for buf values (bytes). This function is called automatically when a Buffer is used in a for..of statement.

Examples:

const buf = Buffer.from('buffer');

// Prints:
//   98
//   117
//   102
//   102
//   101
//   114
for (const value of buf.values()) {
  console.log(value);
}

// Prints:
//   98
//   117
//   102
//   102
//   101
//   114
for (const value of buf) {
  console.log(value);
}

buf.write(string[, offset[, length]][, encoding])#

Added in: v0.1.90
  • string <string> String to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write string. Default: 0
  • length <integer> Number of bytes to write. Default: buf.length - offset
  • encoding <string> The character encoding of string. Default: 'utf8'
  • Returns: <integer> Number of bytes written.

Writes string to buf at offset according to the character encoding in encoding. The length parameter is the number of bytes to write. If buf did not contain enough space to fit the entire string, only a partial amount of string will be written. However, partially encoded characters will not be written.

Example:

const buf = Buffer.allocUnsafe(256);

const len = buf.write('\u00bd + \u00bc = \u00be', 0);

// Prints: 12 bytes: ½ + ¼ = ¾
console.log(`${len} bytes: ${buf.toString('utf8', 0, len)}`);

buf.writeDoubleBE(value, offset[, noAssert])#

buf.writeDoubleLE(value, offset[, noAssert])#

Added in: v0.11.15
  • value <number> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy: 0 <= offset <= buf.length - 8.
  • noAssert <boolean> Skip value and offset validation? Default: false
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with specified endian format (writeDoubleBE() writes big endian, writeDoubleLE() writes little endian). value should be a valid 64-bit double. Behavior is undefined when value is anything other than a 64-bit double.

Setting noAssert to true allows the encoded form of value to extend beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.allocUnsafe(8);

buf.writeDoubleBE(0xdeadbeefcafebabe, 0);

// Prints: <Buffer 43 eb d5 b7 dd f9 5f d7>
console.log(buf);

buf.writeDoubleLE(0xdeadbeefcafebabe, 0);

// Prints: <Buffer d7 5f f9 dd b7 d5 eb 43>
console.log(buf);

buf.writeFloatBE(value, offset[, noAssert])#

buf.writeFloatLE(value, offset[, noAssert])#

Added in: v0.11.15
  • value <number> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy: 0 <= offset <= buf.length - 4.
  • noAssert <boolean> Skip value and offset validation? Default: false
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with specified endian format (writeFloatBE() writes big endian, writeFloatLE() writes little endian). value should be a valid 32-bit float. Behavior is undefined when value is anything other than a 32-bit float.

Setting noAssert to true allows the encoded form of value to extend beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.allocUnsafe(4);

buf.writeFloatBE(0xcafebabe, 0);

// Prints: <Buffer 4f 4a fe bb>
console.log(buf);

buf.writeFloatLE(0xcafebabe, 0);

// Prints: <Buffer bb fe 4a 4f>
console.log(buf);

buf.writeInt8(value, offset[, noAssert])#

Added in: v0.5.0
  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy: 0 <= offset <= buf.length - 1.
  • noAssert <boolean> Skip value and offset validation? Default: false
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset. value should be a valid signed 8-bit integer. Behavior is undefined when value is anything other than a signed 8-bit integer.

Setting noAssert to true allows the encoded form of value to extend beyond the end of buf, but the result should be considered undefined behavior.

value is interpreted and written as a two's complement signed integer.

Examples:

const buf = Buffer.allocUnsafe(2);

buf.writeInt8(2, 0);
buf.writeInt8(-2, 1);

// Prints: <Buffer 02 fe>
console.log(buf);

buf.writeInt16BE(value, offset[, noAssert])#

buf.writeInt16LE(value, offset[, noAssert])#

Added in: v0.5.5
  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy: 0 <= offset <= buf.length - 2.
  • noAssert <boolean> Skip value and offset validation? Default: false
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with specified endian format (writeInt16BE() writes big endian, writeInt16LE() writes little endian). value should be a valid signed 16-bit integer. Behavior is undefined when value is anything other than a signed 16-bit integer.

Setting noAssert to true allows the encoded form of value to extend beyond the end of buf, but the result should be considered undefined behavior.

value is interpreted and written as a two's complement signed integer.

Examples:

const buf = Buffer.allocUnsafe(4);

buf.writeInt16BE(0x0102, 0);
buf.writeInt16LE(0x0304, 2);

// Prints: <Buffer 01 02 04 03>
console.log(buf);

buf.writeInt32BE(value, offset[, noAssert])#

buf.writeInt32LE(value, offset[, noAssert])#

Added in: v0.5.5
  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy: 0 <= offset <= buf.length - 4.
  • noAssert <boolean> Skip value and offset validation? Default: false
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with specified endian format (writeInt32BE() writes big endian, writeInt32LE() writes little endian). value should be a valid signed 32-bit integer. Behavior is undefined when value is anything other than a signed 32-bit integer.

Setting noAssert to true allows the encoded form of value to extend beyond the end of buf, but the result should be considered undefined behavior.

value is interpreted and written as a two's complement signed integer.

Examples:

const buf = Buffer.allocUnsafe(8);

buf.writeInt32BE(0x01020304, 0);
buf.writeInt32LE(0x05060708, 4);

// Prints: <Buffer 01 02 03 04 08 07 06 05>
console.log(buf);

buf.writeIntBE(value, offset, byteLength[, noAssert])#

buf.writeIntLE(value, offset, byteLength[, noAssert])#

Added in: v0.11.15
  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy: 0 <= offset <= buf.length - byteLength.
  • byteLength <integer> Number of bytes to write. Must satisfy: 0 < byteLength <= 6.
  • noAssert <boolean> Skip value, offset, and byteLength validation? Default: false
  • Returns: <integer> offset plus the number of bytes written.

Writes byteLength bytes of value to buf at the specified offset. Supports up to 48 bits of accuracy. Behavior is undefined when value is anything other than a signed integer.

Setting noAssert to true allows the encoded form of value to extend beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.allocUnsafe(6);

buf.writeUIntBE(0x1234567890ab, 0, 6);

// Prints: <Buffer 12 34 56 78 90 ab>
console.log(buf);

buf.writeUIntLE(0x1234567890ab, 0, 6);

// Prints: <Buffer ab 90 78 56 34 12>
console.log(buf);

buf.writeUInt8(value, offset[, noAssert])#

Added in: v0.5.0
  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy: 0 <= offset <= buf.length - 1.
  • noAssert <boolean> Skip value and offset validation? Default: false
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset. value should be a valid unsigned 8-bit integer. Behavior is undefined when value is anything other than an unsigned 8-bit integer.

Setting noAssert to true allows the encoded form of value to extend beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.allocUnsafe(4);

buf.writeUInt8(0x3, 0);
buf.writeUInt8(0x4, 1);
buf.writeUInt8(0x23, 2);
buf.writeUInt8(0x42, 3);

// Prints: <Buffer 03 04 23 42>
console.log(buf);

buf.writeUInt16BE(value, offset[, noAssert])#

buf.writeUInt16LE(value, offset[, noAssert])#

Added in: v0.5.5
  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy: 0 <= offset <= buf.length - 2.
  • noAssert <boolean> Skip value and offset validation? Default: false
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with specified endian format (writeUInt16BE() writes big endian, writeUInt16LE() writes little endian). value should be a valid unsigned 16-bit integer. Behavior is undefined when value is anything other than an unsigned 16-bit integer.

Setting noAssert to true allows the encoded form of value to extend beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.allocUnsafe(4);

buf.writeUInt16BE(0xdead, 0);
buf.writeUInt16BE(0xbeef, 2);

// Prints: <Buffer de ad be ef>
console.log(buf);

buf.writeUInt16LE(0xdead, 0);
buf.writeUInt16LE(0xbeef, 2);

// Prints: <Buffer ad de ef be>
console.log(buf);

buf.writeUInt32BE(value, offset[, noAssert])#

buf.writeUInt32LE(value, offset[, noAssert])#

Added in: v0.5.5
  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy: 0 <= offset <= buf.length - 4.
  • noAssert <boolean> Skip value and offset validation? Default: false
  • Returns: <integer> offset plus the number of bytes written.

Writes value to buf at the specified offset with specified endian format (writeUInt32BE() writes big endian, writeUInt32LE() writes little endian). value should be a valid unsigned 32-bit integer. Behavior is undefined when value is anything other than an unsigned 32-bit integer.

Setting noAssert to true allows the encoded form of value to extend beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.allocUnsafe(4);

buf.writeUInt32BE(0xfeedface, 0);

// Prints: <Buffer fe ed fa ce>
console.log(buf);

buf.writeUInt32LE(0xfeedface, 0);

// Prints: <Buffer ce fa ed fe>
console.log(buf);

buf.writeUIntBE(value, offset, byteLength[, noAssert])#

buf.writeUIntLE(value, offset, byteLength[, noAssert])#

Added in: v0.5.5
  • value <integer> Number to be written to buf.
  • offset <integer> Number of bytes to skip before starting to write. Must satisfy: 0 <= offset <= buf.length - byteLength.
  • byteLength <integer> Number of bytes to write. Must satisfy: 0 < byteLength <= 6.
  • noAssert <boolean> Skip value, offset, and byteLength validation? Default: false
  • Returns: <integer> offset plus the number of bytes written.

Writes byteLength bytes of value to buf at the specified offset. Supports up to 48 bits of accuracy. Behavior is undefined when value is anything other than an unsigned integer.

Setting noAssert to true allows the encoded form of value to extend beyond the end of buf, but the result should be considered undefined behavior.

Examples:

const buf = Buffer.allocUnsafe(6);

buf.writeIntBE(0x1234567890ab, 0, 6);

// Prints: <Buffer 12 34 56 78 90 ab>
console.log(buf);

buf.writeIntLE(0x1234567890ab, 0, 6);

// Prints: <Buffer ab 90 78 56 34 12>
console.log(buf);

buffer.INSPECT_MAX_BYTES#

Added in: v0.5.4

Returns the maximum number of bytes that will be returned when buf.inspect() is called. This can be overridden by user modules. See util.inspect() for more details on buf.inspect() behavior.

Note that this is a property on the buffer module as returned by require('buffer'), not on the Buffer global or a Buffer instance.

buffer.kMaxLength#

Added in: v3.0.0
  • <integer> The largest size allowed for a single Buffer instance.

On 32-bit architectures, this value is (2^30)-1 (~1GB). On 64-bit architectures, this value is (2^31)-1 (~2GB).

Class: SlowBuffer#

Deprecated since: v6.0.0
Stability: 0 - Deprecated: Use Buffer.allocUnsafeSlow() instead.

Returns an un-pooled Buffer.

In order to avoid the garbage collection overhead of creating many individually allocated Buffer instances, by default allocations under 4KB are sliced from a single larger allocated object. This approach improves both performance and memory usage since v8 does not need to track and cleanup as many Persistent objects.

In the case where a developer may need to retain a small chunk of memory from a pool for an indeterminate amount of time, it may be appropriate to create an un-pooled Buffer instance using SlowBuffer then copy out the relevant bits.

Example:

// Need to keep around a few small chunks of memory
const store = [];

socket.on('readable', () => {
  const data = socket.read();

  // Allocate for retained data
  const sb = SlowBuffer(10);

  // Copy the data into the new allocation
  data.copy(sb, 0, 0, 10);

  store.push(sb);
});

Use of SlowBuffer should be used only as a last resort after a developer has observed undue memory retention in their applications.

new SlowBuffer(size)#

Deprecated since: v6.0.0
Stability: 0 - Deprecated: Use Buffer.allocUnsafeSlow() instead.
  • size <integer> The desired length of the new SlowBuffer.

Allocates a new SlowBuffer of size bytes. The size must be less than or equal to the value of buffer.kMaxLength. Otherwise, a RangeError is thrown. A zero-length Buffer will be created if size <= 0.

The underlying memory for SlowBuffer instances is not initialized. The contents of a newly created SlowBuffer are unknown and could contain sensitive data. Use buf.fill(0) to initialize a SlowBuffer to zeroes.

Example:

const SlowBuffer = require('buffer').SlowBuffer;

const buf = new SlowBuffer(5);

// Prints: (contents may vary): <Buffer 78 e0 82 02 01>
console.log(buf);

buf.fill(0);

// Prints: <Buffer 00 00 00 00 00>
console.log(buf);

Child Process#

Stability: 2 - Stable

The child_process module provides the ability to spawn child processes in a manner that is similar, but not identical, to popen(3). This capability is primarily provided by the child_process.spawn() function:

const spawn = require('child_process').spawn;
const ls = spawn('ls', ['-lh', '/usr']);

ls.stdout.on('data', (data) => {
  console.log(`stdout: ${data}`);
});

ls.stderr.on('data', (data) => {
  console.log(`stderr: ${data}`);
});

ls.on('close', (code) => {
  console.log(`child process exited with code ${code}`);
});

By default, pipes for stdin, stdout and stderr are established between the parent Node.js process and the spawned child. It is possible to stream data through these pipes in a non-blocking way. Note, however, that some programs use line-buffered I/O internally. While that does not affect Node.js, it can mean that data sent to the child process may not be immediately consumed.

The child_process.spawn() method spawns the child process asynchronously, without blocking the Node.js event loop. The child_process.spawnSync() function provides equivalent functionality in a synchronous manner that blocks the event loop until the spawned process either exits or is terminated.

For convenience, the child_process module provides a handful of synchronous and asynchronous alternatives to child_process.spawn() and child_process.spawnSync(). Note that each of these alternatives are implemented on top of child_process.spawn() or child_process.spawnSync().

For certain use cases, such as automating shell scripts, the synchronous counterparts may be more convenient. In many cases, however, the synchronous methods can have significant impact on performance due to stalling the event loop while spawned processes complete.

Asynchronous Process Creation#

The child_process.spawn(), child_process.fork(), child_process.exec(), and child_process.execFile() methods all follow the idiomatic asynchronous programming pattern typical of other Node.js APIs.

Each of the methods returns a ChildProcess instance. These objects implement the Node.js EventEmitter API, allowing the parent process to register listener functions that are called when certain events occur during the life cycle of the child process.

The child_process.exec() and child_process.execFile() methods additionally allow for an optional callback function to be specified that is invoked when the child process terminates.

Spawning .bat and .cmd files on Windows#

The importance of the distinction between child_process.exec() and child_process.execFile() can vary based on platform. On Unix-type operating systems (Unix, Linux, macOS) child_process.execFile() can be more efficient because it does not spawn a shell. On Windows, however, .bat and .cmd files are not executable on their own without a terminal, and therefore cannot be launched using child_process.execFile(). When running on Windows, .bat and .cmd files can be invoked using child_process.spawn() with the shell option set, with child_process.exec(), or by spawning cmd.exe and passing the .bat or .cmd file as an argument (which is what the shell option and child_process.exec() do). In any case, if the script filename contains spaces it needs to be quoted.

// On Windows Only ...
const spawn = require('child_process').spawn;
const bat = spawn('cmd.exe', ['/c', 'my.bat']);

bat.stdout.on('data', (data) => {
  console.log(data.toString());
});

bat.stderr.on('data', (data) => {
  console.log(data.toString());
});

bat.on('exit', (code) => {
  console.log(`Child exited with code ${code}`);
});

// OR...
const exec = require('child_process').exec;
exec('my.bat', (err, stdout, stderr) => {
  if (err) {
    console.error(err);
    return;
  }
  console.log(stdout);
});

// Script with spaces in the filename:
const bat = spawn('"my script.cmd"', ['a', 'b'], { shell: true });
// or:
exec('"my script.cmd" a b', (err, stdout, stderr) => {
  // ...
});

child_process.exec(command[, options][, callback])#

Added in: v0.1.90
  • command <string> The command to run, with space-separated arguments.
  • options <Object>
    • cwd <string> Current working directory of the child process.
    • env <Object> Environment key-value pairs.
    • encoding <string> Default: 'utf8'
    • shell <string> Shell to execute the command with. Default: '/bin/sh' on UNIX, 'cmd.exe' on Windows. The shell should understand the -c switch on UNIX or /s /c on Windows. On Windows, command line parsing should be compatible with cmd.exe.
    • timeout <number> Default: 0
    • maxBuffer <number> Largest amount of data (in bytes) allowed on stdout or stderr - if exceeded child process is killed. Default: 200*1024
    • killSignal <string> | <integer> Default: 'SIGTERM'
    • uid <number> Sets the user identity of the process (see setuid(2)).
    • gid <number> Sets the group identity of the process (see setgid(2)).
  • callback <Function> Called with the output when process terminates.
  • Returns: <ChildProcess>

Spawns a shell then executes the command within that shell, buffering any generated output.

Note: Never pass unsanitised user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

const exec = require('child_process').exec;
exec('cat *.js bad_file | wc -l', (error, stdout, stderr) => {
  if (error) {
    console.error(`exec error: ${error}`);
    return;
  }
  console.log(`stdout: ${stdout}`);
  console.log(`stderr: ${stderr}`);
});

If a callback function is provided, it is called with the arguments (error, stdout, stderr). On success, error will be null. On error, error will be an instance of Error. The error.code property will be the exit code of the child process while error.signal will be set to the signal that terminated the process. Any exit code other than 0 is considered to be an error.

The stdout and stderr arguments passed to the callback will contain the stdout and stderr output of the child process. By default, Node.js will decode the output as UTF-8 and pass strings to the callback. The encoding option can be used to specify the character encoding used to decode the stdout and stderr output. If encoding is 'buffer', or an unrecognized character encoding, Buffer objects will be passed to the callback instead.

The options argument may be passed as the second argument to customize how the process is spawned. The default options are:

const defaults = {
  encoding: 'utf8',
  timeout: 0,
  maxBuffer: 200 * 1024,
  killSignal: 'SIGTERM',
  cwd: null,
  env: null
};

If timeout is greater than 0, the parent will send the signal identified by the killSignal property (the default is 'SIGTERM') if the child runs longer than timeout milliseconds.

Note: Unlike the exec(3) POSIX system call, child_process.exec() does not replace the existing process and uses a shell to execute the command.

child_process.execFile(file[, args][, options][, callback])#

Added in: v0.1.91

The child_process.execFile() function is similar to child_process.exec() except that it does not spawn a shell. Rather, the specified executable file is spawned directly as a new process making it slightly more efficient than child_process.exec().

The same options as child_process.exec() are supported. Since a shell is not spawned, behaviors such as I/O redirection and file globbing are not supported.

const execFile = require('child_process').execFile;
const child = execFile('node', ['--version'], (error, stdout, stderr) => {
  if (error) {
    throw error;
  }
  console.log(stdout);
});

The stdout and stderr arguments passed to the callback will contain the stdout and stderr output of the child process. By default, Node.js will decode the output as UTF-8 and pass strings to the callback. The encoding option can be used to specify the character encoding used to decode the stdout and stderr output. If encoding is 'buffer', or an unrecognized character encoding, Buffer objects will be passed to the callback instead.

child_process.fork(modulePath[, args][, options])#

Added in: v0.5.0
  • modulePath <string> The module to run in the child.
  • args <Array> List of string arguments.
  • options <Object>
    • cwd <string> Current working directory of the child process.
    • env <Object> Environment key-value pairs.
    • execPath <string> Executable used to create the child process.
    • execArgv <Array> List of string arguments passed to the executable. Default: process.execArgv
    • silent <boolean> If true, stdin, stdout, and stderr of the child will be piped to the parent, otherwise they will be inherited from the parent, see the 'pipe' and 'inherit' options for child_process.spawn()'s stdio for more details. Default: false
    • stdio <Array> Supports the array version of child_process.spawn()'s stdio option. When this option is provided, it overrides silent. The array must contain exactly one item with value 'ipc' or an error will be thrown. For instance [0, 1, 2, 'ipc'].
    • uid <number> Sets the user identity of the process (see setuid(2)).
    • gid <number> Sets the group identity of the process (see setgid(2)).
  • Returns: <ChildProcess>

The child_process.fork() method is a special case of child_process.spawn() used specifically to spawn new Node.js processes. Like child_process.spawn(), a ChildProcess object is returned. The returned ChildProcess will have an additional communication channel built-in that allows messages to be passed back and forth between the parent and child. See subprocess.send() for details.

It is important to keep in mind that spawned Node.js child processes are independent of the parent with exception of the IPC communication channel that is established between the two. Each process has its own memory, with their own V8 instances. Because of the additional resource allocations required, spawning a large number of child Node.js processes is not recommended.

By default, child_process.fork() will spawn new Node.js instances using the process.execPath of the parent process. The execPath property in the options object allows for an alternative execution path to be used.

Node.js processes launched with a custom execPath will communicate with the parent process using the file descriptor (fd) identified using the environment variable NODE_CHANNEL_FD on the child process. The input and output on this fd is expected to be line delimited JSON objects.

Note: Unlike the fork(2) POSIX system call, child_process.fork() does not clone the current process.

Note: The shell option available in child_process.spawn() is not supported by child_process.fork() and will be ignored if set.

child_process.spawn(command[, args][, options])#

Added in: v0.1.90
  • command <string> The command to run.
  • args <Array> List of string arguments.
  • options <Object>
    • cwd <string> Current working directory of the child process.
    • env <Object> Environment key-value pairs.
    • argv0 <string> Explicitly set the value of argv[0] sent to the child process. This will be set to command if not specified.
    • stdio <Array> | <string> Child's stdio configuration (see options.stdio).
    • detached <boolean> Prepare child to run independently of its parent process. Specific behavior depends on the platform, see options.detached).
    • uid <number> Sets the user identity of the process (see setuid(2)).
    • gid <number> Sets the group identity of the process (see setgid(2)).
    • shell <boolean> | <string> If true, runs command inside of a shell. Uses '/bin/sh' on UNIX, and 'cmd.exe' on Windows. A different shell can be specified as a string. The shell should understand the -c switch on UNIX, or /s /c on Windows. Default: false (no shell).
  • Returns: <ChildProcess>

The child_process.spawn() method spawns a new process using the given command, with command line arguments in args. If omitted, args defaults to an empty array.

Note: If the shell option is enabled, do not pass unsanitised user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

A third argument may be used to specify additional options, with these defaults:

const defaults = {
  cwd: undefined,
  env: process.env
};

Use cwd to specify the working directory from which the process is spawned. If not given, the default is to inherit the current working directory.

Use env to specify environment variables that will be visible to the new process, the default is process.env.

Example of running ls -lh /usr, capturing stdout, stderr, and the exit code:

const spawn = require('child_process').spawn;
const ls = spawn('ls', ['-lh', '/usr']);

ls.stdout.on('data', (data) => {
  console.log(`stdout: ${data}`);
});

ls.stderr.on('data', (data) => {
  console.log(`stderr: ${data}`);
});

ls.on('close', (code) => {
  console.log(`child process exited with code ${code}`);
});

Example: A very elaborate way to run ps ax | grep ssh

const spawn = require('child_process').spawn;
const ps = spawn('ps', ['ax']);
const grep = spawn('grep', ['ssh']);

ps.stdout.on('data', (data) => {
  grep.stdin.write(data);
});

ps.stderr.on('data', (data) => {
  console.log(`ps stderr: ${data}`);
});

ps.on('close', (code) => {
  if (code !== 0) {
    console.log(`ps process exited with code ${code}`);
  }
  grep.stdin.end();
});

grep.stdout.on('data', (data) => {
  console.log(data.toString());
});

grep.stderr.on('data', (data) => {
  console.log(`grep stderr: ${data}`);
});

grep.on('close', (code) => {
  if (code !== 0) {
    console.log(`grep process exited with code ${code}`);
  }
});

Example of checking for failed spawn:

const spawn = require('child_process').spawn;
const subprocess = spawn('bad_command');

subprocess.on('error', (err) => {
  console.log('Failed to start subprocess.');
});

Note: Certain platforms (macOS, Linux) will use the value of argv[0] for the process title while others (Windows, SunOS) will use command.

Note: Node.js currently overwrites argv[0] with process.execPath on startup, so process.argv[0] in a Node.js child process will not match the argv0 parameter passed to spawn from the parent, retrieve it with the process.argv0 property instead.

options.detached#

Added in: v0.7.10

On Windows, setting options.detached to true makes it possible for the child process to continue running after the parent exits. The child will have its own console window. Once enabled for a child process, it cannot be disabled.

On non-Windows platforms, if options.detached is set to true, the child process will be made the leader of a new process group and session. Note that child processes may continue running after the parent exits regardless of whether they are detached or not. See setsid(2) for more information.

By default, the parent will wait for the detached child to exit. To prevent the parent from waiting for a given subprocess, use the subprocess.unref() method. Doing so will cause the parent's event loop to not include the child in its reference count, allowing the parent to exit independently of the child, unless there is an established IPC channel between the child and parent.

When using the detached option to start a long-running process, the process will not stay running in the background after the parent exits unless it is provided with a stdio configuration that is not connected to the parent. If the parent's stdio is inherited, the child will remain attached to the controlling terminal.

Example of a long-running process, by detaching and also ignoring its parent stdio file descriptors, in order to ignore the parent's termination:

const spawn = require('child_process').spawn;

const subprocess = spawn(process.argv[0], ['child_program.js'], {
  detached: true,
  stdio: 'ignore'
});

subprocess.unref();

Alternatively one can redirect the child process' output into files:

const fs = require('fs');
const spawn = require('child_process').spawn;
const out = fs.openSync('./out.log', 'a');
const err = fs.openSync('./out.log', 'a');

const subprocess = spawn('prg', [], {
  detached: true,
  stdio: [ 'ignore', out, err ]
});

subprocess.unref();

options.stdio#

Added in: v0.7.10

The options.stdio option is used to configure the pipes that are established between the parent and child process. By default, the child's stdin, stdout, and stderr are redirected to corresponding subprocess.stdin, subprocess.stdout, and subprocess.stderr streams on the ChildProcess object. This is equivalent to setting the options.stdio equal to ['pipe', 'pipe', 'pipe'].

For convenience, options.stdio may be one of the following strings:

  • 'pipe' - equivalent to ['pipe', 'pipe', 'pipe'] (the default)
  • 'ignore' - equivalent to ['ignore', 'ignore', 'ignore']
  • 'inherit' - equivalent to [process.stdin, process.stdout, process.stderr] or [0,1,2]

Otherwise, the value of options.stdio is an array where each index corresponds to an fd in the child. The fds 0, 1, and 2 correspond to stdin, stdout, and stderr, respectively. Additional fds can be specified to create additional pipes between the parent and child. The value is one of the following:

  1. 'pipe' - Create a pipe between the child process and the parent process. The parent end of the pipe is exposed to the parent as a property on the child_process object as subprocess.stdio[fd]. Pipes created for fds 0 - 2 are also available as subprocess.stdin, subprocess.stdout and subprocess.stderr, respectively.
  2. 'ipc' - Create an IPC channel for passing messages/file descriptors between parent and child. A ChildProcess may have at most one IPC stdio file descriptor. Setting this option enables the subprocess.send() method. If the child writes JSON messages to this file descriptor, the subprocess.on('message') event handler will be triggered in the parent. If the child is a Node.js process, the presence of an IPC channel will enable process.send(), process.disconnect(), process.on('disconnect'), and process.on('message') within the child.
  3. 'ignore' - Instructs Node.js to ignore the fd in the child. While Node.js will always open fds 0 - 2 for the processes it spawns, setting the fd to 'ignore' will cause Node.js to open /dev/null and attach it to the child's fd.
  4. <Stream> object - Share a readable or writable stream that refers to a tty, file, socket, or a pipe with the child process. The stream's underlying file descriptor is duplicated in the child process to the fd that corresponds to the index in the stdio array. Note that the stream must have an underlying descriptor (file streams do not until the 'open' event has occurred).
  5. Positive integer - The integer value is interpreted as a file descriptor that is is currently open in the parent process. It is shared with the child process, similar to how <Stream> objects can be shared.
  6. null, undefined - Use default value. For stdio fds 0, 1 and 2 (in other words, stdin, stdout, and stderr) a pipe is created. For fd 3 and up, the default is 'ignore'.

Example:

const spawn = require('child_process').spawn;

// Child will use parent's stdios
spawn('prg', [], { stdio: 'inherit' });

// Spawn child sharing only stderr
spawn('prg', [], { stdio: ['pipe', 'pipe', process.stderr] });

// Open an extra fd=4, to interact with programs presenting a
// startd-style interface.
spawn('prg', [], { stdio: ['pipe', null, null, null, 'pipe'] });

It is worth noting that when an IPC channel is established between the parent and child processes, and the child is a Node.js process, the child is launched with the IPC channel unreferenced (using unref()) until the child registers an event handler for the process.on('disconnect') event or the process.on('message') event. This allows the child to exit normally without the process being held open by the open IPC channel.

See also: child_process.exec() and child_process.fork()

Synchronous Process Creation#

The child_process.spawnSync(), child_process.execSync(), and child_process.execFileSync() methods are synchronous and WILL block the Node.js event loop, pausing execution of any additional code until the spawned process exits.

Blocking calls like these are mostly useful for simplifying general purpose scripting tasks and for simplifying the loading/processing of application configuration at startup.

child_process.execFileSync(file[, args][, options])#

Added in: v0.11.12
  • file <string> The name or path of the executable file to run.
  • args <string[]> List of string arguments.
  • options <Object>
    • cwd <string> Current working directory of the child process.
    • input <string> | <Buffer> The value which will be passed as stdin to the spawned process.
      • supplying this value will override stdio[0]
    • stdio <string> | <Array> Child's stdio configuration. Default: 'pipe'
      • stderr by default will be output to the parent process' stderr unless stdio is specified
    • env <Object> Environment key-value pairs.
    • uid <number> Sets the user identity of the process (see setuid(2)).
    • gid <number> Sets the group identity of the process (see setgid(2)).
    • timeout <number> In milliseconds the maximum amount of time the process is allowed to run. Default: undefined
    • killSignal <string> | <integer> The signal value to be used when the spawned process will be killed. Default: 'SIGTERM'
    • maxBuffer <number> Largest amount of data (in bytes) allowed on stdout or stderr - if exceeded child process is killed.
    • encoding <string> The encoding used for all stdio inputs and outputs. Default: 'buffer'
  • Returns: <Buffer> | <string> The stdout from the command.

The child_process.execFileSync() method is generally identical to child_process.execFile() with the exception that the method will not return until the child process has fully closed. When a timeout has been encountered and killSignal is sent, the method won't return until the process has completely exited. Note that if the child process intercepts and handles the SIGTERM signal and does not exit, the parent process will still wait until the child process has exited.

If the process times out, or has a non-zero exit code, this method will throw an Error that will include the full result of the underlying child_process.spawnSync().

child_process.execSync(command[, options])#

Added in: v0.11.12
  • command <string> The command to run.
  • options <Object>
    • cwd <string> Current working directory of the child process.
    • input <string> | <Buffer> The value which will be passed as stdin to the spawned process.
      • supplying this value will override stdio[0]
    • stdio <string> | <Array> Child's stdio configuration. Default: 'pipe'
      • stderr by default will be output to the parent process' stderr unless stdio is specified
    • env <Object> Environment key-value pairs.
    • shell <string> Shell to execute the command with. Default: '/bin/sh' on UNIX, 'cmd.exe' on Windows. The shell should understand the -c switch on UNIX or /s /c on Windows. On Windows, command line parsing should be compatible with cmd.exe.
    • uid <number> Sets the user identity of the process. (see setuid(2)).
    • gid <number> Sets the group identity of the process. (see setgid(2)).
    • timeout <number> In milliseconds the maximum amount of time the process is allowed to run. Default: undefined
    • killSignal <string> | <integer> The signal value to be used when the spawned process will be killed. Default: 'SIGTERM'
    • maxBuffer <number> Largest amount of data (in bytes) allowed on stdout or stderr - if exceeded child process is killed.
    • encoding <string> The encoding used for all stdio inputs and outputs. Default: 'buffer'
  • Returns: <Buffer> | <string> The stdout from the command.

The child_process.execSync() method is generally identical to child_process.exec() with the exception that the method will not return until the child process has fully closed. When a timeout has been encountered and killSignal is sent, the method won't return until the process has completely exited. Note that if the child process intercepts and handles the SIGTERM signal and doesn't exit, the parent process will wait until the child process has exited.

If the process times out, or has a non-zero exit code, this method will throw. The Error object will contain the entire result from child_process.spawnSync()

Note: Never pass unsanitised user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

child_process.spawnSync(command[, args][, options])#

Added in: v0.11.12
  • command <string> The command to run.
  • args <Array> List of string arguments.
  • options <Object>
    • cwd <string> Current working directory of the child process.
    • input <string> | <Buffer> The value which will be passed as stdin to the spawned process
      • supplying this value will override stdio[0].
    • stdio <string> | <Array> Child's stdio configuration.
    • env <Object> Environment key-value pairs.
    • uid <number> Sets the user identity of the process (see setuid(2)).
    • gid <number> Sets the group identity of the process (see setgid(2)).
    • timeout <number> In milliseconds the maximum amount of time the process is allowed to run. Default: undefined
    • killSignal <string> | <integer> The signal value to be used when the spawned process will be killed. Default: 'SIGTERM'
    • maxBuffer <number> Largest amount of data (in bytes) allowed on stdout or stderr - if exceeded child process is killed.
    • encoding <string> The encoding used for all stdio inputs and outputs. Default: 'buffer'
    • shell <boolean> | <string> If true, runs command inside of a shell. Uses '/bin/sh' on UNIX, and 'cmd.exe' on Windows. A different shell can be specified as a string. The shell should understand the -c switch on UNIX, or /s /c on Windows. Default: to false (no shell).
  • Returns: <Object>
    • pid <number> Pid of the child process.
    • output <Array> Array of results from stdio output.
    • stdout <Buffer> | <string> The contents of output[1].
    • stderr <Buffer> | <string> The contents of output[2].
    • status <number> The exit code of the child process.
    • signal <string> The signal used to kill the child process.
    • error <Error> The error object if the child process failed or timed out.

The child_process.spawnSync() method is generally identical to child_process.spawn() with the exception that the function will not return until the child process has fully closed. When a timeout has been encountered and killSignal is sent, the method won't return until the process has completely exited. Note that if the process intercepts and handles the SIGTERM signal and doesn't exit, the parent process will wait until the child process has exited.

Note: If the shell option is enabled, do not pass unsanitised user input to this function. Any input containing shell metacharacters may be used to trigger arbitrary command execution.

Class: ChildProcess#

Added in: v2.2.0

Instances of the ChildProcess class are EventEmitters that represent spawned child processes.

Instances of ChildProcess are not intended to be created directly. Rather, use the child_process.spawn(), child_process.exec(), child_process.execFile(), or child_process.fork() methods to create instances of ChildProcess.

Event: 'close'#

Added in: v0.7.7
  • code <number> The exit code if the child exited on its own.
  • signal <string> The signal by which the child process was terminated.

The 'close' event is emitted when the stdio streams of a child process have been closed. This is distinct from the 'exit' event, since multiple processes might share the same stdio streams.

Event: 'disconnect'#

Added in: v0.7.2

The 'disconnect' event is emitted after calling the subprocess.disconnect() method in parent process or process.disconnect() in child process. After disconnecting it is no longer possible to send or receive messages, and the subprocess.connected property is false.

Event: 'error'#

The 'error' event is emitted whenever:

  1. The process could not be spawned, or
  2. The process could not be killed, or
  3. Sending a message to the child process failed.

Note that the 'exit' event may or may not fire after an error has occurred. If you are listening to both the 'exit' and 'error' events, it is important to guard against accidentally invoking handler functions multiple times.

See also subprocess.kill() and subprocess.send().

Event: 'exit'#

Added in: v0.1.90
  • code <number> The exit code if the child exited on its own.
  • signal <string> The signal by which the child process was terminated.

The 'exit' event is emitted after the child process ends. If the process exited, code is the final exit code of the process, otherwise null. If the process terminated due to receipt of a signal, signal is the string name of the signal, otherwise null. One of the two will always be non-null.

Note that when the 'exit' event is triggered, child process stdio streams might still be open.

Also, note that Node.js establishes signal handlers for SIGINT and SIGTERM and Node.js processes will not terminate immediately due to receipt of those signals. Rather, Node.js will perform a sequence of cleanup actions and then will re-raise the handled signal.

See waitpid(2).

Event: 'message'#

Added in: v0.5.9

The 'message' event is triggered when a child process uses process.send() to send messages.

subprocess.connected#

Added in: v0.7.2
  • <boolean> Set to false after subprocess.disconnect() is called.

The subprocess.connected property indicates whether it is still possible to send and receive messages from a child process. When subprocess.connected is false, it is no longer possible to send or receive messages.

subprocess.disconnect()#

Added in: v0.7.2

Closes the IPC channel between parent and child, allowing the child to exit gracefully once there are no other connections keeping it alive. After calling this method the subprocess.connected and process.connected properties in both the parent and child (respectively) will be set to false, and it will be no longer possible to pass messages between the processes.

The 'disconnect' event will be emitted when there are no messages in the process of being received. This will most often be triggered immediately after calling subprocess.disconnect().

Note that when the child process is a Node.js instance (e.g. spawned using child_process.fork()), the process.disconnect() method can be invoked within the child process to close the IPC channel as well.

subprocess.kill([signal])#

Added in: v0.1.90

The subprocess.kill() method sends a signal to the child process. If no argument is given, the process will be sent the 'SIGTERM' signal. See signal(7) for a list of available signals.

const spawn = require('child_process').spawn;
const grep = spawn('grep', ['ssh']);

grep.on('close', (code, signal) => {
  console.log(
    `child process terminated due to receipt of signal ${signal}`);
});

// Send SIGHUP to process
grep.kill('SIGHUP');

The ChildProcess object may emit an 'error' event if the signal cannot be delivered. Sending a signal to a child process that has already exited is not an error but may have unforeseen consequences. Specifically, if the process identifier (PID) has been reassigned to another process, the signal will be delivered to that process instead which can have unexpected results.

Note that while the function is called kill, the signal delivered to the child process may not actually terminate the process.

See kill(2) for reference.

Also note: on Linux, child processes of child processes will not be terminated when attempting to kill their parent. This is likely to happen when running a new process in a shell or with use of the shell option of ChildProcess, such as in this example:

'use strict';
const spawn = require('child_process').spawn;

const subprocess = spawn(
  'sh',
  [
    '-c',
    `node -e "setInterval(() => {
      console.log(process.pid, 'is alive')
    }, 500);"`
  ], {
    stdio: ['inherit', 'inherit', 'inherit']
  }
);

setTimeout(() => {
  subprocess.kill(); // does not terminate the node process in the shell
}, 2000);

subprocess.killed#

Added in: v0.5.10
  • <boolean> Set to true after subprocess.kill() is used to successfully send a signal to the child process.

The subprocess.killed property indicates whether the child process successfully received a signal from subprocess.kill(). The killed property does not indicate that the child process has been terminated.

subprocess.pid#

Added in: v0.1.90

Returns the process identifier (PID) of the child process.

Example:

const spawn = require('child_process').spawn;
const grep = spawn('grep', ['ssh']);

console.log(`Spawned child pid: ${grep.pid}`);
grep.stdin.end();

subprocess.send(message[, sendHandle[, options]][, callback])#

Added in: v0.5.9

When an IPC channel has been established between the parent and child ( i.e. when using child_process.fork()), the subprocess.send() method can be used to send messages to the child process. When the child process is a Node.js instance, these messages can be received via the process.on('message') event.

For example, in the parent script:

const cp = require('child_process');
const n = cp.fork(`${__dirname}/sub.js`);

n.on('message', (m) => {
  console.log('PARENT got message:', m);
});

n.send({ hello: 'world' });

And then the child script, 'sub.js' might look like this:

process.on('message', (m) => {
  console.log('CHILD got message:', m);
});

process.send({ foo: 'bar' });

Child Node.js processes will have a process.send() method of their own that allows the child to send messages back to the parent.

There is a special case when sending a {cmd: 'NODE_foo'} message. All messages containing a NODE_ prefix in its cmd property are considered to be reserved for use within Node.js core and will not be emitted in the child's process.on('message') event. Rather, such messages are emitted using the process.on('internalMessage') event and are consumed internally by Node.js. Applications should avoid using such messages or listening for 'internalMessage' events as it is subject to change without notice.

The optional sendHandle argument that may be passed to subprocess.send() is for passing a TCP server or socket object to the child process. The child will receive the object as the second argument passed to the callback function registered on the process.on('message') event. Any data that is received and buffered in the socket will not be sent to the child.

The options argument, if present, is an object used to parameterize the sending of certain types of handles. options supports the following properties:

  • keepOpen - A Boolean value that can be used when passing instances of net.Socket. When true, the socket is kept open in the sending process. Defaults to false.

The optional callback is a function that is invoked after the message is sent but before the child may have received it. The function is called with a single argument: null on success, or an Error object on failure.

If no callback function is provided and the message cannot be sent, an 'error' event will be emitted by the ChildProcess object. This can happen, for instance, when the child process has already exited.

subprocess.send() will return false if the channel has closed or when the backlog of unsent messages exceeds a threshold that makes it unwise to send more. Otherwise, the method returns true. The callback function can be used to implement flow control.

Example: sending a server object#

The sendHandle argument can be used, for instance, to pass the handle of a TCP server object to the child process as illustrated in the example below:

const subprocess = require('child_process').fork('subprocess.js');

// Open up the server object and send the handle.
const server = require('net').createServer();
server.on('connection', (socket) => {
  socket.end('handled by parent');
});
server.listen(1337, () => {
  subprocess.send('server', server);
});

The child would then receive the server object as:

process.on('message', (m, server) => {
  if (m === 'server') {
    server.on('connection', (socket) => {
      socket.end('handled by child');
    });
  }
});

Once the server is now shared between the parent and child, some connections can be handled by the parent and some by the child.

While the example above uses a server created using the net module, dgram module servers use exactly the same workflow with the exceptions of listening on a 'message' event instead of 'connection' and using server.bind() instead of server.listen(). This is, however, currently only supported on UNIX platforms.

Example: sending a socket object#

Similarly, the sendHandler argument can be used to pass the handle of a socket to the child process. The example below spawns two children that each handle connections with "normal" or "special" priority:

const { fork } = require('child_process');
const normal = fork('subprocess.js', ['normal']);
const special = fork('subprocess.js', ['special']);

// Open up the server and send sockets to child. Use pauseOnConnect to prevent
// the sockets from being read before they are sent to the child process.
const server = require('net').createServer({ pauseOnConnect: true });
server.on('connection', (socket) => {

  // If this is special priority
  if (socket.remoteAddress === '74.125.127.100') {
    special.send('socket', socket);
    return;
  }
  // This is normal priority
  normal.send('socket', socket);
});
server.listen(1337);

The subprocess.js would receive the socket handle as the second argument passed to the event callback function:

process.on('message', (m, socket) => {
  if (m === 'socket') {
    if (socket) {
      // Check that the client socket exists.
      // It is possible for the socket to be closed between the time it is
      // sent and the time it is received in the child process.
      socket.end(`Request handled with ${process.argv[2]} priority`);
    }
  }
});

Once a socket has been passed to a child, the parent is no longer capable of tracking when the socket is destroyed. To indicate this, the .connections property becomes null. It is recommended not to use .maxConnections when this occurs.

It is also recommended that any 'message' handlers in the child process verify that socket exists, as the connection may have been closed during the time it takes to send the connection to the child.

Note: this function uses JSON.stringify() internally to serialize the message.

subprocess.stderr#

Added in: v0.1.90

A Readable Stream that represents the child process's stderr.

If the child was spawned with stdio[2] set to anything other than 'pipe', then this will be null.

subprocess.stderr is an alias for subprocess.stdio[2]. Both properties will refer to the same value.

subprocess.stdin#

Added in: v0.1.90

A Writable Stream that represents the child process's stdin.

Note that if a child process waits to read all of its input, the child will not continue until this stream has been closed via end().

If the child was spawned with stdio[0] set to anything other than 'pipe', then this will be null.

subprocess.stdin is an alias for subprocess.stdio[0]. Both properties will refer to the same value.

subprocess.stdio#

Added in: v0.7.10

A sparse array of pipes to the child process, corresponding with positions in the stdio option passed to child_process.spawn() that have been set to the value 'pipe'. Note that subprocess.stdio[0], subprocess.stdio[1], and subprocess.stdio[2] are also available as subprocess.stdin, subprocess.stdout, and subprocess.stderr, respectively.

In the following example, only the child's fd 1 (stdout) is configured as a pipe, so only the parent's subprocess.stdio[1] is a stream, all other values in the array are null.

const assert = require('assert');
const fs = require('fs');
const child_process = require('child_process');

const subprocess = child_process.spawn('ls', {
  stdio: [
    0, // Use parent's stdin for child
    'pipe', // Pipe child's stdout to parent
    fs.openSync('err.out', 'w') // Direct child's stderr to a file
  ]
});

assert.strictEqual(subprocess.stdio[0], null);
assert.strictEqual(subprocess.stdio[0], subprocess.stdin);

assert(subprocess.stdout);
assert.strictEqual(subprocess.stdio[1], subprocess.stdout);

assert.strictEqual(subprocess.stdio[2], null);
assert.strictEqual(subprocess.stdio[2], subprocess.stderr);

subprocess.stdout#

Added in: v0.1.90

A Readable Stream that represents the child process's stdout.

If the child was spawned with stdio[1] set to anything other than 'pipe', then this will be null.

subprocess.stdout is an alias for subprocess.stdio[1]. Both properties will refer to the same value.

maxBuffer and Unicode#

The maxBuffer option specifies the largest number of bytes allowed on stdout or stderr. If this value is exceeded, then the child process is terminated. This impacts output that includes multibyte character encodings such as UTF-8 or UTF-16. For instance, console.log('中文测试') will send 13 UTF-8 encoded bytes to stdout although there are only 4 characters.

Cluster#

Stability: 2 - Stable

A single instance of Node.js runs in a single thread. To take advantage of multi-core systems, the user will sometimes want to launch a cluster of Node.js processes to handle the load.

The cluster module allows you to easily create child processes that all share server ports.

const cluster = require('cluster');
const http = require('http');
const numCPUs = require('os').cpus().length;

if (cluster.isMaster) {
  console.log(`Master ${process.pid} is running`);

  // Fork workers.
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  cluster.on('exit', (worker, code, signal) => {
    console.log(`worker ${worker.process.pid} died`);
  });
} else {
  // Workers can share any TCP connection
  // In this case it is an HTTP server
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end('hello world\n');
  }).listen(8000);

  console.log(`Worker ${process.pid} started`);
}

Running Node.js will now share port 8000 between the workers:

$ node server.js
Master 3596 is running
Worker 4324 started
Worker 4520 started
Worker 6056 started
Worker 5644 started

Please note that on Windows, it is not yet possible to set up a named pipe server in a worker.

How It Works#

The worker processes are spawned using the child_process.fork() method, so that they can communicate with the parent via IPC and pass server handles back and forth.

The cluster module supports two methods of distributing incoming connections.

The first one (and the default one on all platforms except Windows), is the round-robin approach, where the master process listens on a port, accepts new connections and distributes them across the workers in a round-robin fashion, with some built-in smarts to avoid overloading a worker process.

The second approach is where the master process creates the listen socket and sends it to interested workers. The workers then accept incoming connections directly.

The second approach should, in theory, give the best performance. In practice however, distribution tends to be very unbalanced due to operating system scheduler vagaries. Loads have been observed where over 70% of all connections ended up in just two processes, out of a total of eight.

Because server.listen() hands off most of the work to the master process, there are three cases where the behavior between a normal Node.js process and a cluster worker differs:

  1. server.listen({fd: 7}) Because the message is passed to the master, file descriptor 7 in the parent will be listened on, and the handle passed to the worker, rather than listening to the worker's idea of what the number 7 file descriptor references.
  2. server.listen(handle) Listening on handles explicitly will cause the worker to use the supplied handle, rather than talk to the master process. If the worker already has the handle, then it's presumed that you know what you are doing.
  3. server.listen(0) Normally, this will cause servers to listen on a random port. However, in a cluster, each worker will receive the same "random" port each time they do listen(0). In essence, the port is random the first time, but predictable thereafter. If you want to listen on a unique port, generate a port number based on the cluster worker ID.

There is no routing logic in Node.js, or in your program, and no shared state between the workers. Therefore, it is important to design your program such that it does not rely too heavily on in-memory data objects for things like sessions and login.

Because workers are all separate processes, they can be killed or re-spawned depending on your program's needs, without affecting other workers. As long as there are some workers still alive, the server will continue to accept connections. If no workers are alive, existing connections will be dropped and new connections will be refused. Node.js does not automatically manage the number of workers for you, however. It is your responsibility to manage the worker pool for your application's needs.

Although a primary use case for the cluster module is networking, it can also be used for other use cases requiring worker processes.

Class: Worker#

Added in: v0.7.0

A Worker object contains all public information and method about a worker. In the master it can be obtained using cluster.workers. In a worker it can be obtained using cluster.worker.

Event: 'disconnect'#

Added in: v0.7.7

Similar to the cluster.on('disconnect') event, but specific to this worker.

cluster.fork().on('disconnect', () => {
  // Worker has disconnected
});

Event: 'error'#

Added in: v0.7.3

This event is the same as the one provided by child_process.fork().

In a worker you can also use process.on('error').

Event: 'exit'#

Added in: v0.11.2
  • code <number> The exit code, if it exited normally.
  • signal <string> The name of the signal (e.g. 'SIGHUP') that caused the process to be killed.

Similar to the cluster.on('exit') event, but specific to this worker.

const worker = cluster.fork();
worker.on('exit', (code, signal) => {
  if (signal) {
    console.log(`worker was killed by signal: ${signal}`);
  } else if (code !== 0) {
    console.log(`worker exited with error code: ${code}`);
  } else {
    console.log('worker success!');
  }
});

Event: 'listening'#

Added in: v0.7.0

Similar to the cluster.on('listening') event, but specific to this worker.

cluster.fork().on('listening', (address) => {
  // Worker is listening
});

It is not emitted in the worker.

Event: 'message'#

Added in: v0.7.0

Similar to the cluster.on('message') event, but specific to this worker. In a worker you can also use process.on('message').

See process event: 'message'.

As an example, here is a cluster that keeps count of the number of requests in the master process using the message system:

const cluster = require('cluster');
const http = require('http');

if (cluster.isMaster) {

  // Keep track of http requests
  let numReqs = 0;
  setInterval(() => {
    console.log(`numReqs = ${numReqs}`);
  }, 1000);

  // Count requests
  function messageHandler(msg) {
    if (msg.cmd && msg.cmd === 'notifyRequest') {
      numReqs += 1;
    }
  }

  // Start workers and listen for messages containing notifyRequest
  const numCPUs = require('os').cpus().length;
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  for (const id in cluster.workers) {
    cluster.workers[id].on('message', messageHandler);
  }

} else {

  // Worker processes have a http server.
  http.Server((req, res) => {
    res.writeHead(200);
    res.end('hello world\n');

    // notify master about the request
    process.send({ cmd: 'notifyRequest' });
  }).listen(8000);
}

Event: 'online'#

Added in: v0.7.0

Similar to the cluster.on('online') event, but specific to this worker.

cluster.fork().on('online', () => {
  // Worker is online
});

It is not emitted in the worker.

worker.disconnect()#

Added in: v0.7.7
  • Returns: <Worker> A reference to worker.

In a worker, this function will close all servers, wait for the 'close' event on those servers, and then disconnect the IPC channel.

In the master, an internal message is sent to the worker causing it to call .disconnect() on itself.

Causes .exitedAfterDisconnect to be set.

Note that after a server is closed, it will no longer accept new connections, but connections may be accepted by any other listening worker. Existing connections will be allowed to close as usual. When no more connections exist, see server.close(), the IPC channel to the worker will close allowing it to die gracefully.

The above applies only to server connections, client connections are not automatically closed by workers, and disconnect does not wait for them to close before exiting.

Note that in a worker, process.disconnect exists, but it is not this function, it is disconnect.

Because long living server connections may block workers from disconnecting, it may be useful to send a message, so application specific actions may be taken to close them. It also may be useful to implement a timeout, killing a worker if the 'disconnect' event has not been emitted after some time.

if (cluster.isMaster) {
  const worker = cluster.fork();
  let timeout;

  worker.on('listening', (address) => {
    worker.send('shutdown');
    worker.disconnect();
    timeout = setTimeout(() => {
      worker.kill();
    }, 2000);
  });

  worker.on('disconnect', () => {
    clearTimeout(timeout);
  });

} else if (cluster.isWorker) {
  const net = require('net');
  const server = net.createServer((socket) => {
    // connections never end
  });

  server.listen(8000);

  process.on('message', (msg) => {
    if (msg === 'shutdown') {
      // initiate graceful close of any connections to server
    }
  });
}

worker.exitedAfterDisconnect#

Added in: v6.0.0

Set by calling .kill() or .disconnect(). Until then, it is undefined.

The boolean worker.exitedAfterDisconnect lets you distinguish between voluntary and accidental exit, the master may choose not to respawn a worker based on this value.

cluster.on('exit', (worker, code, signal) => {
  if (worker.exitedAfterDisconnect === true) {
    console.log('Oh, it was just voluntary – no need to worry');
  }
});

// kill worker
worker.kill();

worker.id#

Added in: v0.8.0

Each new worker is given its own unique id, this id is stored in the id.

While a worker is alive, this is the key that indexes it in cluster.workers

worker.isConnected()#

Added in: v0.11.14

This function returns true if the worker is connected to its master via its IPC channel, false otherwise. A worker is connected to its master after it's been created. It is disconnected after the 'disconnect' event is emitted.

worker.isDead()#

Added in: v0.11.14

This function returns true if the worker's process has terminated (either because of exiting or being signaled). Otherwise, it returns false.

worker.kill([signal='SIGTERM'])#

Added in: v0.9.12
  • signal <string> Name of the kill signal to send to the worker process.

This function will kill the worker. In the master, it does this by disconnecting the worker.process, and once disconnected, killing with signal. In the worker, it does it by disconnecting the channel, and then exiting with code 0.

Causes .exitedAfterDisconnect to be set.

This method is aliased as worker.destroy() for backwards compatibility.

Note that in a worker, process.kill() exists, but it is not this function, it is kill.

worker.process#

Added in: v0.7.0

All workers are created using child_process.fork(), the returned object from this function is stored as .process. In a worker, the global process is stored.

See: Child Process module

Note that workers will call process.exit(0) if the 'disconnect' event occurs on process and .exitedAfterDisconnect is not true. This protects against accidental disconnection.

worker.send(message[, sendHandle][, callback])#

Added in: v0.7.0

Send a message to a worker or master, optionally with a handle.

In the master this sends a message to a specific worker. It is identical to ChildProcess.send().

In a worker this sends a message to the master. It is identical to process.send().

This example will echo back all messages from the master:

if (cluster.isMaster) {
  const worker = cluster.fork();
  worker.send('hi there');

} else if (cluster.isWorker) {
  process.on('message', (msg) => {
    process.send(msg);
  });
}

worker.suicide#

Added in: v0.7.0 Deprecated since: v6.0.0
Stability: 0 - Deprecated: Use worker.exitedAfterDisconnect instead.

An alias to worker.exitedAfterDisconnect.

Set by calling .kill() or .disconnect(). Until then, it is undefined.

The boolean worker.suicide lets you distinguish between voluntary and accidental exit, the master may choose not to respawn a worker based on this value.

cluster.on('exit', (worker, code, signal) => {
  if (worker.suicide === true) {
    console.log('Oh, it was just voluntary – no need to worry');
  }
});

// kill worker
worker.kill();

This API only exists for backwards compatibility and will be removed in the future.

Event: 'disconnect'#

Added in: v0.7.9

Emitted after the worker IPC channel has disconnected. This can occur when a worker exits gracefully, is killed, or is disconnected manually (such as with worker.disconnect()).

There may be a delay between the 'disconnect' and 'exit' events. These events can be used to detect if the process is stuck in a cleanup or if there are long-living connections.

cluster.on('disconnect', (worker) => {
  console.log(`The worker #${worker.id} has disconnected`);
});

Event: 'exit'#

Added in: v0.7.9
  • worker <cluster.Worker>
  • code <number> The exit code, if it exited normally.
  • signal <string> The name of the signal (e.g. 'SIGHUP') that caused the process to be killed.

When any of the workers die the cluster module will emit the 'exit' event.

This can be used to restart the worker by calling .fork() again.

cluster.on(
  'exit',
  (worker, code, signal) => {
    console.log('worker %d died (%s). restarting...',
                worker.process.pid, signal || code);
    cluster.fork();
  }
);

See child_process event: 'exit'.

Event: 'fork'#

Added in: v0.7.0

When a new worker is forked the cluster module will emit a 'fork' event. This can be used to log worker activity, and create your own timeout.

const timeouts = [];
function errorMsg() {
  console.error('Something must be wrong with the connection ...');
}

cluster.on('fork', (worker) => {
  timeouts[worker.id] = setTimeout(errorMsg, 2000);
});
cluster.on('listening', (worker, address) => {
  clearTimeout(timeouts[worker.id]);
});
cluster.on('exit', (worker, code, signal) => {
  clearTimeout(timeouts[worker.id]);
  errorMsg();
});

Event: 'listening'#

Added in: v0.7.0

After calling listen() from a worker, when the 'listening' event is emitted on the server, a 'listening' event will also be emitted on cluster in the master.

The event handler is executed with two arguments, the worker contains the worker object and the address object contains the following connection properties: address, port and addressType. This is very useful if the worker is listening on more than one address.

cluster.on('listening', (worker, address) => {
  console.log(
    `A worker is now connected to ${address.address}:${address.port}`);
});

The addressType is one of:

  • 4 (TCPv4)
  • 6 (TCPv6)
  • -1 (unix domain socket)
  • "udp4" or "udp6" (UDP v4 or v6)

Event: 'message'#

Emitted when the cluster master receives a message from any worker.

See child_process event: 'message'.

Before Node.js v6.0, this event emitted only the message and the handle, but not the worker object, contrary to what the documentation stated.

If you need to support older versions and don't need the worker object, you can work around the discrepancy by checking the number of arguments:

cluster.on('message', (worker, message, handle) => {
  if (arguments.length === 2) {
    handle = message;
    message = worker;
    worker = undefined;
  }
  // ...
});

Event: 'online'#

Added in: v0.7.0

After forking a new worker, the worker should respond with an online message. When the master receives an online message it will emit this event. The difference between 'fork' and 'online' is that fork is emitted when the master forks a worker, and 'online' is emitted when the worker is running.

cluster.on('online', (worker) => {
  console.log('Yay, the worker responded after it was forked');
});

Event: 'setup'#

Added in: v0.7.1

Emitted every time .setupMaster() is called.

The settings object is the cluster.settings object at the time .setupMaster() was called and is advisory only, since multiple calls to .setupMaster() can be made in a single tick.

If accuracy is important, use cluster.settings.

cluster.disconnect([callback])#

Added in: v0.7.7
  • callback <Function> Called when all workers are disconnected and handles are closed.

Calls .disconnect() on each worker in cluster.workers.

When they are disconnected all internal handles will be closed, allowing the master process to die gracefully if no other event is waiting.

The method takes an optional callback argument which will be called when finished.

This can only be called from the master process.

cluster.fork([env])#

Added in: v0.6.0

Spawn a new worker process.

This can only be called from the master process.

cluster.isMaster#

Added in: v0.8.1

True if the process is a master. This is determined by the process.env.NODE_UNIQUE_ID. If process.env.NODE_UNIQUE_ID is undefined, then isMaster is true.

cluster.isWorker#

Added in: v0.6.0

True if the process is not a master (it is the negation of cluster.isMaster).

cluster.schedulingPolicy#

Added in: v0.11.2

The scheduling policy, either cluster.SCHED_RR for round-robin or cluster.SCHED_NONE to leave it to the operating system. This is a global setting and effectively frozen once you spawn the first worker or call cluster.setupMaster(), whatever comes first.

SCHED_RR is the default on all operating systems except Windows. Windows will change to SCHED_RR once libuv is able to effectively distribute IOCP handles without incurring a large performance hit.

cluster.schedulingPolicy can also be set through the NODE_CLUSTER_SCHED_POLICY environment variable. Valid values are "rr" and "none".

cluster.settings#

Added in: v0.7.1
  • <Object>
    • execArgv <Array> list of string arguments passed to the Node.js executable. Default: process.execArgv
    • exec <string> file path to worker file. Default: process.argv[1]
    • args <Array> string arguments passed to worker. Default:: process.argv.slice(2)
    • silent <boolean> whether or not to send output to parent's stdio. Default: false
    • stdio <Array> Configures the stdio of forked processes. Because the cluster module relies on IPC to function, this configuration must contain an 'ipc' entry. When this option is provided, it overrides silent.
    • uid <number> Sets the user identity of the process. (see setuid(2))
    • gid <number> Sets the group identity of the process. (see setgid(2))

After calling .setupMaster() (or .fork()) this settings object will contain the settings, including the default values.

This object is not supposed to be changed or set manually, by you.

cluster.setupMaster([settings])#

Added in: v0.7.1
  • settings <Object>
    • exec <string> file path to worker file. Default: process.argv[1]
    • args <Array> string arguments passed to worker. Default:: process.argv.slice(2)
    • silent <boolean> whether or not to send output to parent's stdio. Default: false
    • stdio <Array> Configures the stdio of forked processes. When this option is provided, it overrides silent.

setupMaster is used to change the default 'fork' behavior. Once called, the settings will be present in cluster.settings.

Note that:

  • any settings changes only affect future calls to .fork() and have no effect on workers that are already running
  • The only attribute of a worker that cannot be set via .setupMaster() is the env passed to .fork()
  • the defaults above apply to the first call only, the defaults for later calls is the current value at the time of cluster.setupMaster() is called

Example:

const cluster = require('cluster');
cluster.setupMaster({
  exec: 'worker.js',
  args: ['--use', 'https'],
  silent: true
});
cluster.fork(); // https worker
cluster.setupMaster({
  exec: 'worker.js',
  args: ['--use', 'http']
});
cluster.fork(); // http worker

This can only be called from the master process.

cluster.worker#

Added in: v0.7.0

A reference to the current worker object. Not available in the master process.

const cluster = require('cluster');

if (cluster.isMaster) {
  console.log('I am master');
  cluster.fork();
  cluster.fork();
} else if (cluster.isWorker) {
  console.log(`I am worker #${cluster.worker.id}`);
}

cluster.workers#

Added in: v0.7.0

A hash that stores the active worker objects, keyed by id field. Makes it easy to loop through all the workers. It is only available in the master process.

A worker is removed from cluster.workers after the worker has disconnected and exited. The order between these two events cannot be determined in advance. However, it is guaranteed that the removal from the cluster.workers list happens before last 'disconnect' or 'exit' event is emitted.

// Go through all workers
function eachWorker(callback) {
  for (const id in cluster.workers) {
    callback(cluster.workers[id]);
  }
}
eachWorker((worker) => {
  worker.send('big announcement to all workers');
});

Should you wish to reference a worker over a communication channel, using the worker's unique id is the easiest way to find the worker.

socket.on('data', (id) => {
  const worker = cluster.workers[id];
});

Command Line Options#

Node.js comes with a variety of CLI options. These options expose built-in debugging, multiple ways to execute scripts, and other helpful runtime options.

To view this documentation as a manual page in your terminal, run man node.

Synopsis#

node [options] [v8 options] [script.js | -e "script"] [--] [arguments]

node debug [script.js | -e "script" | <host>:<port>] …

node --v8-options

Execute without arguments to start the REPL.

For more info about node debug, please see the debugger documentation.

Options#

-v, --version#

Added in: v0.1.3

Print node's version.

-h, --help#

Added in: v0.1.3

Print node command line options. The output of this option is less detailed than this document.

-e, --eval "script"#

Added in: v0.5.2

Evaluate the following argument as JavaScript. The modules which are predefined in the REPL can also be used in script.

Note: On Windows, using cmd.exe a single quote will not work correctly because it only recognizes double " for quoting. In Powershell or Git bash, both ' and " are usable.

-p, --print "script"#

Added in: v0.6.4

Identical to -e but prints the result.

-c, --check#

Added in: v5.0.0

Syntax check the script without executing.

-i, --interactive#

Added in: v0.7.7

Opens the REPL even if stdin does not appear to be a terminal.

-r, --require module#

Added in: v1.6.0

Preload the specified module at startup.

Follows require()'s module resolution rules. module may be either a path to a file, or a node module name.

--no-deprecation#

Added in: v0.8.0

Silence deprecation warnings.

--trace-deprecation#

Added in: v0.8.0

Print stack traces for deprecations.

--throw-deprecation#

Added in: v0.11.14

Throw errors for deprecations.

--no-warnings#

Added in: v6.0.0

Silence all process warnings (including deprecations).

--trace-warnings#

Added in: v6.0.0

Print stack traces for process warnings (including deprecations).

--redirect-warnings=file#

Added in: v6.12.0

Write process warnings to the given file instead of printing to stderr. The file will be created if it does not exist, and will be appended to if it does. If an error occurs while attempting to write the warning to the file, the warning will be written to stderr instead.

--trace-sync-io#

Added in: v2.1.0

Prints a stack trace whenever synchronous I/O is detected after the first turn of the event loop.

--zero-fill-buffers#

Added in: v6.0.0

Automatically zero-fills all newly allocated Buffer and SlowBuffer instances.

--preserve-symlinks#

Added in: v6.3.0

Instructs the module loader to preserve symbolic links when resolving and caching modules.

By default, when Node.js loads a module from a path that is symbolically linked to a different on-disk location, Node.js will dereference the link and use the actual on-disk "real path" of the module as both an identifier and as a root path to locate other dependency modules. In most cases, this default behavior is acceptable. However, when using symbolically linked peer dependencies, as illustrated in the example below, the default behavior causes an exception to be thrown if moduleA attempts to require moduleB as a peer dependency:

{appDir}
 ├── app
 │   ├── index.js
 │   └── node_modules
 │       ├── moduleA -> {appDir}/moduleA
 │       └── moduleB
 │           ├── index.js
 │           └── package.json
 └── moduleA
     ├── index.js
     └── package.json

The --preserve-symlinks command line flag instructs Node.js to use the symlink path for modules as opposed to the real path, allowing symbolically linked peer dependencies to be found.

Note, however, that using --preserve-symlinks can have other side effects. Specifically, symbolically linked native modules can fail to load if those are linked from more than one location in the dependency tree (Node.js would see those as two separate modules and would attempt to load the module multiple times, causing an exception to be thrown).

--track-heap-objects#

Added in: v2.4.0

Track heap object allocations for heap snapshots.

--prof-process#

Added in: v6.0.0

Process v8 profiler output generated using the v8 option --prof.

--v8-options#

Added in: v0.1.3

Print v8 command line options.

Note: v8 options allow words to be separated by both dashes (-) or underscores (_).

For example, --stack-trace-limit is equivalent to --stack_trace_limit.

--tls-cipher-list=list#

Added in: v4.0.0

Specify an alternative default TLS cipher list. (Requires Node.js to be built with crypto support. (Default))

--enable-fips#

Added in: v6.0.0

Enable FIPS-compliant crypto at startup. (Requires Node.js to be built with ./configure --openssl-fips)

--force-fips#

Added in: v6.0.0

Force FIPS-compliant crypto on startup. (Cannot be disabled from script code.) (Same requirements as --enable-fips)

--openssl-config=file#

Added in: v6.9.0

Load an OpenSSL configuration file on startup. Among other uses, this can be used to enable FIPS-compliant crypto if Node.js is built with ./configure --openssl-fips.

--use-openssl-ca, --use-bundled-ca#

Added in: v6.11.0

Use OpenSSL's default CA store or use bundled Mozilla CA store as supplied by current Node.js version. The default store is selectable at build-time.

Using OpenSSL store allows for external modifications of the store. For most Linux and BSD distributions, this store is maintained by the distribution maintainers and system administrators. OpenSSL CA store location is dependent on configuration of the OpenSSL library but this can be altered at runtime using environment variables.

The bundled CA store, as supplied by Node.js, is a snapshot of Mozilla CA store that is fixed at release time. It is identical on all supported platforms.

See SSL_CERT_DIR and SSL_CERT_FILE.

--icu-data-dir=file#

Added in: v0.11.15

Specify ICU data load path. (overrides NODE_ICU_DATA)

--#

Added in: v6.11.0

Indicate the end of node options. Pass the rest of the arguments to the script. If no script filename or eval/print script is supplied prior to this, then the next argument will be used as a script filename.

Environment Variables#

NODE_DEBUG=module[,…]#

Added in: v0.1.32

','-separated list of core modules that should print debug information.

NODE_PATH=path[:…]#

Added in: v0.1.32

':'-separated list of directories prefixed to the module search path.

Note: on Windows, this is a ';'-separated list instead.

NODE_DISABLE_COLORS=1#

Added in: v0.3.0

When set to 1 colors will not be used in the REPL.

NODE_ICU_DATA=file#

Added in: v0.11.15

Data path for ICU (Intl object) data. Will extend linked-in data when compiled with small-icu support.

NODE_NO_WARNINGS=1#

Added in: v6.11.0

When set to 1, process warnings are silenced.

NODE_OPTIONS=options...#

Added in: v6.12.0

options... are interpreted as if they had been specified on the command line before the actual command line (so they can be overriden). Node will exit with an error if an option that is not allowed in the environment is used, such as -p or a script file.

Node options that are allowed are:

  • --enable-fips
  • --force-fips
  • --icu-data-dir
  • --debug-brk
  • --debug-port
  • --debug
  • --napi-modules
  • --no-deprecation
  • --no-warnings
  • --openssl-config
  • --redirect-warnings
  • --require, -r
  • --throw-deprecation
  • --tls-cipher-list
  • --trace-deprecation
  • --trace-sync-io
  • --trace-warnings
  • --track-heap-objects
  • --use-bundled-ca
  • --use-openssl-ca
  • --v8-pool-size
  • --zero-fill-buffers

V8 options that are allowed are:

  • --abort-on-uncaught-exception
  • --max-old-space-size

NODE_REPL_HISTORY=file#

Added in: v3.0.0

Path to the file used to store the persistent REPL history. The default path is ~/.node_repl_history, which is overridden by this variable. Setting the value to an empty string ("" or " ") disables persistent REPL history.

NODE_TTY_UNSAFE_ASYNC=1#

Added in: 6.4.0

When set to 1, writes to stdout and stderr will be non-blocking and asynchronous when outputting to a TTY on platforms which support async stdio. Setting this will void any guarantee that stdio will not be interleaved or dropped at program exit. Use of this mode is not recommended.

NODE_EXTRA_CA_CERTS=file#

When set, the well known "root" CAs (like VeriSign) will be extended with the extra certificates in file. The file should consist of one or more trusted certificates in PEM format. A message will be emitted (once) with process.emitWarning() if the file is missing or malformed, but any errors are otherwise ignored.

Note that neither the well known nor extra certificates are used when the ca options property is explicitly specified for a TLS or HTTPS client or server.

OPENSSL_CONF=file#

Added in: v6.11.0

Load an OpenSSL configuration file on startup. Among other uses, this can be used to enable FIPS-compliant crypto if Node.js is built with ./configure --openssl-fips.

If the --openssl-config command line option is used, the environment variable is ignored.

SSL_CERT_DIR=dir#

If --use-openssl-ca is enabled, this overrides and sets OpenSSL's directory containing trusted certificates.

Note: Be aware that unless the child environment is explicitly set, this evironment variable will be inherited by any child processes, and if they use OpenSSL, it may cause them to trust the same CAs as node.

SSL_CERT_FILE=file#

If --use-openssl-ca is enabled, this overrides and sets OpenSSL's file containing trusted certificates.

Note: Be aware that unless the child environment is explicitly set, this evironment variable will be inherited by any child processes, and if they use OpenSSL, it may cause them to trust the same CAs as node.

NODE_REDIRECT_WARNINGS=file#

Added in: v6.12.0

When set, process warnings will be emitted to the given file instead of printing to stderr. The file will be created if it does not exist, and will be appended to if it does. If an error occurs while attempting to write the warning to the file, the warning will be written to stderr instead. This is equivalent to using the --redirect-warnings=file command-line flag.

Console#

Stability: 2 - Stable

The console module provides a simple debugging console that is similar to the JavaScript console mechanism provided by web browsers.

The module exports two specific components:

  • A Console class with methods such as console.log(), console.error() and console.warn() that can be used to write to any Node.js stream.
  • A global console instance configured to write to process.stdout and process.stderr. The global console can be used without calling require('console').

Warning: The global console object's methods are neither consistently synchronous like the browser APIs they resemble, nor are they consistently asynchronous like all other Node.js streams. See the note on process I/O for more information.

Example using the global console:

console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to stderr

const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr

Example using the Console class:

const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);

myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err

const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err

Class: Console#

The Console class can be used to create a simple logger with configurable output streams and can be accessed using either require('console').Console or console.Console:

const Console = require('console').Console;

const Console = console.Console;

new Console(stdout[, stderr])#

Creates a new Console with one or two writable stream instances. stdout is a writable stream to print log or info output. stderr is used for warning or error output. If stderr is not provided, stdout is used for stderr.

const output = fs.createWriteStream('./stdout.log');
const errorOutput = fs.createWriteStream('./stderr.log');
// custom simple logger
const logger = new Console(output, errorOutput);
// use it like console
const count = 5;
logger.log('count: %d', count);
// in stdout.log: count 5

The global console is a special Console whose output is sent to process.stdout and process.stderr. It is equivalent to calling:

new Console(process.stdout, process.stderr);

console.assert(value[, message][, ...args])#

Added in: v0.1.101

A simple assertion test that verifies whether value is truthy. If it is not, an AssertionError is thrown. If provided, the error message is formatted using util.format() and used as the error message.

console.assert(true, 'does nothing');
// OK
console.assert(false, 'Whoops %s', 'didn\'t work');
// AssertionError: Whoops didn't work

Note: the console.assert() method is implemented differently in Node.js than the console.assert() method available in browsers.

Specifically, in browsers, calling console.assert() with a falsy assertion will cause the message to be printed to the console without interrupting execution of subsequent code. In Node.js, however, a falsy assertion will cause an AssertionError to be thrown.

Functionality approximating that implemented by browsers can be implemented by extending Node.js' console and overriding the console.assert() method.

In the following example, a simple module is created that extends and overrides the default behavior of console in Node.js.

'use strict';

// Creates a simple extension of console with a
// new impl for assert without monkey-patching.
const myConsole = Object.create(console, {
  assert: {
    value(assertion, message, ...args) {
      try {
        console.assert(assertion, message, ...args);
      } catch (err) {
        console.error(err.stack);
      }
    },
    configurable: true,
    enumerable: true,
    writable: true,
  },
});

module.exports = myConsole;

This can then be used as a direct replacement for the built in console:

const console = require('./myConsole');
console.assert(false, 'this message will print, but no error thrown');
console.log('this will also print');

console.clear()#

Added in: REPLACEME

When stdout is a TTY, calling console.clear() will attempt to clear the TTY. When stdout is not a TTY, this method does nothing.

Note: The specific operation of console.clear() can vary across operating systems and terminal types. For most Linux operating systems, console.clear() operates similarly to the clear shell command. On Windows, console.clear() will clear only the output in the current terminal viewport for the Node.js binary.

console.count([label])#

Added in: REPLACEME
  • label <string> The display label for the counter. Defaults to 'default'.

Maintains an internal counter specific to label and outputs to stdout the number of times console.count() has been called with the given label.

> console.count()
default: 1
undefined
> console.count('default')
default: 2
undefined
> console.count('abc')
abc: 1
undefined
> console.count('xyz')
xyz: 1
undefined
> console.count('abc')
abc: 2
undefined
> console.count()
default: 3
undefined
>

console.countReset([label = 'default'])#

Added in: REPLACEME
  • label <string> The display label for the counter. Defaults to 'default'.

Resets the internal counter specific to label.

> console.count('abc');
abc: 1
undefined
> console.countReset('abc');
undefined
> console.count('abc');
abc: 1
undefined
>

console.dir(obj[, options])#

Added in: v0.1.101

Uses util.inspect() on obj and prints the resulting string to stdout. This function bypasses any custom inspect() function defined on obj. An optional options object may be passed to alter certain aspects of the formatted string:

  • showHidden - if true then the object's non-enumerable and symbol properties will be shown too. Defaults to false.

  • depth - tells util.inspect() how many times to recurse while formatting the object. This is useful for inspecting large complicated objects. Defaults to 2. To make it recurse indefinitely, pass null.

  • colors - if true, then the output will be styled with ANSI color codes. Defaults to false. Colors are customizable; see customizing util.inspect() colors.

console.error([data][, ...args])#

Added in: v0.1.100

Prints to stderr with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to printf(3) (the arguments are all passed to util.format()).

const code = 5;
console.error('error #%d', code);
// Prints: error #5, to stderr
console.error('error', code);
// Prints: error 5, to stderr

If formatting elements (e.g. %d) are not found in the first string then util.inspect() is called on each argument and the resulting string values are concatenated. See util.format() for more information.

console.info([data][, ...args])#

Added in: v0.1.100

The console.info() function is an alias for console.log().

console.log([data][, ...args])#

Added in: v0.1.100

Prints to stdout with newline. Multiple arguments can be passed, with the first used as the primary message and all additional used as substitution values similar to printf(3) (the arguments are all passed to util.format()).

const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout

See util.format() for more information.

console.time(label)#

Added in: v0.1.104

Starts a timer that can be used to compute the duration of an operation. Timers are identified by a unique label. Use the same label when you call console.timeEnd() to stop the timer and output the elapsed time in milliseconds to stdout. Timer durations are accurate to the sub-millisecond.

console.timeEnd(label)#

Added in: v0.1.104

Stops a timer that was previously started by calling console.time() and prints the result to stdout:

console.time('100-elements');
for (let i = 0; i < 100; i++) ;
console.timeEnd('100-elements');
// prints 100-elements: 225.438ms

Note: As of Node.js v6.0.0, console.timeEnd() deletes the timer to avoid leaking it. On older versions, the timer persisted. This allowed console.timeEnd() to be called multiple times for the same label. This functionality was unintended and is no longer supported.

console.trace(message[, ...args])#

Added in: v0.1.104

Prints to stderr the string 'Trace :', followed by the util.format() formatted message and stack trace to the current position in the code.

console.trace('Show me');
// Prints: (stack trace will vary based on where trace is called)
//  Trace: Show me
//    at repl:2:9
//    at REPLServer.defaultEval (repl.js:248:27)
//    at bound (domain.js:287:14)
//    at REPLServer.runBound [as eval] (domain.js:300:12)
//    at REPLServer.<anonymous> (repl.js:412:12)
//    at emitOne (events.js:82:20)
//    at REPLServer.emit (events.js:169:7)
//    at REPLServer.Interface._onLine (readline.js:210:10)
//    at REPLServer.Interface._line (readline.js:549:8)
//    at REPLServer.Interface._ttyWrite (readline.js:826:14)

console.warn([data][, ...args])#

Added in: v0.1.100

The console.warn() function is an alias for console.error().

Crypto#

Stability: 2 - Stable

The crypto module provides cryptographic functionality that includes a set of wrappers for OpenSSL's hash, HMAC, cipher, decipher, sign and verify functions.

Use require('crypto') to access this module.

const crypto = require('crypto');

const secret = 'abcdefg';
const hash = crypto.createHmac('sha256', secret)
                   .update('I love cupcakes')
                   .digest('hex');
console.log(hash);
// Prints:
//   c0fa1bc00531bd78ef38c628449c5102aeabd49b5dc3a2a516ea6ea959d6658e

Determining if crypto support is unavailable#

It is possible for Node.js to be built without including support for the crypto module. In such cases, calling require('crypto') will result in an error being thrown.

let crypto;
try {
  crypto = require('crypto');
} catch (err) {
  console.log('crypto support is disabled!');
}

Class: Certificate#

Added in: v0.11.8

SPKAC is a Certificate Signing Request mechanism originally implemented by Netscape and now specified formally as part of HTML5's keygen element.

The crypto module provides the Certificate class for working with SPKAC data. The most common usage is handling output generated by the HTML5 <keygen> element. Node.js uses OpenSSL's SPKAC implementation internally.

new crypto.Certificate()#

Instances of the Certificate class can be created using the new keyword or by calling crypto.Certificate() as a function:

const crypto = require('crypto');

const cert1 = new crypto.Certificate();
const cert2 = crypto.Certificate();

certificate.exportChallenge(spkac)#

Added in: v0.11.8

The spkac data structure includes a public key and a challenge. The certificate.exportChallenge() returns the challenge component in the form of a Node.js Buffer. The spkac argument can be either a string or a Buffer.

const cert = require('crypto').Certificate();
const spkac = getSpkacSomehow();
const challenge = cert.exportChallenge(spkac);
console.log(challenge.toString('utf8'));
// Prints: the challenge as a UTF8 string

certificate.exportPublicKey(spkac)#

Added in: v0.11.8

The spkac data structure includes a public key and a challenge. The certificate.exportPublicKey() returns the public key component in the form of a Node.js Buffer. The spkac argument can be either a string or a Buffer.

const cert = require('crypto').Certificate();
const spkac = getSpkacSomehow();
const publicKey = cert.exportPublicKey(spkac);
console.log(publicKey);
// Prints: the public key as <Buffer ...>

certificate.verifySpkac(spkac)#

Added in: v0.11.8

Returns true if the given spkac data structure is valid, false otherwise. The spkac argument must be a Node.js Buffer.

const cert = require('crypto').Certificate();
const spkac = getSpkacSomehow();
console.log(cert.verifySpkac(Buffer.from(spkac)));
// Prints: true or false

Class: Cipher#

Added in: v0.1.94

Instances of the Cipher class are used to encrypt data. The class can be used in one of two ways:

  • As a stream that is both readable and writable, where plain unencrypted data is written to produce encrypted data on the readable side, or
  • Using the cipher.update() and cipher.final() methods to produce the encrypted data.

The crypto.createCipher() or crypto.createCipheriv() methods are used to create Cipher instances. Cipher objects are not to be created directly using the new keyword.

Example: Using Cipher objects as streams:

const crypto = require('crypto');
const cipher = crypto.createCipher('aes192', 'a password');

let encrypted = '';
cipher.on('readable', () => {
  const data = cipher.read();
  if (data)
    encrypted += data.toString('hex');
});
cipher.on('end', () => {
  console.log(encrypted);
  // Prints: ca981be48e90867604588e75d04feabb63cc007a8f8ad89b10616ed84d815504
});

cipher.write('some clear text data');
cipher.end();

Example: Using Cipher and piped streams:

const crypto = require('crypto');
const fs = require('fs');
const cipher = crypto.createCipher('aes192', 'a password');

const input = fs.createReadStream('test.js');
const output = fs.createWriteStream('test.enc');

input.pipe(cipher).pipe(output);

Example: Using the cipher.update() and cipher.final() methods:

const crypto = require('crypto');
const cipher = crypto.createCipher('aes192', 'a password');

let encrypted = cipher.update('some clear text data', 'utf8', 'hex');
encrypted += cipher.final('hex');
console.log(encrypted);
// Prints: ca981be48e90867604588e75d04feabb63cc007a8f8ad89b10616ed84d815504

cipher.final([output_encoding])#

Added in: v0.1.94

Returns any remaining enciphered contents. If output_encoding parameter is one of 'latin1', 'base64' or 'hex', a string is returned. If an output_encoding is not provided, a Buffer is returned.

Once the cipher.final() method has been called, the Cipher object can no longer be used to encrypt data. Attempts to call cipher.final() more than once will result in an error being thrown.

cipher.setAAD(buffer)#

Added in: v1.0.0

When using an authenticated encryption mode (only GCM is currently supported), the cipher.setAAD() method sets the value used for the additional authenticated data (AAD) input parameter.

Returns this for method chaining.

cipher.getAuthTag()#

Added in: v1.0.0

When using an authenticated encryption mode (only GCM is currently supported), the cipher.getAuthTag() method returns a Buffer containing the authentication tag that has been computed from the given data.

The cipher.getAuthTag() method should only be called after encryption has been completed using the cipher.final() method.

cipher.setAutoPadding(auto_padding=true)#

Added in: v0.7.1

When using block encryption algorithms, the Cipher class will automatically add padding to the input data to the appropriate block size. To disable the default padding call cipher.setAutoPadding(false).

When auto_padding is false, the length of the entire input data must be a multiple of the cipher's block size or cipher.final() will throw an Error. Disabling automatic padding is useful for non-standard padding, for instance using 0x0 instead of PKCS padding.

The cipher.setAutoPadding() method must be called before cipher.final().

Returns this for method chaining.

cipher.update(data[, input_encoding][, output_encoding])#

Added in: v0.1.94

Updates the cipher with data. If the input_encoding argument is given, its value must be one of 'utf8', 'ascii', or 'latin1' and the data argument is a string using the specified encoding. If the input_encoding argument is not given, data must be a Buffer. If data is a Buffer then input_encoding is ignored.

The output_encoding specifies the output format of the enciphered data, and can be 'latin1', 'base64' or 'hex'. If the output_encoding is specified, a string using the specified encoding is returned. If no output_encoding is provided, a Buffer is returned.

The cipher.update() method can be called multiple times with new data until cipher.final() is called. Calling cipher.update() after cipher.final() will result in an error being thrown.

Class: Decipher#

Added in: v0.1.94

Instances of the Decipher class are used to decrypt data. The class can be used in one of two ways:

  • As a stream that is both readable and writable, where plain encrypted data is written to produce unencrypted data on the readable side, or
  • Using the decipher.update() and decipher.final() methods to produce the unencrypted data.

The crypto.createDecipher() or crypto.createDecipheriv() methods are used to create Decipher instances. Decipher objects are not to be created directly using the new keyword.

Example: Using Decipher objects as streams:

const crypto = require('crypto');
const decipher = crypto.createDecipher('aes192', 'a password');

let decrypted = '';
decipher.on('readable', () => {
  const data = decipher.read();
  if (data)
    decrypted += data.toString('utf8');
});
decipher.on('end', () => {
  console.log(decrypted);
  // Prints: some clear text data
});

const encrypted =
  'ca981be48e90867604588e75d04feabb63cc007a8f8ad89b10616ed84d815504';
decipher.write(encrypted, 'hex');
decipher.end();

Example: Using Decipher and piped streams:

const crypto = require('crypto');
const fs = require('fs');
const decipher = crypto.createDecipher('aes192', 'a password');

const input = fs.createReadStream('test.enc');
const output = fs.createWriteStream('test.js');

input.pipe(decipher).pipe(output);

Example: Using the decipher.update() and decipher.final() methods:

const crypto = require('crypto');
const decipher = crypto.createDecipher('aes192', 'a password');

const encrypted =
  'ca981be48e90867604588e75d04feabb63cc007a8f8ad89b10616ed84d815504';
let decrypted = decipher.update(encrypted, 'hex', 'utf8');
decrypted += decipher.final('utf8');
console.log(decrypted);
// Prints: some clear text data

decipher.final([output_encoding])#

Added in: v0.1.94

Returns any remaining deciphered contents. If output_encoding parameter is one of 'latin1', 'ascii' or 'utf8', a string is returned. If an output_encoding is not provided, a Buffer is returned.

Once the decipher.final() method has been called, the Decipher object can no longer be used to decrypt data. Attempts to call decipher.final() more than once will result in an error being thrown.

decipher.setAAD(buffer)#

Added in: v1.0.0

When using an authenticated encryption mode (only GCM is currently supported), the decipher.setAAD() method sets the value used for the additional authenticated data (AAD) input parameter.

Returns this for method chaining.

decipher.setAuthTag(buffer)#

Added in: v1.0.0

When using an authenticated encryption mode (only GCM is currently supported), the decipher.setAuthTag() method is used to pass in the received authentication tag. If no tag is provided, or if the cipher text has been tampered with, decipher.final() will throw, indicating that the cipher text should be discarded due to failed authentication.

Returns this for method chaining.

decipher.setAutoPadding(auto_padding=true)#

Added in: v0.7.1

When data has been encrypted without standard block padding, calling decipher.setAutoPadding(false) will disable automatic padding to prevent decipher.final() from checking for and removing padding.

Turning auto padding off will only work if the input data's length is a multiple of the ciphers block size.

The decipher.setAutoPadding() method must be called before decipher.update().

Returns this for method chaining.

decipher.update(data[, input_encoding][, output_encoding])#

Added in: v0.1.94

Updates the decipher with data. If the input_encoding argument is given, its value must be one of 'latin1', 'base64', or 'hex' and the data argument is a string using the specified encoding. If the input_encoding argument is not given, data must be a Buffer. If data is a Buffer then input_encoding is ignored.

The output_encoding specifies the output format of the enciphered data, and can be 'latin1', 'ascii' or 'utf8'. If the output_encoding is specified, a string using the specified encoding is returned. If no output_encoding is provided, a Buffer is returned.

The decipher.update() method can be called multiple times with new data until decipher.final() is called. Calling decipher.update() after decipher.final() will result in an error being thrown.

Class: DiffieHellman#

Added in: v0.5.0

The DiffieHellman class is a utility for creating Diffie-Hellman key exchanges.

Instances of the DiffieHellman class can be created using the crypto.createDiffieHellman() function.

const crypto = require('crypto');
const assert = require('assert');

// Generate Alice's keys...
const alice = crypto.createDiffieHellman(2048);
const aliceKey = alice.generateKeys();

// Generate Bob's keys...
const bob = crypto.createDiffieHellman(alice.getPrime(), alice.getGenerator());
const bobKey = bob.generateKeys();

// Exchange and generate the secret...
const aliceSecret = alice.computeSecret(bobKey);
const bobSecret = bob.computeSecret(aliceKey);

// OK
assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));

diffieHellman.computeSecret(other_public_key[, input_encoding][, output_encoding])#

Added in: v0.5.0

Computes the shared secret using other_public_key as the other party's public key and returns the computed shared secret. The supplied key is interpreted using the specified input_encoding, and secret is encoded using specified output_encoding. Encodings can be 'latin1', 'hex', or 'base64'. If the input_encoding is not provided, other_public_key is expected to be a Buffer.

If output_encoding is given a string is returned; otherwise, a Buffer is returned.

diffieHellman.generateKeys([encoding])#

Added in: v0.5.0

Generates private and public Diffie-Hellman key values, and returns the public key in the specified encoding. This key should be transferred to the other party. Encoding can be 'latin1', 'hex', or 'base64'. If encoding is provided a string is returned; otherwise a Buffer is returned.

diffieHellman.getGenerator([encoding])#

Added in: v0.5.0

Returns the Diffie-Hellman generator in the specified encoding, which can be 'latin1', 'hex', or 'base64'. If encoding is provided a string is returned; otherwise a Buffer is returned.

diffieHellman.getPrime([encoding])#

Added in: v0.5.0

Returns the Diffie-Hellman prime in the specified encoding, which can be 'latin1', 'hex', or 'base64'. If encoding is provided a string is returned; otherwise a Buffer is returned.

diffieHellman.getPrivateKey([encoding])#

Added in: v0.5.0

Returns the Diffie-Hellman private key in the specified encoding, which can be 'latin1', 'hex', or 'base64'. If encoding is provided a string is returned; otherwise a Buffer is returned.

diffieHellman.getPublicKey([encoding])#

Added in: v0.5.0

Returns the Diffie-Hellman public key in the specified encoding, which can be 'latin1', 'hex', or 'base64'. If encoding is provided a string is returned; otherwise a Buffer is returned.

diffieHellman.setPrivateKey(private_key[, encoding])#

Added in: v0.5.0

Sets the Diffie-Hellman private key. If the encoding argument is provided and is either 'latin1', 'hex', or 'base64', private_key is expected to be a string. If no encoding is provided, private_key is expected to be a Buffer.

diffieHellman.setPublicKey(public_key[, encoding])#

Added in: v0.5.0

Sets the Diffie-Hellman public key. If the encoding argument is provided and is either 'latin1', 'hex' or 'base64', public_key is expected to be a string. If no encoding is provided, public_key is expected to be a Buffer.

diffieHellman.verifyError#

Added in: v0.11.12

A bit field containing any warnings and/or errors resulting from a check performed during initialization of the DiffieHellman object.

The following values are valid for this property (as defined in constants module):

  • DH_CHECK_P_NOT_SAFE_PRIME
  • DH_CHECK_P_NOT_PRIME
  • DH_UNABLE_TO_CHECK_GENERATOR
  • DH_NOT_SUITABLE_GENERATOR

Class: ECDH#

Added in: v0.11.14

The ECDH class is a utility for creating Elliptic Curve Diffie-Hellman (ECDH) key exchanges.

Instances of the ECDH class can be created using the crypto.createECDH() function.

const crypto = require('crypto');
const assert = require('assert');

// Generate Alice's keys...
const alice = crypto.createECDH('secp521r1');
const aliceKey = alice.generateKeys();

// Generate Bob's keys...
const bob = crypto.createECDH('secp521r1');
const bobKey = bob.generateKeys();

// Exchange and generate the secret...
const aliceSecret = alice.computeSecret(bobKey);
const bobSecret = bob.computeSecret(aliceKey);

assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));
// OK

ecdh.computeSecret(other_public_key[, input_encoding][, output_encoding])#

Added in: v0.11.14

Computes the shared secret using other_public_key as the other party's public key and returns the computed shared secret. The supplied key is interpreted using specified input_encoding, and the returned secret is encoded using the specified output_encoding. Encodings can be 'latin1', 'hex', or 'base64'. If the input_encoding is not provided, other_public_key is expected to be a Buffer.

If output_encoding is given a string will be returned; otherwise a Buffer is returned.

ecdh.generateKeys([encoding[, format]])#

Added in: v0.11.14

Generates private and public EC Diffie-Hellman key values, and returns the public key in the specified format and encoding. This key should be transferred to the other party.

The format arguments specifies point encoding and can be 'compressed', 'uncompressed', or 'hybrid'. If format is not specified, the point will be returned in 'uncompressed' format.

The encoding argument can be 'latin1', 'hex', or 'base64'. If encoding is provided a string is returned; otherwise a Buffer is returned.

ecdh.getPrivateKey([encoding])#

Added in: v0.11.14

Returns the EC Diffie-Hellman private key in the specified encoding, which can be 'latin1', 'hex', or 'base64'. If encoding is provided a string is returned; otherwise a Buffer is returned.

ecdh.getPublicKey([encoding[, format]])#

Added in: v0.11.14

Returns the EC Diffie-Hellman public key in the specified encoding and format.

The format argument specifies point encoding and can be 'compressed', 'uncompressed', or 'hybrid'. If format is not specified the point will be returned in 'uncompressed' format.

The encoding argument can be 'latin1', 'hex', or 'base64'. If encoding is specified, a string is returned; otherwise a Buffer is returned.

ecdh.setPrivateKey(private_key[, encoding])#

Added in: v0.11.14

Sets the EC Diffie-Hellman private key. The encoding can be 'latin1', 'hex' or 'base64'. If encoding is provided, private_key is expected to be a string; otherwise private_key is expected to be a Buffer. If private_key is not valid for the curve specified when the ECDH object was created, an error is thrown. Upon setting the private key, the associated public point (key) is also generated and set in the ECDH object.

ecdh.setPublicKey(public_key[, encoding])#

Added in: v0.11.14 Deprecated since: v5.2.0
Stability: 0 - Deprecated

Sets the EC Diffie-Hellman public key. Key encoding can be 'latin1', 'hex' or 'base64'. If encoding is provided public_key is expected to be a string; otherwise a Buffer is expected.

Note that there is not normally a reason to call this method because ECDH only requires a private key and the other party's public key to compute the shared secret. Typically either ecdh.generateKeys() or ecdh.setPrivateKey() will be called. The ecdh.setPrivateKey() method attempts to generate the public point/key associated with the private key being set.

Example (obtaining a shared secret):

const crypto = require('crypto');
const alice = crypto.createECDH('secp256k1');
const bob = crypto.createECDH('secp256k1');

// Note: This is a shortcut way to specify one of Alice's previous private
// keys. It would be unwise to use such a predictable private key in a real
// application.
alice.setPrivateKey(
  crypto.createHash('sha256').update('alice', 'utf8').digest()
);

// Bob uses a newly generated cryptographically strong
// pseudorandom key pair
bob.generateKeys();

const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex');
const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex');

// aliceSecret and bobSecret should be the same shared secret value
console.log(aliceSecret === bobSecret);

Class: Hash#

Added in: v0.1.92

The Hash class is a utility for creating hash digests of data. It can be used in one of two ways:

  • As a stream that is both readable and writable, where data is written to produce a computed hash digest on the readable side, or
  • Using the hash.update() and hash.digest() methods to produce the computed hash.

The crypto.createHash() method is used to create Hash instances. Hash objects are not to be created directly using the new keyword.

Example: Using Hash objects as streams:

const crypto = require('crypto');
const hash = crypto.createHash('sha256');

hash.on('readable', () => {
  const data = hash.read();
  if (data) {
    console.log(data.toString('hex'));
    // Prints:
    //   6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50
  }
});

hash.write('some data to hash');
hash.end();

Example: Using Hash and piped streams:

const crypto = require('crypto');
const fs = require('fs');
const hash = crypto.createHash('sha256');

const input = fs.createReadStream('test.js');
input.pipe(hash).pipe(process.stdout);

Example: Using the hash.update() and hash.digest() methods:

const crypto = require('crypto');
const hash = crypto.createHash('sha256');

hash.update('some data to hash');
console.log(hash.digest('hex'));
// Prints:
//   6a2da20943931e9834fc12cfe5bb47bbd9ae43489a30726962b576f4e3993e50

hash.digest([encoding])#

Added in: v0.1.92

Calculates the digest of all of the data passed to be hashed (using the hash.update() method). The encoding can be 'hex', 'latin1' or 'base64'. If encoding is provided a string will be returned; otherwise a Buffer is returned.

The Hash object can not be used again after hash.digest() method has been called. Multiple calls will cause an error to be thrown.

hash.update(data[, input_encoding])#

Added in: v0.1.92

Updates the hash content with the given data, the encoding of which is given in input_encoding and can be 'utf8', 'ascii' or 'latin1'. If encoding is not provided, and the data is a string, an encoding of 'utf8' is enforced. If data is a Buffer then input_encoding is ignored.

This can be called many times with new data as it is streamed.

Class: Hmac#

Added in: v0.1.94

The Hmac Class is a utility for creating cryptographic HMAC digests. It can be used in one of two ways:

  • As a stream that is both readable and writable, where data is written to produce a computed HMAC digest on the readable side, or
  • Using the hmac.update() and hmac.digest() methods to produce the computed HMAC digest.

The crypto.createHmac() method is used to create Hmac instances. Hmac objects are not to be created directly using the new keyword.

Example: Using Hmac objects as streams:

const crypto = require('crypto');
const hmac = crypto.createHmac('sha256', 'a secret');

hmac.on('readable', () => {
  const data = hmac.read();
  if (data) {
    console.log(data.toString('hex'));
    // Prints:
    //   7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e
  }
});

hmac.write('some data to hash');
hmac.end();

Example: Using Hmac and piped streams:

const crypto = require('crypto');
const fs = require('fs');
const hmac = crypto.createHmac('sha256', 'a secret');

const input = fs.createReadStream('test.js');
input.pipe(hmac).pipe(process.stdout);

Example: Using the hmac.update() and hmac.digest() methods:

const crypto = require('crypto');
const hmac = crypto.createHmac('sha256', 'a secret');

hmac.update('some data to hash');
console.log(hmac.digest('hex'));
// Prints:
//   7fd04df92f636fd450bc841c9418e5825c17f33ad9c87c518115a45971f7f77e

hmac.digest([encoding])#

Added in: v0.1.94

Calculates the HMAC digest of all of the data passed using hmac.update(). The encoding can be 'hex', 'latin1' or 'base64'. If encoding is provided a string is returned; otherwise a Buffer is returned;

The Hmac object can not be used again after hmac.digest() has been called. Multiple calls to hmac.digest() will result in an error being thrown.

hmac.update(data[, input_encoding])#

Added in: v0.1.94

Updates the Hmac content with the given data, the encoding of which is given in input_encoding and can be 'utf8', 'ascii' or 'latin1'. If encoding is not provided, and the data is a string, an encoding of 'utf8' is enforced. If data is a Buffer then input_encoding is ignored.

This can be called many times with new data as it is streamed.

Class: Sign#

Added in: v0.1.92

The Sign Class is a utility for generating signatures. It can be used in one of two ways:

  • As a writable stream, where data to be signed is written and the sign.sign() method is used to generate and return the signature, or
  • Using the sign.update() and sign.sign() methods to produce the signature.

The crypto.createSign() method is used to create Sign instances. The argument is the string name of the hash function to use. Sign objects are not to be created directly using the new keyword.

Example: Using Sign objects as streams:

const crypto = require('crypto');
const sign = crypto.createSign('SHA256');

sign.write('some data to sign');
sign.end();

const privateKey = getPrivateKeySomehow();
console.log(sign.sign(privateKey, 'hex'));
// Prints: the calculated signature using the specified private key and
// SHA-256. For RSA keys, the algorithm is RSASSA-PKCS1-v1_5 (see padding
// parameter below for RSASSA-PSS). For EC keys, the algorithm is ECDSA.

Example: Using the sign.update() and sign.sign() methods:

const crypto = require('crypto');
const sign = crypto.createSign('SHA256');

sign.update('some data to sign');

const privateKey = getPrivateKeySomehow();
console.log(sign.sign(privateKey, 'hex'));
// Prints: the calculated signature

In some cases, a Sign instance can also be created by passing in a signature algorithm name, such as 'RSA-SHA256'. This will use the corresponding digest algorithm. This does not work for all signature algorithms, such as 'ecdsa-with-SHA256'. Use digest names instead.

Example: signing using legacy signature algorithm name

const crypto = require('crypto');
const sign = crypto.createSign('RSA-SHA256');

sign.update('some data to sign');

const privateKey = getPrivateKeySomehow();
console.log(sign.sign(privateKey, 'hex'));
// Prints: the calculated signature

sign.sign(private_key[, output_format])#

Added in: v0.1.92

Calculates the signature on all the data passed through using either sign.update() or sign.write().

The private_key argument can be an object or a string. If private_key is a string, it is treated as a raw key with no passphrase. If private_key is an object, it must contain one or more of the following properties:

  • key: <string> - PEM encoded private key (required)
  • passphrase: <string> - passphrase for the private key

  • padding: <integer> - Optional padding value for RSA, one of the following:

    • crypto.constants.RSA_PKCS1_PADDING (default)
    • crypto.constants.RSA_PKCS1_PSS_PADDING

    Note that RSA_PKCS1_PSS_PADDING will use MGF1 with the same hash function used to sign the message as specified in section 3.1 of RFC 4055.

  • saltLength: <integer> - salt length for when padding is RSA_PKCS1_PSS_PADDING. The special value crypto.constants.RSA_PSS_SALTLEN_DIGEST sets the salt length to the digest size, crypto.constants.RSA_PSS_SALTLEN_MAX_SIGN (default) sets it to the maximum permissible value.

The output_format can specify one of 'latin1', 'hex' or 'base64'. If output_format is provided a string is returned; otherwise a Buffer is returned.

The Sign object can not be again used after sign.sign() method has been called. Multiple calls to sign.sign() will result in an error being thrown.

sign.update(data[, input_encoding])#

Added in: v0.1.92

Updates the Sign content with the given data, the encoding of which is given in input_encoding and can be 'utf8', 'ascii' or 'latin1'. If encoding is not provided, and the data is a string, an encoding of 'utf8' is enforced. If data is a Buffer then input_encoding is ignored.

This can be called many times with new data as it is streamed.

Class: Verify#

Added in: v0.1.92

The Verify class is a utility for verifying signatures. It can be used in one of two ways:

The crypto.createVerify() method is used to create Verify instances. Verify objects are not to be created directly using the new keyword.

Example: Using Verify objects as streams:

const crypto = require('crypto');
const verify = crypto.createVerify('SHA256');

verify.write('some data to sign');
verify.end();

const publicKey = getPublicKeySomehow();
const signature = getSignatureToVerify();
console.log(verify.verify(publicKey, signature));
// Prints: true or false

Example: Using the verify.update() and verify.verify() methods:

const crypto = require('crypto');
const verify = crypto.createVerify('SHA256');

verify.update('some data to sign');

const publicKey = getPublicKeySomehow();
const signature = getSignatureToVerify();
console.log(verify.verify(publicKey, signature));
// Prints: true or false

verifier.update(data[, input_encoding])#

Added in: v0.1.92

Updates the Verify content with the given data, the encoding of which is given in input_encoding and can be 'utf8', 'ascii' or 'latin1'. If encoding is not provided, and the data is a string, an encoding of 'utf8' is enforced. If data is a Buffer then input_encoding is ignored.

This can be called many times with new data as it is streamed.

verifier.verify(object, signature[, signature_format])#

Added in: v0.1.92

Verifies the provided data using the given object and signature. The object argument can be either a string containing a PEM encoded object, which can be an RSA public key, a DSA public key, or an X.509 certificate, or an object with one or more of the following properties:

  • key: <string> - PEM encoded public key (required)

  • padding: <integer> - Optional padding value for RSA, one of the following:

    • crypto.constants.RSA_PKCS1_PADDING (default)
    • crypto.constants.RSA_PKCS1_PSS_PADDING

    Note that RSA_PKCS1_PSS_PADDING will use MGF1 with the same hash function used to verify the message as specified in section 3.1 of RFC 4055.

  • saltLength: <integer> - salt length for when padding is RSA_PKCS1_PSS_PADDING. The special value crypto.constants.RSA_PSS_SALTLEN_DIGEST sets the salt length to the digest size, crypto.constants.RSA_PSS_SALTLEN_AUTO (default) causes it to be determined automatically.

The signature argument is the previously calculated signature for the data, in the signature_format which can be 'latin1', 'hex' or 'base64'. If a signature_format is specified, the signature is expected to be a string; otherwise signature is expected to be a Buffer.

Returns true or false depending on the validity of the signature for the data and public key.

The verifier object can not be used again after verify.verify() has been called. Multiple calls to verify.verify() will result in an error being thrown.

crypto module methods and properties#

crypto.constants#

Added in: v6.3.0

Returns an object containing commonly used constants for crypto and security related operations. The specific constants currently defined are described in Crypto Constants.

crypto.DEFAULT_ENCODING#

Added in: v0.9.3

The default encoding to use for functions that can take either strings or buffers. The default value is 'buffer', which makes methods default to Buffer objects.

The crypto.DEFAULT_ENCODING mechanism is provided for backwards compatibility with legacy programs that expect 'latin1' to be the default encoding.

New applications should expect the default to be 'buffer'. This property may become deprecated in a future Node.js release.

crypto.fips#

Added in: v6.0.0

Property for checking and controlling whether a FIPS compliant crypto provider is currently in use. Setting to true requires a FIPS build of Node.js.

crypto.createCipher(algorithm, password)#

Added in: v0.1.94

Creates and returns a Cipher object that uses the given algorithm and password.

The algorithm is dependent on OpenSSL, examples are 'aes192', etc. On recent OpenSSL releases, openssl list-cipher-algorithms will display the available cipher algorithms.

The password is used to derive the cipher key and initialization vector (IV). The value must be either a 'latin1' encoded string or a Buffer.

The implementation of crypto.createCipher() derives keys using the OpenSSL function EVP_BytesToKey with the digest algorithm set to MD5, one iteration, and no salt. The lack of salt allows dictionary attacks as the same password always creates the same key. The low iteration count and non-cryptographically secure hash algorithm allow passwords to be tested very rapidly.

In line with OpenSSL's recommendation to use pbkdf2 instead of EVP_BytesToKey it is recommended that developers derive a key and IV on their own using crypto.pbkdf2() and to use crypto.createCipheriv() to create the Cipher object. Users should not use ciphers with counter mode (e.g. CTR, GCM or CCM) in crypto.createCipher(). A warning is emitted when they are used in order to avoid the risk of IV reuse that causes vulnerabilities. For the case when IV is reused in GCM, see Nonce-Disrespecting Adversaries for details.

crypto.createCipheriv(algorithm, key, iv)#

Creates and returns a Cipher object, with the given algorithm, key and initialization vector (iv).

The algorithm is dependent on OpenSSL, examples are 'aes192', etc. On recent OpenSSL releases, openssl list-cipher-algorithms will display the available cipher algorithms.

The key is the raw key used by the algorithm and iv is an initialization vector. Both arguments must be 'utf8' encoded strings or buffers.

crypto.createCredentials(details)#

Added in: v0.1.92 Deprecated since: v0.11.13
Stability: 0 - Deprecated: Use tls.createSecureContext() instead.

The crypto.createCredentials() method is a deprecated function for creating and returning a tls.SecureContext. It should not be used. Replace it with tls.createSecureContext() which has the exact same arguments and return value.

Returns a tls.SecureContext, as-if tls.createSecureContext() had been called.

crypto.createDecipher(algorithm, password)#

Added in: v0.1.94

Creates and returns a Decipher object that uses the given algorithm and password (key).

The implementation of crypto.createDecipher() derives keys using the OpenSSL function EVP_BytesToKey with the digest algorithm set to MD5, one iteration, and no salt. The lack of salt allows dictionary attacks as the same password always creates the same key. The low iteration count and non-cryptographically secure hash algorithm allow passwords to be tested very rapidly.

In line with OpenSSL's recommendation to use pbkdf2 instead of EVP_BytesToKey it is recommended that developers derive a key and IV on their own using crypto.pbkdf2() and to use crypto.createDecipheriv() to create the Decipher object.

crypto.createDecipheriv(algorithm, key, iv)#

Added in: v0.1.94

Creates and returns a Decipher object that uses the given algorithm, key and initialization vector (iv).

The algorithm is dependent on OpenSSL, examples are 'aes192', etc. On recent OpenSSL releases, openssl list-cipher-algorithms will display the available cipher algorithms.

The key is the raw key used by the algorithm and iv is an initialization vector. Both arguments must be 'utf8' encoded strings or buffers.

crypto.createDiffieHellman(prime[, prime_encoding][, generator][, generator_encoding])#

Added in: v0.11.12

Creates a DiffieHellman key exchange object using the supplied prime and an optional specific generator.

The generator argument can be a number, string, or Buffer. If generator is not specified, the value 2 is used.

The prime_encoding and generator_encoding arguments can be 'latin1', 'hex', or 'base64'.

If prime_encoding is specified, prime is expected to be a string; otherwise a Buffer is expected.

If generator_encoding is specified, generator is expected to be a string; otherwise either a number or Buffer is expected.

crypto.createDiffieHellman(prime_length[, generator])#

Added in: v0.5.0

Creates a DiffieHellman key exchange object and generates a prime of prime_length bits using an optional specific numeric generator. If generator is not specified, the value 2 is used.

crypto.createECDH(curve_name)#

Added in: v0.11.14

Creates an Elliptic Curve Diffie-Hellman (ECDH) key exchange object using a predefined curve specified by the curve_name string. Use crypto.getCurves() to obtain a list of available curve names. On recent OpenSSL releases, openssl ecparam -list_curves will also display the name and description of each available elliptic curve.

crypto.createHash(algorithm)#

Added in: v0.1.92

Creates and returns a Hash object that can be used to generate hash digests using the given algorithm.

The algorithm is dependent on the available algorithms supported by the version of OpenSSL on the platform. Examples are 'sha256', 'sha512', etc. On recent releases of OpenSSL, openssl list-message-digest-algorithms will display the available digest algorithms.

Example: generating the sha256 sum of a file

const filename = process.argv[2];
const crypto = require('crypto');
const fs = require('fs');

const hash = crypto.createHash('sha256');

const input = fs.createReadStream(filename);
input.on('readable', () => {
  const data = input.read();
  if (data)
    hash.update(data);
  else {
    console.log(`${hash.digest('hex')} ${filename}`);
  }
});

crypto.createHmac(algorithm, key)#

Added in: v0.1.94

Creates and returns an Hmac object that uses the given algorithm and key.

The algorithm is dependent on the available algorithms supported by the version of OpenSSL on the platform. Examples are 'sha256', 'sha512', etc. On recent releases of OpenSSL, openssl list-message-digest-algorithms will display the available digest algorithms.

The key is the HMAC key used to generate the cryptographic HMAC hash.

Example: generating the sha256 HMAC of a file

const filename = process.argv[2];
const crypto = require('crypto');
const fs = require('fs');

const hmac = crypto.createHmac('sha256', 'a secret');

const input = fs.createReadStream(filename);
input.on('readable', () => {
  const data = input.read();
  if (data)
    hmac.update(data);
  else {
    console.log(`${hmac.digest('hex')} ${filename}`);
  }
});

crypto.createSign(algorithm)#

Added in: v0.1.92

Creates and returns a Sign object that uses the given algorithm. Use crypto.getHashes() to obtain an array of names of the available signing algorithms.

crypto.createVerify(algorithm)#

Added in: v0.1.92

Creates and returns a Verify object that uses the given algorithm. Use crypto.getHashes() to obtain an array of names of the available signing algorithms.

crypto.getCiphers()#

Added in: v0.9.3

Returns an array with the names of the supported cipher algorithms.

Example:

const ciphers = crypto.getCiphers();
console.log(ciphers); // ['aes-128-cbc', 'aes-128-ccm', ...]

crypto.getCurves()#

Added in: v2.3.0

Returns an array with the names of the supported elliptic curves.

Example:

const curves = crypto.getCurves();
console.log(curves); // ['Oakley-EC2N-3', 'Oakley-EC2N-4', ...]

crypto.getDiffieHellman(group_name)#

Added in: v0.7.5

Creates a predefined DiffieHellman key exchange object. The supported groups are: 'modp1', 'modp2', 'modp5' (defined in RFC 2412, but see Caveats) and 'modp14', 'modp15', 'modp16', 'modp17', 'modp18' (defined in RFC 3526). The returned object mimics the interface of objects created by crypto.createDiffieHellman(), but will not allow changing the keys (with diffieHellman.setPublicKey() for example). The advantage of using this method is that the parties do not have to generate nor exchange a group modulus beforehand, saving both processor and communication time.

Example (obtaining a shared secret):

const crypto = require('crypto');
const alice = crypto.getDiffieHellman('modp14');
const bob = crypto.getDiffieHellman('modp14');

alice.generateKeys();
bob.generateKeys();

const aliceSecret = alice.computeSecret(bob.getPublicKey(), null, 'hex');
const bobSecret = bob.computeSecret(alice.getPublicKey(), null, 'hex');

/* aliceSecret and bobSecret should be the same */
console.log(aliceSecret === bobSecret);

crypto.getHashes()#

Added in: v0.9.3

Returns an array of the names of the supported hash algorithms, such as RSA-SHA256.

Example:

const hashes = crypto.getHashes();
console.log(hashes); // ['DSA', 'DSA-SHA', 'DSA-SHA1', ...]

crypto.pbkdf2(password, salt, iterations, keylen, digest, callback)#

Added in: v0.5.5

Provides an asynchronous Password-Based Key Derivation Function 2 (PBKDF2) implementation. A selected HMAC digest algorithm specified by digest is applied to derive a key of the requested byte length (keylen) from the password, salt and iterations.

The supplied callback function is called with two arguments: err and derivedKey. If an error occurs, err will be set; otherwise err will be null. The successfully generated derivedKey will be passed as a Buffer.

The iterations argument must be a number set as high as possible. The higher the number of iterations, the more secure the derived key will be, but will take a longer amount of time to complete.

The salt should also be as unique as possible. It is recommended that the salts are random and their lengths are at least 16 bytes. See NIST SP 800-132 for details.

Example:

const crypto = require('crypto');
crypto.pbkdf2('secret', 'salt', 100000, 512, 'sha512', (err, key) => {
  if (err) throw err;
  console.log(key.toString('hex'));  // '3745e48...aa39b34'
});

An array of supported digest functions can be retrieved using crypto.getHashes().

crypto.pbkdf2Sync(password, salt, iterations, keylen, digest)#

Added in: v0.9.3

Provides a synchronous Password-Based Key Derivation Function 2 (PBKDF2) implementation. A selected HMAC digest algorithm specified by digest is applied to derive a key of the requested byte length (keylen) from the password, salt and iterations.

If an error occurs an Error will be thrown, otherwise the derived key will be returned as a Buffer.

The iterations argument must be a number set as high as possible. The higher the number of iterations, the more secure the derived key will be, but will take a longer amount of time to complete.

The salt should also be as unique as possible. It is recommended that the salts are random and their lengths are at least 16 bytes. See NIST SP 800-132 for details.

Example:

const crypto = require('crypto');
const key = crypto.pbkdf2Sync('secret', 'salt', 100000, 512, 'sha512');
console.log(key.toString('hex'));  // '3745e48...aa39b34'

An array of supported digest functions can be retrieved using crypto.getHashes().

crypto.privateDecrypt(private_key, buffer)#

Added in: v0.11.14

Decrypts buffer with private_key.

private_key can be an object or a string. If private_key is a string, it is treated as the key with no passphrase and will use RSA_PKCS1_OAEP_PADDING. If private_key is an object, it is interpreted as a hash object with the keys:

  • key: <string> - PEM encoded private key
  • passphrase: <string> - Optional passphrase for the private key
  • padding : An optional padding value, one of the following:
    • crypto.constants.RSA_NO_PADDING
    • crypto.constants.RSA_PKCS1_PADDING
    • crypto.constants.RSA_PKCS1_OAEP_PADDING

All paddings are defined in crypto.constants.

crypto.timingSafeEqual(a, b)#

Added in: v6.6.0

This function is based on a constant-time algorithm. Returns true if a is equal to b, without leaking timing information that would allow an attacker to guess one of the values. This is suitable for comparing HMAC digests or secret values like authentication cookies or capability urls.

a and b must both be Buffers, and they must have the same length.

Note: Use of crypto.timingSafeEqual does not guarantee that the surrounding code is timing-safe. Care should be taken to ensure that the surrounding code does not introduce timing vulnerabilities.

crypto.privateEncrypt(private_key, buffer)#

Added in: v1.1.0

Encrypts buffer with private_key.

private_key can be an object or a string. If private_key is a string, it is treated as the key with no passphrase and will use RSA_PKCS1_PADDING. If private_key is an object, it is interpreted as a hash object with the keys:

  • key: <string> - PEM encoded private key
  • passphrase: <string> - Optional passphrase for the private key
  • padding : An optional padding value, one of the following:
    • crypto.constants.RSA_NO_PADDING
    • crypto.constants.RSA_PKCS1_PADDING

All paddings are defined in crypto.constants.

crypto.publicDecrypt(public_key, buffer)#

Added in: v1.1.0

Decrypts buffer with public_key.

public_key can be an object or a string. If public_key is a string, it is treated as the key with no passphrase and will use RSA_PKCS1_PADDING. If public_key is an object, it is interpreted as a hash object with the keys:

  • key: <string> - PEM encoded public key
  • passphrase: <string> - Optional passphrase for the private key
  • padding : An optional padding value, one of the following:
    • crypto.constants.RSA_NO_PADDING
    • crypto.constants.RSA_PKCS1_PADDING
    • crypto.constants.RSA_PKCS1_OAEP_PADDING

Because RSA public keys can be derived from private keys, a private key may be passed instead of a public key.

All paddings are defined in crypto.constants.

crypto.publicEncrypt(public_key, buffer)#

Added in: v0.11.14

Encrypts buffer with public_key.

public_key can be an object or a string. If public_key is a string, it is treated as the key with no passphrase and will use RSA_PKCS1_OAEP_PADDING. If public_key is an object, it is interpreted as a hash object with the keys:

  • key: <string> - PEM encoded public key
  • passphrase: <string> - Optional passphrase for the private key
  • padding : An optional padding value, one of the following:
    • crypto.constants.RSA_NO_PADDING
    • crypto.constants.RSA_PKCS1_PADDING
    • crypto.constants.RSA_PKCS1_OAEP_PADDING

Because RSA public keys can be derived from private keys, a private key may be passed instead of a public key.

All paddings are defined in crypto.constants.

crypto.randomBytes(size[, callback])#

Added in: v0.5.8

Generates cryptographically strong pseudo-random data. The size argument is a number indicating the number of bytes to generate.

If a callback function is provided, the bytes are generated asynchronously and the callback function is invoked with two arguments: err and buf. If an error occurs, err will be an Error object; otherwise it is null. The buf argument is a Buffer containing the generated bytes.

// Asynchronous
const crypto = require('crypto');
crypto.randomBytes(256, (err, buf) => {
  if (err) throw err;
  console.log(`${buf.length} bytes of random data: ${buf.toString('hex')}`);
});

If the callback function is not provided, the random bytes are generated synchronously and returned as a Buffer. An error will be thrown if there is a problem generating the bytes.

// Synchronous
const buf = crypto.randomBytes(256);
console.log(
  `${buf.length} bytes of random data: ${buf.toString('hex')}`);

The crypto.randomBytes() method will not complete until there is sufficient entropy available. This should normally never take longer than a few milliseconds. The only time when generating the random bytes may conceivably block for a longer period of time is right after boot, when the whole system is still low on entropy.

crypto.randomFillSync(buffer[, offset][, size])#

Added in: REPLACEME

Synchronous version of crypto.randomFill().

Returns buffer

const buf = Buffer.alloc(10);
console.log(crypto.randomFillSync(buf).toString('hex'));

crypto.randomFillSync(buf, 5);
console.log(buf.toString('hex'));

// The above is equivalent to the following:
crypto.randomFillSync(buf, 5, 5);
console.log(buf.toString('hex'));

crypto.randomFill(buffer[, offset][, size], callback)#

Added in: REPLACEME

This function is similar to crypto.randomBytes() but requires the first argument to be a Buffer that will be filled. It also requires that a callback is passed in.

If the callback function is not provided, an error will be thrown.

const buf = Buffer.alloc(10);
crypto.randomFill(buf, (err, buf) => {
  if (err) throw err;
  console.log(buf.toString('hex'));
});

crypto.randomFill(buf, 5, (err, buf) => {
  if (err) throw err;
  console.log(buf.toString('hex'));
});

// The above is equivalent to the following:
crypto.randomFill(buf, 5, 5, (err, buf) => {
  if (err) throw err;
  console.log(buf.toString('hex'));
});

crypto.setEngine(engine[, flags])#

Added in: v0.11.11

Load and set the engine for some or all OpenSSL functions (selected by flags).

engine could be either an id or a path to the engine's shared library.

The optional flags argument uses ENGINE_METHOD_ALL by default. The flags is a bit field taking one of or a mix of the following flags (defined in crypto.constants):

  • crypto.constants.ENGINE_METHOD_RSA
  • crypto.constants.ENGINE_METHOD_DSA
  • crypto.constants.ENGINE_METHOD_DH
  • crypto.constants.ENGINE_METHOD_RAND
  • crypto.constants.ENGINE_METHOD_ECDH
  • crypto.constants.ENGINE_METHOD_ECDSA
  • crypto.constants.ENGINE_METHOD_CIPHERS
  • crypto.constants.ENGINE_METHOD_DIGESTS
  • crypto.constants.ENGINE_METHOD_STORE
  • crypto.constants.ENGINE_METHOD_PKEY_METHS
  • crypto.constants.ENGINE_METHOD_PKEY_ASN1_METHS
  • crypto.constants.ENGINE_METHOD_ALL
  • crypto.constants.ENGINE_METHOD_NONE

Notes#

Legacy Streams API (pre Node.js v0.10)#

The Crypto module was added to Node.js before there was the concept of a unified Stream API, and before there were Buffer objects for handling binary data. As such, the many of the crypto defined classes have methods not typically found on other Node.js classes that implement the streams API (e.g. update(), final(), or digest()). Also, many methods accepted and returned 'latin1' encoded strings by default rather than Buffers. This default was changed after Node.js v0.8 to use Buffer objects by default instead.

Recent ECDH Changes#

Usage of ECDH with non-dynamically generated key pairs has been simplified. Now, ecdh.setPrivateKey() can be called with a preselected private key and the associated public point (key) will be computed and stored in the object. This allows code to only store and provide the private part of the EC key pair. ecdh.setPrivateKey() now also validates that the private key is valid for the selected curve.

The ecdh.setPublicKey() method is now deprecated as its inclusion in the API is not useful. Either a previously stored private key should be set, which automatically generates the associated public key, or ecdh.generateKeys() should be called. The main drawback of using ecdh.setPublicKey() is that it can be used to put the ECDH key pair into an inconsistent state.

Support for weak or compromised algorithms#

The crypto module still supports some algorithms which are already compromised and are not currently recommended for use. The API also allows the use of ciphers and hashes with a small key size that are considered to be too weak for safe use.

Users should take full responsibility for selecting the crypto algorithm and key size according to their security requirements.

Based on the recommendations of NIST SP 800-131A:

  • MD5 and SHA-1 are no longer acceptable where collision resistance is required such as digital signatures.
  • The key used with RSA, DSA and DH algorithms is recommended to have at least 2048 bits and that of the curve of ECDSA and ECDH at least 224 bits, to be safe to use for several years.
  • The DH groups of modp1, modp2 and modp5 have a key size smaller than 2048 bits and are not recommended.

See the reference for other recommendations and details.

Crypto Constants#

The following constants exported by crypto.constants apply to various uses of the crypto, tls, and https modules and are generally specific to OpenSSL.

OpenSSL Options#

Constant Description
SSL_OP_ALL Applies multiple bug workarounds within OpenSSL. See https://www.openssl.org/docs/man1.0.2/ssl/SSL_CTX_set_options.html for detail.
SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION Allows legacy insecure renegotiation between OpenSSL and unpatched clients or servers. See https://www.openssl.org/docs/man1.0.2/ssl/SSL_CTX_set_options.html.
SSL_OP_CIPHER_SERVER_PREFERENCE Attempts to use the server's preferences instead of the client's when selecting a cipher. Behavior depends on protocol version. See https://www.openssl.org/docs/man1.0.2/ssl/SSL_CTX_set_options.html.
SSL_OP_CISCO_ANYCONNECT Instructs OpenSSL to use Cisco's "speshul" version of DTLS_BAD_VER.
SSL_OP_COOKIE_EXCHANGE Instructs OpenSSL to turn on cookie exchange.
SSL_OP_CRYPTOPRO_TLSEXT_BUG Instructs OpenSSL to add server-hello extension from an early version of the cryptopro draft.
SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS Instructs OpenSSL to disable a SSL 3.0/TLS 1.0 vulnerability workaround added in OpenSSL 0.9.6d.
SSL_OP_EPHEMERAL_RSA Instructs OpenSSL to always use the tmp_rsa key when performing RSA operations.
SSL_OP_LEGACY_SERVER_CONNECT Allows initial connection to servers that do not support RI.
SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
SSL_OP_MICROSOFT_SESS_ID_BUG
SSL_OP_MSIE_SSLV2_RSA_PADDING Instructs OpenSSL to disable the workaround for a man-in-the-middle protocol-version vulnerability in the SSL 2.0 server implementation.
SSL_OP_NETSCAPE_CA_DN_BUG
SSL_OP_NETSCAPE_CHALLENGE_BUG
SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG
SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG
SSL_OP_NO_COMPRESSION Instructs OpenSSL to disable support for SSL/TLS compression.
SSL_OP_NO_QUERY_MTU
SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION Instructs OpenSSL to always start a new session when performing renegotiation.
SSL_OP_NO_SSLv2 Instructs OpenSSL to turn off SSL v2
SSL_OP_NO_SSLv3 Instructs OpenSSL to turn off SSL v3
SSL_OP_NO_TICKET Instructs OpenSSL to disable use of RFC4507bis tickets.
SSL_OP_NO_TLSv1 Instructs OpenSSL to turn off TLS v1
SSL_OP_NO_TLSv1_1 Instructs OpenSSL to turn off TLS v1.1
SSL_OP_NO_TLSv1_2 Instructs OpenSSL to turn off TLS v1.2
SSL_OP_PKCS1_CHECK_1
SSL_OP_PKCS1_CHECK_2
SSL_OP_SINGLE_DH_USE Instructs OpenSSL to always create a new key when using temporary/ephemeral DH parameters.
SSL_OP_SINGLE_ECDH_USE Instructs OpenSSL to always create a new key when using temporary/ephemeral ECDH parameters.
SSL_OP_SSLEAY_080_CLIENT_DH_BUG
SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG
SSL_OP_TLS_BLOCK_PADDING_BUG
SSL_OP_TLS_D5_BUG
SSL_OP_TLS_ROLLBACK_BUG Instructs OpenSSL to disable version rollback attack detection.

OpenSSL Engine Constants#

Constant Description
ENGINE_METHOD_RSA Limit engine usage to RSA
ENGINE_METHOD_DSA Limit engine usage to DSA
ENGINE_METHOD_DH Limit engine usage to DH
ENGINE_METHOD_RAND Limit engine usage to RAND
ENGINE_METHOD_ECDH Limit engine usage to ECDH
ENGINE_METHOD_ECDSA Limit engine usage to ECDSA
ENGINE_METHOD_CIPHERS Limit engine usage to CIPHERS
ENGINE_METHOD_DIGESTS Limit engine usage to DIGESTS
ENGINE_METHOD_STORE Limit engine usage to STORE
ENGINE_METHOD_PKEY_METHS Limit engine usage to PKEY_METHDS
ENGINE_METHOD_PKEY_ASN1_METHS Limit engine usage to PKEY_ASN1_METHS
ENGINE_METHOD_ALL
ENGINE_METHOD_NONE

Other OpenSSL Constants#

Constant Description
DH_CHECK_P_NOT_SAFE_PRIME
DH_CHECK_P_NOT_PRIME
DH_UNABLE_TO_CHECK_GENERATOR
DH_NOT_SUITABLE_GENERATOR
NPN_ENABLED
ALPN_ENABLED
RSA_PKCS1_PADDING
RSA_SSLV23_PADDING
RSA_NO_PADDING
RSA_PKCS1_OAEP_PADDING
RSA_X931_PADDING
RSA_PKCS1_PSS_PADDING
RSA_PSS_SALTLEN_DIGEST Sets the salt length for RSA_PKCS1_PSS_PADDING to the digest size when signing or verifying.
RSA_PSS_SALTLEN_MAX_SIGN Sets the salt length for RSA_PKCS1_PSS_PADDING to the maximum permissible value when signing data.
RSA_PSS_SALTLEN_AUTO Causes the salt length for RSA_PKCS1_PSS_PADDING to be determined automatically when verifying a signature.
POINT_CONVERSION_COMPRESSED
POINT_CONVERSION_UNCOMPRESSED
POINT_CONVERSION_HYBRID

Node.js Crypto Constants#

Constant Description
defaultCoreCipherList Specifies the built-in default cipher list used by Node.js.
defaultCipherList Specifies the active default cipher list used by the current Node.js process.

Debugger#

Stability: 2 - Stable

Node.js includes an out-of-process debugging utility accessible via a TCP-based protocol and built-in debugging client. To use it, start Node.js with the debug argument followed by the path to the script to debug; a prompt will be displayed indicating successful launch of the debugger:

$ node debug myscript.js
< Debugger listening on [::]:5858
connecting to 127.0.0.1:5858 ... ok
break in /home/indutny/Code/git/indutny/myscript.js:1
> 1 global.x = 5;
  2 setTimeout(() => {
  3   debugger;
debug>

Node.js's debugger client is not a full-featured debugger, but simple step and inspection are possible.

Inserting the statement debugger; into the source code of a script will enable a breakpoint at that position in the code:

// myscript.js
global.x = 5;
setTimeout(() => {
  debugger;
  console.log('world');
}, 1000);
console.log('hello');

Once the debugger is run, a breakpoint will occur at line 3:

$ node debug myscript.js
< Debugger listening on [::]:5858
connecting to 127.0.0.1:5858 ... ok
break in /home/indutny/Code/git/indutny/myscript.js:1
> 1 global.x = 5;
  2 setTimeout(() => {
  3   debugger;
debug> cont
< hello
break in /home/indutny/Code/git/indutny/myscript.js:3
  1 global.x = 5;
  2 setTimeout(() => {
> 3   debugger;
  4   console.log('world');
  5 }, 1000);
debug> next
break in /home/indutny/Code/git/indutny/myscript.js:4
  2 setTimeout(() => {
  3   debugger;
> 4   console.log('world');
  5 }, 1000);
  6 console.log('hello');
debug> repl
Press Ctrl + C to leave debug repl
> x
5
> 2+2
4
debug> next
break in /home/indutny/Code/git/indutny/myscript.js:5
< world
  3   debugger;
  4   console.log('world');
> 5 }, 1000);
  6 console.log('hello');
  7
debug> quit

The repl command allows code to be evaluated remotely. The next command steps to the next line. Type help to see what other commands are available.

Pressing enter without typing a command will repeat the previous debugger command.

Watchers#

It is possible to watch expression and variable values while debugging. On every breakpoint, each expression from the watchers list will be evaluated in the current context and displayed immediately before the breakpoint's source code listing.

To begin watching an expression, type watch('my_expression'). The command watchers will print the active watchers. To remove a watcher, type unwatch('my_expression').

Command reference#

Stepping#

  • cont, c - Continue execution
  • next, n - Step next
  • step, s - Step in
  • out, o - Step out
  • pause - Pause running code (like pause button in Developer Tools)

Breakpoints#

  • setBreakpoint(), sb() - Set breakpoint on current line
  • setBreakpoint(line), sb(line) - Set breakpoint on specific line
  • setBreakpoint('fn()'), sb(...) - Set breakpoint on a first statement in functions body
  • setBreakpoint('script.js', 1), sb(...) - Set breakpoint on first line of script.js
  • clearBreakpoint('script.js', 1), cb(...) - Clear breakpoint in script.js on line 1

It is also possible to set a breakpoint in a file (module) that is not loaded yet:

$ node debug test/fixtures/break-in-module/main.js
< Debugger listening on [::]:5858
connecting to 127.0.0.1:5858 ... ok
break in test/fixtures/break-in-module/main.js:1
> 1 var mod = require('./mod.js');
  2 mod.hello();
  3 mod.hello();
debug> setBreakpoint('mod.js', 2)
Warning: script 'mod.js' was not loaded yet.
> 1 var mod = require('./mod.js');
  2 mod.hello();
  3 mod.hello();
  4 debugger;
  5
  6 });
debug> c
break in test/fixtures/break-in-module/mod.js:2
  1 exports.hello = function() {
> 2   return 'hello from module';
  3 };
  4
debug>

Information#

  • backtrace, bt - Print backtrace of current execution frame
  • list(5) - List scripts source code with 5 line context (5 lines before and after)
  • watch(expr) - Add expression to watch list
  • unwatch(expr) - Remove expression from watch list
  • watchers - List all watchers and their values (automatically listed on each breakpoint)
  • repl - Open debugger's repl for evaluation in debugging script's context
  • exec expr - Execute an expression in debugging script's context

Execution control#

  • run - Run script (automatically runs on debugger's start)
  • restart - Restart script
  • kill - Kill script

Various#

  • scripts - List all loaded scripts
  • version - Display V8's version

Advanced Usage#

An alternative way of enabling and accessing the debugger is to start Node.js with the --debug command-line flag or by signaling an existing Node.js process with SIGUSR1.

Once a process has been set in debug mode this way, it can be inspected using the Node.js debugger by either connecting to the pid of the running process or via URI reference to the listening debugger:

  • node debug -p <pid> - Connects to the process via the pid
  • node debug <URI> - Connects to the process via the URI such as localhost:5858

V8 Inspector Integration for Node.js#

NOTE: This is an experimental feature.

V8 Inspector integration allows attaching Chrome DevTools to Node.js instances for debugging and profiling.

V8 Inspector can be enabled by passing the --inspect flag when starting a Node.js application. It is also possible to supply a custom port with that flag, e.g. --inspect=9222 will accept DevTools connections on port 9222.

To break on the first line of the application code, provide the --debug-brk flag in addition to --inspect.

$ node --inspect index.js
Debugger listening on port 9229.
Warning: This is an experimental feature and could change at any time.
To start debugging, open the following URL in Chrome:
    chrome-devtools://devtools/remote/serve_file/@60cd6e859b9f557d2312f5bf532f6aec5f284980/inspector.html?experiments=true&v8only=true&ws=127.0.0.1:9229/3a6d0a9e-0707-48f8-a7c6-48f157b67ab5

(In the example above, the UUID 3a6d0a9e-0707-48f8-a7c6-48f157b67ab5 at the end of the URL is generated on the fly, it varies in different debugging sessions.)

UDP / Datagram Sockets#

Stability: 2 - Stable

The dgram module provides an implementation of UDP Datagram sockets.

const dgram = require('dgram');
const server = dgram.createSocket('udp4');

server.on('error', (err) => {
  console.log(`server error:\n${err.stack}`);
  server.close();
});

server.on('message', (msg, rinfo) => {
  console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
});

server.on('listening', () => {
  const address = server.address();
  console.log(`server listening ${address.address}:${address.port}`);
});

server.bind(41234);
// server listening 0.0.0.0:41234

Class: dgram.Socket#

Added in: v0.1.99

The dgram.Socket object is an EventEmitter that encapsulates the datagram functionality.

New instances of dgram.Socket are created using dgram.createSocket(). The new keyword is not to be used to create dgram.Socket instances.

Event: 'close'#

Added in: v0.1.99

The 'close' event is emitted after a socket is closed with close(). Once triggered, no new 'message' events will be emitted on this socket.

Event: 'error'#

Added in: v0.1.99

The 'error' event is emitted whenever any error occurs. The event handler function is passed a single Error object.

Event: 'listening'#

Added in: v0.1.99

The 'listening' event is emitted whenever a socket begins listening for datagram messages. This occurs as soon as UDP sockets are created.

Event: 'message'#

Added in: v0.1.99

The 'message' event is emitted when a new datagram is available on a socket. The event handler function is passed two arguments: msg and rinfo.

socket.addMembership(multicastAddress[, multicastInterface])#

Added in: v0.6.9

Tells the kernel to join a multicast group at the given multicastAddress and multicastInterface using the IP_ADD_MEMBERSHIP socket option. If the multicastInterface argument is not specified, the operating system will choose one interface and will add membership to it. To add membership to every available interface, call addMembership multiple times, once per interface.

socket.address()#

Added in: v0.1.99

Returns an object containing the address information for a socket. For UDP sockets, this object will contain address, family and port properties.

socket.bind([port][, address][, callback])#

Added in: v0.1.99

For UDP sockets, causes the dgram.Socket to listen for datagram messages on a named port and optional address. If port is not specified or is 0, the operating system will attempt to bind to a random port. If address is not specified, the operating system will attempt to listen on all addresses. Once binding is complete, a 'listening' event is emitted and the optional callback function is called.

Note that specifying both a 'listening' event listener and passing a callback to the socket.bind() method is not harmful but not very useful.

A bound datagram socket keeps the Node.js process running to receive datagram messages.

If binding fails, an 'error' event is generated. In rare case (e.g. attempting to bind with a closed socket), an Error may be thrown.

Example of a UDP server listening on port 41234:

const dgram = require('dgram');
const server = dgram.createSocket('udp4');

server.on('error', (err) => {
  console.log(`server error:\n${err.stack}`);
  server.close();
});

server.on('message', (msg, rinfo) => {
  console.log(`server got: ${msg} from ${rinfo.address}:${rinfo.port}`);
});

server.on('listening', () => {
  const address = server.address();
  console.log(`server listening ${address.address}:${address.port}`);
});

server.bind(41234);
// server listening 0.0.0.0:41234

socket.bind(options[, callback])#

Added in: v0.11.14

For UDP sockets, causes the dgram.Socket to listen for datagram messages on a named port and optional address that are passed as properties of an options object passed as the first argument. If port is not specified or is 0, the operating system will attempt to bind to a random port. If address is not specified, the operating system will attempt to listen on all addresses. Once binding is complete, a 'listening' event is emitted and the optional callback function is called.

Note that specifying both a 'listening' event listener and passing a callback to the socket.bind() method is not harmful but not very useful.

The options object may contain an additional exclusive property that is use when using dgram.Socket objects with the cluster module. When exclusive is set to false (the default), cluster workers will use the same underlying socket handle allowing connection handling duties to be shared. When exclusive is true, however, the handle is not shared and attempted port sharing results in an error.

A bound datagram socket keeps the Node.js process running to receive datagram messages.

If binding fails, an 'error' event is generated. In rare case (e.g. attempting to bind with a closed socket), an Error may be thrown.

An example socket listening on an exclusive port is shown below.

socket.bind({
  address: 'localhost',
  port: 8000,
  exclusive: true
});

socket.close([callback])#

Added in: v0.1.99

Close the underlying socket and stop listening for data on it. If a callback is provided, it is added as a listener for the 'close' event.

socket.dropMembership(multicastAddress[, multicastInterface])#

Added in: v0.6.9

Instructs the kernel to leave a multicast group at multicastAddress using the IP_DROP_MEMBERSHIP socket option. This method is automatically called by the kernel when the socket is closed or the process terminates, so most apps will never have reason to call this.

If multicastInterface is not specified, the operating system will attempt to drop membership on all valid interfaces.

socket.send(msg, [offset, length,] port, address[, callback])#

Added in: v0.1.99
  • msg <Buffer> | <string> | <array> Message to be sent.
  • offset <number> Integer. Offset in the buffer where the message starts.
  • length <number> Integer. Number of bytes in the message.
  • port <number> Integer. Destination port.
  • address <string> Destination hostname or IP address.
  • callback <Function> Called when the message has been sent.

Broadcasts a datagram on the socket. The destination port and address must be specified.

The msg argument contains the message to be sent. Depending on its type, different behavior can apply. If msg is a Buffer, the offset and length specify the offset within the Buffer where the message begins and the number of bytes in the message, respectively. If msg is a String, then it is automatically converted to a Buffer with 'utf8' encoding. With messages that contain multi-byte characters, offset and length will be calculated with respect to byte length and not the character position. If msg is an array, offset and length must not be specified.

The address argument is a string. If the value of address is a host name, DNS will be used to resolve the address of the host. If the address is not specified or is an empty string, '127.0.0.1' or '::1' will be used instead.

If the socket has not been previously bound with a call to bind, the socket is assigned a random port number and is bound to the "all interfaces" address ('0.0.0.0' for udp4 sockets, '::0' for udp6 sockets.)

An optional callback function may be specified to as a way of reporting DNS errors or for determining when it is safe to reuse the buf object. Note that DNS lookups delay the time to send for at least one tick of the Node.js event loop.

The only way to know for sure that the datagram has been sent is by using a callback. If an error occurs and a callback is given, the error will be passed as the first argument to the callback. If a callback is not given, the error is emitted as an 'error' event on the socket object.

Offset and length are optional, but if you specify one you would need to specify the other. Also, they are supported only when the first argument is a Buffer.

Example of sending a UDP packet to a random port on localhost;

const dgram = require('dgram');
const message = Buffer.from('Some bytes');
const client = dgram.createSocket('udp4');
client.send(message, 41234, 'localhost', (err) => {
  client.close();
});

Example of sending a UDP packet composed of multiple buffers to a random port on localhost;

const dgram = require('dgram');
const buf1 = Buffer.from('Some ');
const buf2 = Buffer.from('bytes');
const client = dgram.createSocket('udp4');
client.send([buf1, buf2], 41234, 'localhost', (err) => {
  client.close();
});

Sending multiple buffers might be faster or slower depending on your application and operating system: benchmark it. Usually it is faster.

A Note about UDP datagram size

The maximum size of an IPv4/v6 datagram depends on the MTU (Maximum Transmission Unit) and on the Payload Length field size.

  • The Payload Length field is 16 bits wide, which means that a normal payload exceed 64K octets including the internet header and data (65,507 bytes = 65,535 − 8 bytes UDP header − 20 bytes IP header); this is generally true for loopback interfaces, but such long datagram messages are impractical for most hosts and networks.

  • The MTU is the largest size a given link layer technology can support for datagram messages. For any link, IPv4 mandates a minimum MTU of 68 octets, while the recommended MTU for IPv4 is 576 (typically recommended as the MTU for dial-up type applications), whether they arrive whole or in fragments.

    For IPv6, the minimum MTU is 1280 octets, however, the mandatory minimum fragment reassembly buffer size is 1500 octets. The value of 68 octets is very small, since most current link layer technologies, like Ethernet, have a minimum MTU of 1500.

It is impossible to know in advance the MTU of each link through which a packet might travel. Sending a datagram greater than the receiver MTU will not work because the packet will get silently dropped without informing the source that the data did not reach its intended recipient.

socket.setBroadcast(flag)#

Added in: v0.6.9

Sets or clears the SO_BROADCAST socket option. When set to true, UDP packets may be sent to a local interface's broadcast address.

socket.setMulticastInterface(multicastInterface)#

Added in: REPLACEME

Note: All references to scope in this section are refering to IPv6 Zone Indices, which are defined by RFC 4007. In string form, an IP with a scope index is written as 'IP%scope' where scope is an interface name or interface number.

Sets the default outgoing multicast interface of the socket to a chosen interface or back to system interface selection. The multicastInterface must be a valid string representation of an IP from the socket's family.

For IPv4 sockets, this should be the IP configured for the desired physical interface. All packets sent to multicast on the socket will be sent on the interface determined by the most recent successful use of this call.

For IPv6 sockets, multicastInterface should include a scope to indicate the interface as in the examples that follow. In IPv6, individual send calls can also use explicit scope in addresses, so only packets sent to a multicast address without specifying an explicit scope are affected by the most recent successful use of this call.

Examples: IPv6 Outgoing Multicast Interface#

On most systems, where scope format uses the interface name:

const socket = dgram.createSocket('udp6');

socket.bind(1234, () => {
  socket.setMulticastInterface('::%eth1');
});

On Windows, where scope format uses an interface number:

const socket = dgram.createSocket('udp6');

socket.bind(1234, () => {
  socket.setMulticastInterface('::%2');
});

Example: IPv4 Outgoing Multicast Interface#

All systems use an IP of the host on the desired physical interface:

const socket = dgram.createSocket('udp4');

socket.bind(1234, () => {
  socket.setMulticastInterface('10.0.0.2');
});

Call Results#

A call on a socket that is not ready to send or no longer open may throw a Not running Error.

If multicastInterface can not be parsed into an IP then an EINVAL System Error is thrown.

On IPv4, if multicastInterface is a valid address but does not match any interface, or if the address does not match the family then a System Error such as EADDRNOTAVAIL or EPROTONOSUP is thrown.

On IPv6, most errors with specifying or omiting scope will result in the socket continuing to use (or returning to) the system's default interface selection.

A socket's address family's ANY address (IPv4 '0.0.0.0' or IPv6 '::') can be used to return control of the sockets default outgoing interface to the system for future multicast packets.

socket.setMulticastLoopback(flag)#

Added in: v0.3.8

Sets or clears the IP_MULTICAST_LOOP socket option. When set to true, multicast packets will also be received on the local interface.

socket.setMulticastTTL(ttl)#

Added in: v0.3.8

Sets the IP_MULTICAST_TTL socket option. While TTL generally stands for "Time to Live", in this context it specifies the number of IP hops that a packet is allowed to travel through, specifically for multicast traffic. Each router or gateway that forwards a packet decrements the TTL. If the TTL is decremented to 0 by a router, it will not be forwarded.

The argument passed to to socket.setMulticastTTL() is a number of hops between 0 and 255. The default on most systems is 1 but can vary.

socket.setTTL(ttl)#

Added in: v0.1.101

Sets the IP_TTL socket option. While TTL generally stands for "Time to Live", in this context it specifies the number of IP hops that a packet is allowed to travel through. Each router or gateway that forwards a packet decrements the TTL. If the TTL is decremented to 0 by a router, it will not be forwarded. Changing TTL values is typically done for network probes or when multicasting.

The argument to socket.setTTL() is a number of hops between 1 and 255. The default on most systems is 64 but can vary.

socket.ref()#

Added in: v0.9.1

By default, binding a socket will cause it to block the Node.js process from exiting as long as the socket is open. The socket.unref() method can be used to exclude the socket from the reference counting that keeps the Node.js process active. The socket.ref() method adds the socket back to the reference counting and restores the default behavior.

Calling socket.ref() multiples times will have no additional effect.

The socket.ref() method returns a reference to the socket so calls can be chained.

socket.unref()#

Added in: v0.9.1

By default, binding a socket will cause it to block the Node.js process from exiting as long as the socket is open. The socket.unref() method can be used to exclude the socket from the reference counting that keeps the Node.js process active, allowing the process to exit even if the socket is still listening.

Calling socket.unref() multiple times will have no addition effect.

The socket.unref() method returns a reference to the socket so calls can be chained.

Change to asynchronous socket.bind() behavior#

As of Node.js v0.10, dgram.Socket#bind() changed to an asynchronous execution model. Legacy code that assumes synchronous behavior, as in the following example:

const s = dgram.createSocket('udp4');
s.bind(1234);
s.addMembership('224.0.0.114');

Must be changed to pass a callback function to the dgram.Socket#bind() function:

const s = dgram.createSocket('udp4');
s.bind(1234, () => {
  s.addMembership('224.0.0.114');
});

dgram module functions#

dgram.createSocket(options[, callback])#

Added in: v0.11.13

Creates a dgram.Socket object. The options argument is an object that should contain a type field of either udp4 or udp6 and an optional boolean reuseAddr field.

When reuseAddr is true socket.bind() will reuse the address, even if another process has already bound a socket on it. reuseAddr defaults to false. The optional callback function is added as a listener for 'message' events.

Once the socket is created, calling socket.bind() will instruct the socket to begin listening for datagram messages. When address and port are not passed to socket.bind() the method will bind the socket to the "all interfaces" address on a random port (it does the right thing for both udp4 and udp6 sockets). The bound address and port can be retrieved using socket.address().address and socket.address().port.

dgram.createSocket(type[, callback])#

Added in: v0.1.99

Creates a dgram.Socket object of the specified type. The type argument can be either udp4 or udp6. An optional callback function can be passed which is added as a listener for 'message' events.

Once the socket is created, calling socket.bind() will instruct the socket to begin listening for datagram messages. When address and port are not passed to socket.bind() the method will bind the socket to the "all interfaces" address on a random port (it does the right thing for both udp4 and udp6 sockets). The bound address and port can be retrieved using socket.address().address and socket.address().port.

DNS#

Stability: 2 - Stable

The dns module contains functions belonging to two different categories:

1) Functions that use the underlying operating system facilities to perform name resolution, and that do not necessarily perform any network communication. This category contains only one function: dns.lookup(). Developers looking to perform name resolution in the same way that other applications on the same operating system behave should use dns.lookup().

For example, looking up iana.org.

const dns = require('dns');

dns.lookup('nodejs.org', (err, addresses, family) => {
  console.log('addresses:', addresses);
});
// address: "192.0.43.8" family: IPv4

2) Functions that connect to an actual DNS server to perform name resolution, and that always use the network to perform DNS queries. This category contains all functions in the dns module except dns.lookup(). These functions do not use the same set of configuration files used by dns.lookup() (e.g. /etc/hosts). These functions should be used by developers who do not want to use the underlying operating system's facilities for name resolution, and instead want to always perform DNS queries.

Below is an example that resolves 'archive.org' then reverse resolves the IP addresses that are returned.

const dns = require('dns');

dns.resolve4('archive.org', (err, addresses) => {
  if (err) throw err;

  console.log(`addresses: ${JSON.stringify(addresses)}`);

  addresses.forEach((a) => {
    dns.reverse(a, (err, hostnames) => {
      if (err) {
        throw err;
      }
      console.log(`reverse for ${a}: ${JSON.stringify(hostnames)}`);
    });
  });
});

There are subtle consequences in choosing one over the other, please consult the Implementation considerations section for more information.

dns.getServers()#

Added in: v0.11.3

Returns an array of IP address strings that are being used for name resolution.

dns.lookup(hostname[, options], callback)#

Added in: v0.1.90

Resolves a hostname (e.g. 'nodejs.org') into the first found A (IPv4) or AAAA (IPv6) record. options can be an object or integer. If options is not provided, then IPv4 and IPv6 addresses are both valid. If options is an integer, then it must be 4 or 6.

Alternatively, options can be an object containing these properties:

  • family <number> - The record family. If present, must be the integer 4 or 6. If not provided, both IP v4 and v6 addresses are accepted.
  • hints: <number> - If present, it should be one or more of the supported getaddrinfo flags. If hints is not provided, then no flags are passed to getaddrinfo. Multiple flags can be passed through hints by bitwise ORing their values. See supported getaddrinfo flags for more information on supported flags.
  • all: <boolean> - When true, the callback returns all resolved addresses in an array, otherwise returns a single address. Defaults to false.

All properties are optional.

The callback function has arguments (err, address, family). address is a string representation of an IPv4 or IPv6 address. family is either the integer 4 or 6 and denotes the family of address (not necessarily the value initially passed to lookup).

With the all option set to true, the arguments change to (err, addresses), with addresses being an array of objects with the properties address and family.

On error, err is an Error object, where err.code is the error code. Keep in mind that err.code will be set to 'ENOENT' not only when the hostname does not exist but also when the lookup fails in other ways such as no available file descriptors.

dns.lookup() does not necessarily have anything to do with the DNS protocol. The implementation uses an operating system facility that can associate names with addresses, and vice versa. This implementation can have subtle but important consequences on the behavior of any Node.js program. Please take some time to consult the Implementation considerations section before using dns.lookup().

Example usage:

const dns = require('dns');
const options = {
  family: 6,
  hints: dns.ADDRCONFIG | dns.V4MAPPED,
};
dns.lookup('example.com', options, (err, address, family) =>
  console.log('address: %j family: IPv%s', address, family));
// address: "2606:2800:220:1:248:1893:25c8:1946" family: IPv6

// When options.all is true, the result will be an Array.
options.all = true;
dns.lookup('example.com', options, (err, addresses) =>
  console.log('addresses: %j', addresses));
// addresses: [{"address":"2606:2800:220:1:248:1893:25c8:1946","family":6}]

Supported getaddrinfo flags#

The following flags can be passed as hints to dns.lookup().

  • dns.ADDRCONFIG: Returned address types are determined by the types of addresses supported by the current system. For example, IPv4 addresses are only returned if the current system has at least one IPv4 address configured. Loopback addresses are not considered.
  • dns.V4MAPPED: If the IPv6 family was specified, but no IPv6 addresses were found, then return IPv4 mapped IPv6 addresses. Note that it is not supported on some operating systems (e.g FreeBSD 10.1).

dns.lookupService(address, port, callback)#

Added in: v0.11.14

Resolves the given address and port into a hostname and service using the operating system's underlying getnameinfo implementation.

If address is not a valid IP address, a TypeError will be thrown. The port will be coerced to a number. If it is not a legal port, a TypeError will be thrown.

The callback has arguments (err, hostname, service). The hostname and service arguments are strings (e.g. 'localhost' and 'http' respectively).

On error, err is an Error object, where err.code is the error code.

const dns = require('dns');
dns.lookupService('127.0.0.1', 22, (err, hostname, service) => {
  console.log(hostname, service);
  // Prints: localhost ssh
});

dns.resolve(hostname[, rrtype], callback)#

Added in: v0.1.27

Uses the DNS protocol to resolve a hostname (e.g. 'nodejs.org') into an array of the resource records. The callback function has arguments (err, records). When successful, records will be an array of resource records. The type and structure of individual results varies based on rrtype:

rrtype records contains Result type Shorthand method
'A' IPv4 addresses (default) <string> dns.resolve4()
'AAAA' IPv6 addresses <string> dns.resolve6()
'CNAME' canonical name records <string> dns.resolveCname()
'MX' mail exchange records <Object> dns.resolveMx()
'NAPTR' name authority pointer records <Object> dns.resolveNaptr()
'NS' name server records <string> dns.resolveNs()
'PTR' pointer records <string> dns.resolvePtr()
'SOA' start of authority records <Object> dns.resolveSoa()
'SRV' service records <Object> dns.resolveSrv()
'TXT' text records <string[]> dns.resolveTxt()

On error, err is an Error object, where err.code is one of the DNS error codes.

dns.resolve4(hostname[, options], callback)#

Added in: v0.1.16

Uses the DNS protocol to resolve a IPv4 addresses (A records) for the hostname. The addresses argument passed to the callback function will contain an array of IPv4 addresses (e.g. ['74.125.79.104', '74.125.79.105', '74.125.79.106']).

  • hostname <string> Hostname to resolve.
  • options <Object>
    • ttl <boolean> Retrieve the Time-To-Live value (TTL) of each record. The callback receives an array of { address: '1.2.3.4', ttl: 60 } objects rather than an array of strings. The TTL is expressed in seconds.
  • callback <Function> An (err, result) callback function.

dns.resolve6(hostname[, options], callback)#

Added in: v0.1.16

Uses the DNS protocol to resolve a IPv6 addresses (AAAA records) for the hostname. The addresses argument passed to the callback function will contain an array of IPv6 addresses.

  • hostname <string> Hostname to resolve.
  • options <Object>
    • ttl <boolean> Retrieve the Time-To-Live value (TTL) of each record. The callback receives an array of { address: '0:1:2:3:4:5:6:7', ttl: 60 } objects rather than an array of strings. The TTL is expressed in seconds.
  • callback <Function> An (err, result) callback function.

dns.resolveCname(hostname, callback)#

Added in: v0.3.2

Uses the DNS protocol to resolve CNAME records for the hostname. The addresses argument passed to the callback function will contain an array of canonical name records available for the hostname (e.g. ['bar.example.com']).

dns.resolveMx(hostname, callback)#

Added in: v0.1.27

Uses the DNS protocol to resolve mail exchange records (MX records) for the hostname. The addresses argument passed to the callback function will contain an array of objects containing both a priority and exchange property (e.g. [{priority: 10, exchange: 'mx.example.com'}, ...]).

dns.resolveNaptr(hostname, callback)#

Added in: v0.9.12

Uses the DNS protocol to resolve regular expression based records (NAPTR records) for the hostname. The callback function has arguments (err, addresses). The addresses argument passed to the callback function will contain an array of objects with the following properties:

  • flags
  • service
  • regexp
  • replacement
  • order
  • preference

For example:

{
  flags: 's',
  service: 'SIP+D2U',
  regexp: '',
  replacement: '_sip._udp.example.com',
  order: 30,
  preference: 100
}

dns.resolveNs(hostname, callback)#

Added in: v0.1.90

Uses the DNS protocol to resolve name server records (NS records) for the hostname. The addresses argument passed to the callback function will contain an array of name server records available for hostname (e.g. ['ns1.example.com', 'ns2.example.com']).

dns.resolveSoa(hostname, callback)#

Added in: v0.11.10

Uses the DNS protocol to resolve a start of authority record (SOA record) for the hostname. The addresses argument passed to the callback function will be an object with the following properties:

  • nsname
  • hostmaster
  • serial
  • refresh
  • retry
  • expire
  • minttl
{
  nsname: 'ns.example.com',
  hostmaster: 'root.example.com',
  serial: 2013101809,
  refresh: 10000,
  retry: 2400,
  expire: 604800,
  minttl: 3600
}

dns.resolveSrv(hostname, callback)#

Added in: v0.1.27

Uses the DNS protocol to resolve service records (SRV records) for the hostname. The addresses argument passed to the callback function will be an array of objects with the following properties:

  • priority
  • weight
  • port
  • name
{
  priority: 10,
  weight: 5,
  port: 21223,
  name: 'service.example.com'
}

dns.resolvePtr(hostname, callback)#

Added in: v6.0.0

Uses the DNS protocol to resolve pointer records (PTR records) for the hostname. The addresses argument passed to the callback function will be an array of strings containing the reply records.

dns.resolveTxt(hostname, callback)#

Added in: v0.1.27

Uses the DNS protocol to resolve text queries (TXT records) for the hostname. The records argument passed to the callback function is a two-dimensional array of the text records available for hostname (e.g., [ ['v=spf1 ip4:0.0.0.0 ', '~all' ] ]). Each sub-array contains TXT chunks of one record. Depending on the use case, these could be either joined together or treated separately.

dns.reverse(ip, callback)#

Added in: v0.1.16

Performs a reverse DNS query that resolves an IPv4 or IPv6 address to an array of hostnames.

The callback function has arguments (err, hostnames), where hostnames is an array of resolved hostnames for the given ip.

On error, err is an Error object, where err.code is one of the DNS error codes.

dns.setServers(servers)#

Added in: v0.11.3

Sets the IP addresses of the servers to be used when resolving. The servers argument is an array of IPv4 or IPv6 addresses.

If a port specified on the address it will be removed.

An error will be thrown if an invalid address is provided.

The dns.setServers() method must not be called while a DNS query is in progress.

Error codes#

Each DNS query can return one of the following error codes:

  • dns.NODATA: DNS server returned answer with no data.
  • dns.FORMERR: DNS server claims query was misformatted.
  • dns.SERVFAIL: DNS server returned general failure.
  • dns.NOTFOUND: Domain name not found.
  • dns.NOTIMP: DNS server does not implement requested operation.
  • dns.REFUSED: DNS server refused query.
  • dns.BADQUERY: Misformatted DNS query.
  • dns.BADNAME: Misformatted hostname.
  • dns.BADFAMILY: Unsupported address family.
  • dns.BADRESP: Misformatted DNS reply.
  • dns.CONNREFUSED: Could not contact DNS servers.
  • dns.TIMEOUT: Timeout while contacting DNS servers.
  • dns.EOF: End of file.
  • dns.FILE: Error reading file.
  • dns.NOMEM: Out of memory.
  • dns.DESTRUCTION: Channel is being destroyed.
  • dns.BADSTR: Misformatted string.
  • dns.BADFLAGS: Illegal flags specified.
  • dns.NONAME: Given hostname is not numeric.
  • dns.BADHINTS: Illegal hints flags specified.
  • dns.NOTINITIALIZED: c-ares library initialization not yet performed.
  • dns.LOADIPHLPAPI: Error loading iphlpapi.dll.
  • dns.ADDRGETNETWORKPARAMS: Could not find GetNetworkParams function.
  • dns.CANCELLED: DNS query cancelled.

Implementation considerations#

Although dns.lookup() and the various dns.resolve*()/dns.reverse() functions have the same goal of associating a network name with a network address (or vice versa), their behavior is quite different. These differences can have subtle but significant consequences on the behavior of Node.js programs.

dns.lookup()#

Under the hood, dns.lookup() uses the same operating system facilities as most other programs. For instance, dns.lookup() will almost always resolve a given name the same way as the ping command. On most POSIX-like operating systems, the behavior of the dns.lookup() function can be modified by changing settings in nsswitch.conf(5) and/or resolv.conf(5), but note that changing these files will change the behavior of all other programs running on the same operating system.

Though the call to dns.lookup() will be asynchronous from JavaScript's perspective, it is implemented as a synchronous call to getaddrinfo(3) that runs on libuv's threadpool. Because libuv's threadpool has a fixed size, it means that if for whatever reason the call to getaddrinfo(3) takes a long time, other operations that could run on libuv's threadpool (such as filesystem operations) will experience degraded performance. In order to mitigate this issue, one potential solution is to increase the size of libuv's threadpool by setting the 'UV_THREADPOOL_SIZE' environment variable to a value greater than 4 (its current default value). For more information on libuv's threadpool, see the official libuv documentation.

dns.resolve(), dns.resolve*() and dns.reverse()#

These functions are implemented quite differently than dns.lookup(). They do not use getaddrinfo(3) and they always perform a DNS query on the network. This network communication is always done asynchronously, and does not use libuv's threadpool.

As a result, these functions cannot have the same negative impact on other processing that happens on libuv's threadpool that dns.lookup() can have.

They do not use the same set of configuration files than what dns.lookup() uses. For instance, they do not use the configuration from /etc/hosts.

Domain#

Stability: 0 - Deprecated

This module is pending deprecation. Once a replacement API has been finalized, this module will be fully deprecated. Most end users should not have cause to use this module. Users who absolutely must have the functionality that domains provide may rely on it for the time being but should expect to have to migrate to a different solution in the future.

Domains provide a way to handle multiple different IO operations as a single group. If any of the event emitters or callbacks registered to a domain emit an 'error' event, or throw an error, then the domain object will be notified, rather than losing the context of the error in the process.on('uncaughtException') handler, or causing the program to exit immediately with an error code.

Warning: Don't Ignore Errors!#

Domain error handlers are not a substitute for closing down your process when an error occurs.

By the very nature of how throw works in JavaScript, there is almost never any way to safely "pick up where you left off", without leaking references, or creating some other sort of undefined brittle state.

The safest way to respond to a thrown error is to shut down the process. Of course, in a normal web server, you might have many connections open, and it is not reasonable to abruptly shut those down because an error was triggered by someone else.

The better approach is to send an error response to the request that triggered the error, while letting the others finish in their normal time, and stop listening for new requests in that worker.

In this way, domain usage goes hand-in-hand with the cluster module, since the master process can fork a new worker when a worker encounters an error. For Node.js programs that scale to multiple machines, the terminating proxy or service registry can take note of the failure, and react accordingly.

For example, this is not a good idea:

// XXX WARNING!  BAD IDEA!

const d = require('domain').create();
d.on('error', (er) => {
  // The error won't crash the process, but what it does is worse!
  // Though we've prevented abrupt process restarting, we are leaking
  // resources like crazy if this ever happens.
  // This is no better than process.on('uncaughtException')!
  console.log(`error, but oh well ${er.message}`);
});
d.run(() => {
  require('http').createServer((req, res) => {
    handleRequest(req, res);
  }).listen(PORT);
});

By using the context of a domain, and the resilience of separating our program into multiple worker processes, we can react more appropriately, and handle errors with much greater safety.

// Much better!

const cluster = require('cluster');
const PORT = +process.env.PORT || 1337;

if (cluster.isMaster) {
  // In real life, you'd probably use more than just 2 workers,
  // and perhaps not put the master and worker in the same file.
  //
  // You can also of course get a bit fancier about logging, and
  // implement whatever custom logic you need to prevent DoS
  // attacks and other bad behavior.
  //
  // See the options in the cluster documentation.
  //
  // The important thing is that the master does very little,
  // increasing our resilience to unexpected errors.

  cluster.fork();
  cluster.fork();

  cluster.on('disconnect', (worker) => {
    console.error('disconnect!');
    cluster.fork();
  });

} else {
  // the worker
  //
  // This is where we put our bugs!

  const domain = require('domain');

  // See the cluster documentation for more details about using
  // worker processes to serve requests.  How it works, caveats, etc.

  const server = require('http').createServer((req, res) => {
    const d = domain.create();
    d.on('error', (er) => {
      console.error(`error ${er.stack}`);

      // Note: we're in dangerous territory!
      // By definition, something unexpected occurred,
      // which we probably didn't want.
      // Anything can happen now!  Be very careful!

      try {
        // make sure we close down within 30 seconds
        const killtimer = setTimeout(() => {
          process.exit(1);
        }, 30000);
        // But don't keep the process open just for that!
        killtimer.unref();

        // stop taking new requests.
        server.close();

        // Let the master know we're dead.  This will trigger a
        // 'disconnect' in the cluster master, and then it will fork
        // a new worker.
        cluster.worker.disconnect();

        // try to send an error to the request that triggered the problem
        res.statusCode = 500;
        res.setHeader('content-type', 'text/plain');
        res.end('Oops, there was a problem!\n');
      } catch (er2) {
        // oh well, not much we can do at this point.
        console.error(`Error sending 500! ${er2.stack}`);
      }
    });

    // Because req and res were created before this domain existed,
    // we need to explicitly add them.
    // See the explanation of implicit vs explicit binding below.
    d.add(req);
    d.add(res);

    // Now run the handler function in the domain.
    d.run(() => {
      handleRequest(req, res);
    });
  });
  server.listen(PORT);
}

// This part is not important.  Just an example routing thing.
// You'd put your fancy application logic here.
function handleRequest(req, res) {
  switch (req.url) {
    case '/error':
      // We do some async stuff, and then...
      setTimeout(() => {
        // Whoops!
        flerb.bark();
      }, timeout);
      break;
    default:
      res.end('ok');
  }
}

Additions to Error objects#

Any time an Error object is routed through a domain, a few extra fields are added to it.

  • error.domain The domain that first handled the error.
  • error.domainEmitter The event emitter that emitted an 'error' event with the error object.
  • error.domainBound The callback function which was bound to the domain, and passed an error as its first argument.
  • error.domainThrown A boolean indicating whether the error was thrown, emitted, or passed to a bound callback function.

Implicit Binding#

If domains are in use, then all new EventEmitter objects (including Stream objects, requests, responses, etc.) will be implicitly bound to the active domain at the time of their creation.

Additionally, callbacks passed to lowlevel event loop requests (such as to fs.open, or other callback-taking methods) will automatically be bound to the active domain. If they throw, then the domain will catch the error.

In order to prevent excessive memory usage, Domain objects themselves are not implicitly added as children of the active domain. If they were, then it would be too easy to prevent request and response objects from being properly garbage collected.

If you want to nest Domain objects as children of a parent Domain, then you must explicitly add them.

Implicit binding routes thrown errors and 'error' events to the Domain's 'error' event, but does not register the EventEmitter on the Domain, so domain.dispose() will not shut down the EventEmitter. Implicit binding only takes care of thrown errors and 'error' events.

Explicit Binding#

Sometimes, the domain in use is not the one that ought to be used for a specific event emitter. Or, the event emitter could have been created in the context of one domain, but ought to instead be bound to some other domain.

For example, there could be one domain in use for an HTTP server, but perhaps we would like to have a separate domain to use for each request.

That is possible via explicit binding.

For example:

// create a top-level domain for the server
const domain = require('domain');
const http = require('http');
const serverDomain = domain.create();

serverDomain.run(() => {
  // server is created in the scope of serverDomain
  http.createServer((req, res) => {
    // req and res are also created in the scope of serverDomain
    // however, we'd prefer to have a separate domain for each request.
    // create it first thing, and add req and res to it.
    const reqd = domain.create();
    reqd.add(req);
    reqd.add(res);
    reqd.on('error', (er) => {
      console.error('Error', er, req.url);
      try {
        res.writeHead(500);
        res.end('Error occurred, sorry.');
      } catch (er2) {
        console.error('Error sending 500', er2, req.url);
      }
    });
  }).listen(1337);
});

domain.create()#

  • Returns: <Domain>

Returns a new Domain object.

Class: Domain#

The Domain class encapsulates the functionality of routing errors and uncaught exceptions to the active Domain object.

Domain is a child class of EventEmitter. To handle the errors that it catches, listen to its 'error' event.

domain.run(fn[, ...args])#

Run the supplied function in the context of the domain, implicitly binding all event emitters, timers, and lowlevel requests that are created in that context. Optionally, arguments can be passed to the function.

This is the most basic way to use a domain.

Example:

const domain = require('domain');
const fs = require('fs');
const d = domain.create();
d.on('error', (er) => {
  console.error('Caught error!', er);
});
d.run(() => {
  process.nextTick(() => {
    setTimeout(() => { // simulating some various async stuff
      fs.open('non-existent file', 'r', (er, fd) => {
        if (er) throw er;
        // proceed...
      });
    }, 100);
  });
});

In this example, the d.on('error') handler will be triggered, rather than crashing the program.

domain.members#

An array of timers and event emitters that have been explicitly added to the domain.

domain.add(emitter)#

Explicitly adds an emitter to the domain. If any event handlers called by the emitter throw an error, or if the emitter emits an 'error' event, it will be routed to the domain's 'error' event, just like with implicit binding.

This also works with timers that are returned from setInterval() and setTimeout(). If their callback function throws, it will be caught by the domain 'error' handler.

If the Timer or EventEmitter was already bound to a domain, it is removed from that one, and bound to this one instead.

domain.remove(emitter)#

The opposite of domain.add(emitter). Removes domain handling from the specified emitter.

domain.bind(callback)#

The returned function will be a wrapper around the supplied callback function. When the returned function is called, any errors that are thrown will be routed to the domain's 'error' event.

Example#

const d = domain.create();

function readSomeFile(filename, cb) {
  fs.readFile(filename, 'utf8', d.bind((er, data) => {
    // if this throws, it will also be passed to the domain
    return cb(er, data ? JSON.parse(data) : null);
  }));
}

d.on('error', (er) => {
  // an error occurred somewhere.
  // if we throw it now, it will crash the program
  // with the normal line number and stack message.
});

domain.intercept(callback)#

This method is almost identical to domain.bind(callback). However, in addition to catching thrown errors, it will also intercept Error objects sent as the first argument to the function.

In this way, the common if (err) return callback(err); pattern can be replaced with a single error handler in a single place.

Example#

const d = domain.create();

function readSomeFile(filename, cb) {
  fs.readFile(filename, 'utf8', d.intercept((data) => {
    // note, the first argument is never passed to the
    // callback since it is assumed to be the 'Error' argument
    // and thus intercepted by the domain.

    // if this throws, it will also be passed to the domain
    // so the error-handling logic can be moved to the 'error'
    // event on the domain instead of being repeated throughout
    // the program.
    return cb(null, JSON.parse(data));
  }));
}

d.on('error', (er) => {
  // an error occurred somewhere.
  // if we throw it now, it will crash the program
  // with the normal line number and stack message.
});

domain.enter()#

The enter method is plumbing used by the run, bind, and intercept methods to set the active domain. It sets domain.active and process.domain to the domain, and implicitly pushes the domain onto the domain stack managed by the domain module (see domain.exit() for details on the domain stack). The call to enter delimits the beginning of a chain of asynchronous calls and I/O operations bound to a domain.

Calling enter changes only the active domain, and does not alter the domain itself. enter and exit can be called an arbitrary number of times on a single domain.

If the domain on which enter is called has been disposed, enter will return without setting the domain.

domain.exit()#

The exit method exits the current domain, popping it off the domain stack. Any time execution is going to switch to the context of a different chain of asynchronous calls, it's important to ensure that the current domain is exited. The call to exit delimits either the end of or an interruption to the chain of asynchronous calls and I/O operations bound to a domain.

If there are multiple, nested domains bound to the current execution context, exit will exit any domains nested within this domain.

Calling exit changes only the active domain, and does not alter the domain itself. enter and exit can be called an arbitrary number of times on a single domain.

If the domain on which exit is called has been disposed, exit will return without exiting the domain.

domain.dispose()#

Stability: 0 - Deprecated. Please recover from failed IO actions explicitly via error event handlers set on the domain.

Once dispose has been called, the domain will no longer be used by callbacks bound into the domain via run, bind, or intercept, and a 'dispose' event is emitted.

Errors#

Applications running in Node.js will generally experience four categories of errors:

  • Standard JavaScript errors such as:
    • <EvalError> : thrown when a call to eval() fails.
    • <SyntaxError> : thrown in response to improper JavaScript language syntax.
    • <RangeError> : thrown when a value is not within an expected range
    • <ReferenceError> : thrown when using undefined variables
    • <TypeError> : thrown when passing arguments of the wrong type
    • <URIError> : thrown when a global URI handling function is misused.
  • System errors triggered by underlying operating system constraints such as attempting to open a file that does not exist, attempting to send data over a closed socket, etc;
  • And User-specified errors triggered by application code.
  • Assertion Errors are a special class of error that can be triggered whenever Node.js detects an exceptional logic violation that should never occur. These are raised typically by the assert module.

All JavaScript and System errors raised by Node.js inherit from, or are instances of, the standard JavaScript <Error> class and are guaranteed to provide at least the properties available on that class.

Error Propagation and Interception#

Node.js supports several mechanisms for propagating and handling errors that occur while an application is running. How these errors are reported and handled depends entirely on the type of Error and the style of the API that is called.

All JavaScript errors are handled as exceptions that immediately generate and throw an error using the standard JavaScript throw mechanism. These are handled using the try / catch construct provided by the JavaScript language.

// Throws with a ReferenceError because z is undefined
try {
  const m = 1;
  const n = m + z;
} catch (err) {
  // Handle the error here.
}

Any use of the JavaScript throw mechanism will raise an exception that must be handled using try / catch or the Node.js process will exit immediately.

With few exceptions, Synchronous APIs (any blocking method that does not accept a callback function, such as fs.readFileSync), will use throw to report errors.

Errors that occur within Asynchronous APIs may be reported in multiple ways:

  • Most asynchronous methods that accept a callback function will accept an Error object passed as the first argument to that function. If that first argument is not null and is an instance of Error, then an error occurred that should be handled.
  const fs = require('fs');
  fs.readFile('a file that does not exist', (err, data) => {
    if (err) {
      console.error('There was an error reading the file!', err);
      return;
    }
    // Otherwise handle the data
  });

  • When an asynchronous method is called on an object that is an EventEmitter, errors can be routed to that object's 'error' event.

    const net = require('net');
const connection = net.connect('localhost');

// Adding an 'error' event handler to a stream:
connection.on('error', (err) => {
  // If the connection is reset by the server, or if it can't
  // connect at all, or on any sort of error encountered by
  // the connection, the error will be sent here.
  console.error(err);
});

connection.pipe(process.stdout);

  • A handful of typically asynchronous methods in the Node.js API may still use the throw mechanism to raise exceptions that must be handled using try / catch. There is no comprehensive list of such methods; please refer to the documentation of each method to determine the appropriate error handling mechanism required.

The use of the 'error' event mechanism is most common for stream-based and event emitter-based APIs, which themselves represent a series of asynchronous operations over time (as opposed to a single operation that may pass or fail).

For all EventEmitter objects, if an 'error' event handler is not provided, the error will be thrown, causing the Node.js process to report an unhandled exception and crash unless either: The domain module is used appropriately or a handler has been registered for the process.on('uncaughtException') event.

const EventEmitter = require('events');
const ee = new EventEmitter();

setImmediate(() => {
  // This will crash the process because no 'error' event
  // handler has been added.
  ee.emit('error', new Error('This will crash'));
});

Errors generated in this way cannot be intercepted using try / catch as they are thrown after the calling code has already exited.

Developers must refer to the documentation for each method to determine exactly how errors raised by those methods are propagated.

Node.js style callbacks#

Most asynchronous methods exposed by the Node.js core API follow an idiomatic pattern referred to as a "Node.js style callback". With this pattern, a callback function is passed to the method as an argument. When the operation either completes or an error is raised, the callback function is called with the Error object (if any) passed as the first argument. If no error was raised, the first argument will be passed as null.

const fs = require('fs');

function nodeStyleCallback(err, data) {
  if (err) {
    console.error('There was an error', err);
    return;
  }
  console.log(data);
}

fs.readFile('/some/file/that/does-not-exist', nodeStyleCallback);
fs.readFile('/some/file/that/does-exist', nodeStyleCallback);

The JavaScript try / catch mechanism cannot be used to intercept errors generated by asynchronous APIs. A common mistake for beginners is to try to use throw inside a Node.js style callback:

// THIS WILL NOT WORK:
const fs = require('fs');

try {
  fs.readFile('/some/file/that/does-not-exist', (err, data) => {
    // mistaken assumption: throwing here...
    if (err) {
      throw err;
    }
  });
} catch (err) {
  // This will not catch the throw!
  console.error(err);
}

This will not work because the callback function passed to fs.readFile() is called asynchronously. By the time the callback has been called, the surrounding code (including the try { } catch (err) { } block will have already exited. Throwing an error inside the callback can crash the Node.js process in most cases. If domains are enabled, or a handler has been registered with process.on('uncaughtException'), such errors can be intercepted.

Class: Error#

A generic JavaScript Error object that does not denote any specific circumstance of why the error occurred. Error objects capture a "stack trace" detailing the point in the code at which the Error was instantiated, and may provide a text description of the error.

All errors generated by Node.js, including all System and JavaScript errors, will either be instances of, or inherit from, the Error class.

new Error(message)#

Creates a new Error object and sets the error.message property to the provided text message. If an object is passed as message, the text message is generated by calling message.toString(). The error.stack property will represent the point in the code at which new Error() was called. Stack traces are dependent on V8's stack trace API. Stack traces extend only to either (a) the beginning of synchronous code execution, or (b) the number of frames given by the property Error.stackTraceLimit, whichever is smaller.

Error.captureStackTrace(targetObject[, constructorOpt])#

Creates a .stack property on targetObject, which when accessed returns a string representing the location in the code at which Error.captureStackTrace() was called.

const myObject = {};
Error.captureStackTrace(myObject);
myObject.stack;  // similar to `new Error().stack`

The first line of the trace, instead of being prefixed with ErrorType: message, will be the result of calling targetObject.toString().

The optional constructorOpt argument accepts a function. If given, all frames above constructorOpt, including constructorOpt, will be omitted from the generated stack trace.

The constructorOpt argument is useful for hiding implementation details of error generation from an end user. For instance:

function MyError() {
  Error.captureStackTrace(this, MyError);
}

// Without passing MyError to captureStackTrace, the MyError
// frame would show up in the .stack property. By passing
// the constructor, we omit that frame, and retain all frames below it.
new MyError().stack;

Error.stackTraceLimit#

The Error.stackTraceLimit property specifies the number of stack frames collected by a stack trace (whether generated by new Error().stack or Error.captureStackTrace(obj)).

The default value is 10 but may be set to any valid JavaScript number. Changes will affect any stack trace captured after the value has been changed.

If set to a non-number value, or set to a negative number, stack traces will not capture any frames.

error.message#

The error.message property is the string description of the error as set by calling new Error(message). The message passed to the constructor will also appear in the first line of the stack trace of the Error, however changing this property after the Error object is created may not change the first line of the stack trace (for example, when error.stack is read before this property is changed).

const err = new Error('The message');
console.error(err.message);
// Prints: The message

error.stack#

The error.stack property is a string describing the point in the code at which the Error was instantiated.

For example:

Error: Things keep happening!
   at /home/gbusey/file.js:525:2
   at Frobnicator.refrobulate (/home/gbusey/business-logic.js:424:21)
   at Actor.<anonymous> (/home/gbusey/actors.js:400:8)
   at increaseSynergy (/home/gbusey/actors.js:701:6)

The first line is formatted as <error class name>: <error message>, and is followed by a series of stack frames (each line beginning with "at "). Each frame describes a call site within the code that lead to the error being generated. V8 attempts to display a name for each function (by variable name, function name, or object method name), but occasionally it will not be able to find a suitable name. If V8 cannot determine a name for the function, only location information will be displayed for that frame. Otherwise, the determined function name will be displayed with location information appended in parentheses.

It is important to note that frames are only generated for JavaScript functions. If, for example, execution synchronously passes through a C++ addon function called cheetahify, which itself calls a JavaScript function, the frame representing the cheetahify call will not be present in the stack traces:

const cheetahify = require('./native-binding.node');

function makeFaster() {
  // cheetahify *synchronously* calls speedy.
  cheetahify(function speedy() {
    throw new Error('oh no!');
  });
}

makeFaster(); // will throw:
// /home/gbusey/file.js:6
//     throw new Error('oh no!');
//           ^
// Error: oh no!
//     at speedy (/home/gbusey/file.js:6:11)
//     at makeFaster (/home/gbusey/file.js:5:3)
//     at Object.<anonymous> (/home/gbusey/file.js:10:1)
//     at Module._compile (module.js:456:26)
//     at Object.Module._extensions..js (module.js:474:10)
//     at Module.load (module.js:356:32)
//     at Function.Module._load (module.js:312:12)
//     at Function.Module.runMain (module.js:497:10)
//     at startup (node.js:119:16)
//     at node.js:906:3

The location information will be one of:

  • native, if the frame represents a call internal to V8 (as in [].forEach).
  • plain-filename.js:line:column, if the frame represents a call internal to Node.js.
  • /absolute/path/to/file.js:line:column, if the frame represents a call in a user program, or its dependencies.

The string representing the stack trace is lazily generated when the error.stack property is accessed.

The number of frames captured by the stack trace is bounded by the smaller of Error.stackTraceLimit or the number of available frames on the current event loop tick.

System-level errors are generated as augmented Error instances, which are detailed here.

Class: RangeError#

A subclass of Error that indicates that a provided argument was not within the set or range of acceptable values for a function; whether that is a numeric range, or outside the set of options for a given function parameter.

For example:

require('net').connect(-1);
// throws "RangeError: "port" option should be >= 0 and < 65536: -1"

Node.js will generate and throw RangeError instances immediately as a form of argument validation.

Class: ReferenceError#

A subclass of Error that indicates that an attempt is being made to access a variable that is not defined. Such errors commonly indicate typos in code, or an otherwise broken program.

While client code may generate and propagate these errors, in practice, only V8 will do so.

doesNotExist;
// throws ReferenceError, doesNotExist is not a variable in this program.

Unless an application is dynamically generating and running code, ReferenceError instances should always be considered a bug in the code or its dependencies.

Class: SyntaxError#

A subclass of Error that indicates that a program is not valid JavaScript. These errors may only be generated and propagated as a result of code evaluation. Code evaluation may happen as a result of eval, Function, require, or vm. These errors are almost always indicative of a broken program.

try {
  require('vm').runInThisContext('binary ! isNotOk');
} catch (err) {
  // err will be a SyntaxError
}

SyntaxError instances are unrecoverable in the context that created them – they may only be caught by other contexts.

Class: TypeError#

A subclass of Error that indicates that a provided argument is not an allowable type. For example, passing a function to a parameter which expects a string would be considered a TypeError.

require('url').parse(() => { });
// throws TypeError, since it expected a string

Node.js will generate and throw TypeError instances immediately as a form of argument validation.

Exceptions vs. Errors#

A JavaScript exception is a value that is thrown as a result of an invalid operation or as the target of a throw statement. While it is not required that these values are instances of Error or classes which inherit from Error, all exceptions thrown by Node.js or the JavaScript runtime will be instances of Error.

Some exceptions are unrecoverable at the JavaScript layer. Such exceptions will always cause the Node.js process to crash. Examples include assert() checks or abort() calls in the C++ layer.

System Errors#

System errors are generated when exceptions occur within the program's runtime environment. Typically, these are operational errors that occur when an application violates an operating system constraint such as attempting to read a file that does not exist or when the user does not have sufficient permissions.

System errors are typically generated at the syscall level: an exhaustive list of error codes and their meanings is available by running man 2 intro or man 3 errno on most Unices; or online.

In Node.js, system errors are represented as augmented Error objects with added properties.

Class: System Error#

error.code#

The error.code property is a string representing the error code, which is always E followed by a sequence of capital letters.

error.errno#

The error.errno property is a number or a string. The number is a negative value which corresponds to the error code defined in libuv Error handling. See uv-errno.h header file (deps/uv/include/uv-errno.h in the Node.js source tree) for details. In case of a string, it is the same as error.code.

error.syscall#

The error.syscall property is a string describing the syscall that failed.

error.path#

When present (e.g. in fs or child_process), the error.path property is a string containing a relevant invalid pathname.

error.address#

When present (e.g. in net or dgram), the error.address property is a string describing the address to which the connection failed.

error.port#

When present (e.g. in net or dgram), the error.port property is a number representing the connection's port that is not available.

Common System Errors#

This list is not exhaustive, but enumerates many of the common system errors encountered when writing a Node.js program. An exhaustive list may be found here.

  • EACCES (Permission denied): An attempt was made to access a file in a way forbidden by its file access permissions.

  • EADDRINUSE (Address already in use): An attempt to bind a server (net, http, or https) to a local address failed due to another server on the local system already occupying that address.

  • ECONNREFUSED (Connection refused): No connection could be made because the target machine actively refused it. This usually results from trying to connect to a service that is inactive on the foreign host.

  • ECONNRESET (Connection reset by peer): A connection was forcibly closed by a peer. This normally results from a loss of the connection on the remote socket due to a timeout or reboot. Commonly encountered via the http and net modules.

  • EEXIST (File exists): An existing file was the target of an operation that required that the target not exist.

  • EISDIR (Is a directory): An operation expected a file, but the given pathname was a directory.

  • EMFILE (Too many open files in system): Maximum number of file descriptors allowable on the system has been reached, and requests for another descriptor cannot be fulfilled until at least one has been closed. This is encountered when opening many files at once in parallel, especially on systems (in particular, macOS) where there is a low file descriptor limit for processes. To remedy a low limit, run ulimit -n 2048 in the same shell that will run the Node.js process.

  • ENOENT (No such file or directory): Commonly raised by fs operations to indicate that a component of the specified pathname does not exist -- no entity (file or directory) could be found by the given path.

  • ENOTDIR (Not a directory): A component of the given pathname existed, but was not a directory as expected. Commonly raised by fs.readdir.

  • ENOTEMPTY (Directory not empty): A directory with entries was the target of an operation that requires an empty directory -- usually fs.unlink.

  • EPERM (Operation not permitted): An attempt was made to perform an operation that requires elevated privileges.

  • EPIPE (Broken pipe): A write on a pipe, socket, or FIFO for which there is no process to read the data. Commonly encountered at the net and http layers, indicative that the remote side of the stream being written to has been closed.

  • ETIMEDOUT (Operation timed out): A connect or send request failed because the connected party did not properly respond after a period of time. Usually encountered by http or net -- often a sign that a socket.end() was not properly called.

Events#

Stability: 2 - Stable

Much of the Node.js core API is built around an idiomatic asynchronous event-driven architecture in which certain kinds of objects (called "emitters") periodically emit named events that cause Function objects ("listeners") to be called.

For instance: a net.Server object emits an event each time a peer connects to it; a fs.ReadStream emits an event when the file is opened; a stream emits an event whenever data is available to be read.

All objects that emit events are instances of the EventEmitter class. These objects expose an eventEmitter.on() function that allows one or more functions to be attached to named events emitted by the object. Typically, event names are camel-cased strings but any valid JavaScript property key can be used.

When the EventEmitter object emits an event, all of the functions attached to that specific event are called synchronously. Any values returned by the called listeners are ignored and will be discarded.

The following example shows a simple EventEmitter instance with a single listener. The eventEmitter.on() method is used to register listeners, while the eventEmitter.emit() method is used to trigger the event.

const EventEmitter = require('events');

class MyEmitter extends EventEmitter {}

const myEmitter = new MyEmitter();
myEmitter.on('event', () => {
  console.log('an event occurred!');
});
myEmitter.emit('event');

Passing arguments and this to listeners#

The eventEmitter.emit() method allows an arbitrary set of arguments to be passed to the listener functions. It is important to keep in mind that when an ordinary listener function is called by the EventEmitter, the standard this keyword is intentionally set to reference the EventEmitter to which the listener is attached.

const myEmitter = new MyEmitter();
myEmitter.on('event', function(a, b) {
  console.log(a, b, this);
  // Prints:
  //   a b MyEmitter {
  //     domain: null,
  //     _events: { event: [Function] },
  //     _eventsCount: 1,
  //     _maxListeners: undefined }
});
myEmitter.emit('event', 'a', 'b');

It is possible to use ES6 Arrow Functions as listeners, however, when doing so, the this keyword will no longer reference the EventEmitter instance:

const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
  console.log(a, b, this);
  // Prints: a b {}
});
myEmitter.emit('event', 'a', 'b');

Asynchronous vs. Synchronous#

The EventEmitter calls all listeners synchronously in the order in which they were registered. This is important to ensure the proper sequencing of events and to avoid race conditions or logic errors. When appropriate, listener functions can switch to an asynchronous mode of operation using the setImmediate() or process.nextTick() methods:

const myEmitter = new MyEmitter();
myEmitter.on('event', (a, b) => {
  setImmediate(() => {
    console.log('this happens asynchronously');
  });
});
myEmitter.emit('event', 'a', 'b');

Handling events only once#

When a listener is registered using the eventEmitter.on() method, that listener will be invoked every time the named event is emitted.

const myEmitter = new MyEmitter();
let m = 0;
myEmitter.on('event', () => {
  console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Prints: 2

Using the eventEmitter.once() method, it is possible to register a listener that is called at most once for a particular event. Once the event is emitted, the listener is unregistered and then called.

const myEmitter = new MyEmitter();
let m = 0;
myEmitter.once('event', () => {
  console.log(++m);
});
myEmitter.emit('event');
// Prints: 1
myEmitter.emit('event');
// Ignored

Error events#

When an error occurs within an EventEmitter instance, the typical action is for an 'error' event to be emitted. These are treated as special cases within Node.js.

If an EventEmitter does not have at least one listener registered for the 'error' event, and an 'error' event is emitted, the error is thrown, a stack trace is printed, and the Node.js process exits.

const myEmitter = new MyEmitter();
myEmitter.emit('error', new Error('whoops!'));
// Throws and crashes Node.js

To guard against crashing the Node.js process, a listener can be registered on the process object's uncaughtException event or the domain module can be used. (Note, however, that the domain module has been deprecated)

const myEmitter = new MyEmitter();

process.on('uncaughtException', (err) => {
  console.error('whoops! there was an error');
});

myEmitter.emit('error', new Error('whoops!'));
// Prints: whoops! there was an error

As a best practice, listeners should always be added for the 'error' events.

const myEmitter = new MyEmitter();
myEmitter.on('error', (err) => {
  console.error('whoops! there was an error');
});
myEmitter.emit('error', new Error('whoops!'));
// Prints: whoops! there was an error

Class: EventEmitter#

Added in: v0.1.26

The EventEmitter class is defined and exposed by the events module:

const EventEmitter = require('events');

All EventEmitters emit the event 'newListener' when new listeners are added and 'removeListener' when existing listeners are removed.

Event: 'newListener'#

Added in: v0.1.26

The EventEmitter instance will emit its own 'newListener' event before a listener is added to its internal array of listeners.

Listeners registered for the 'newListener' event will be passed the event name and a reference to the listener being added.

The fact that the event is triggered before adding the listener has a subtle but important side effect: any additional listeners registered to the same name within the 'newListener' callback will be inserted before the listener that is in the process of being added.

const myEmitter = new MyEmitter();
// Only do this once so we don't loop forever
myEmitter.once('newListener', (event, listener) => {
  if (event === 'event') {
    // Insert a new listener in front
    myEmitter.on('event', () => {
      console.log('B');
    });
  }
});
myEmitter.on('event', () => {
  console.log('A');
});
myEmitter.emit('event');
// Prints:
//   B
//   A

Event: 'removeListener'#

Added in: v0.9.3

The 'removeListener' event is emitted after the listener is removed.

EventEmitter.listenerCount(emitter, eventName)#

Added in: v0.9.12 Deprecated since: v4.0.0
Stability: 0 - Deprecated: Use emitter.listenerCount() instead.

A class method that returns the number of listeners for the given eventName registered on the given emitter.

const myEmitter = new MyEmitter();
myEmitter.on('event', () => {});
myEmitter.on('event', () => {});
console.log(EventEmitter.listenerCount(myEmitter, 'event'));
// Prints: 2

EventEmitter.defaultMaxListeners#

Added in: v0.11.2

By default, a maximum of 10 listeners can be registered for any single event. This limit can be changed for individual EventEmitter instances using the emitter.setMaxListeners(n) method. To change the default for all EventEmitter instances, the EventEmitter.defaultMaxListeners property can be used.

Take caution when setting the EventEmitter.defaultMaxListeners because the change affects all EventEmitter instances, including those created before the change is made. However, calling emitter.setMaxListeners(n) still has precedence over EventEmitter.defaultMaxListeners.

Note that this is not a hard limit. The EventEmitter instance will allow more listeners to be added but will output a trace warning to stderr indicating that a "possible EventEmitter memory leak" has been detected. For any single EventEmitter, the emitter.getMaxListeners() and emitter.setMaxListeners() methods can be used to temporarily avoid this warning:

emitter.setMaxListeners(emitter.getMaxListeners() + 1);
emitter.once('event', () => {
  // do stuff
  emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));
});

The --trace-warnings command line flag can be used to display the stack trace for such warnings.

The emitted warning can be inspected with process.on('warning') and will have the additional emitter, type and count properties, referring to the event emitter instance, the event’s name and the number of attached listeners, respectively.

emitter.addListener(eventName, listener)#

Added in: v0.1.26

Alias for emitter.on(eventName, listener).

emitter.emit(eventName[, ...args])#

Added in: v0.1.26

Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.

Returns true if the event had listeners, false otherwise.

emitter.eventNames()#

Added in: v6.0.0

Returns an array listing the events for which the emitter has registered listeners. The values in the array will be strings or Symbols.

const EventEmitter = require('events');
const myEE = new EventEmitter();
myEE.on('foo', () => {});
myEE.on('bar', () => {});

const sym = Symbol('symbol');
myEE.on(sym, () => {});

console.log(myEE.eventNames());
// Prints: [ 'foo', 'bar', Symbol(symbol) ]

emitter.getMaxListeners()#

Added in: v1.0.0

Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to EventEmitter.defaultMaxListeners.

emitter.listenerCount(eventName)#

Added in: v3.2.0

Returns the number of listeners listening to the event named eventName.

emitter.listeners(eventName)#

Added in: v0.1.26

Returns a copy of the array of listeners for the event named eventName.

server.on('connection', (stream) => {
  console.log('someone connected!');
});
console.log(util.inspect(server.listeners('connection')));
// Prints: [ [Function] ]

emitter.on(eventName, listener)#

Added in: v0.1.101

Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.

server.on('connection', (stream) => {
  console.log('someone connected!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

const myEE = new EventEmitter();
myEE.on('foo', () => console.log('a'));
myEE.prependListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a

emitter.once(eventName, listener)#

Added in: v0.3.0

Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.

server.once('connection', (stream) => {
  console.log('Ah, we have our first user!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

By default, event listeners are invoked in the order they are added. The emitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

const myEE = new EventEmitter();
myEE.once('foo', () => console.log('a'));
myEE.prependOnceListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a

emitter.prependListener(eventName, listener)#

Added in: v6.0.0

Adds the listener function to the beginning of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.

server.prependListener('connection', (stream) => {
  console.log('someone connected!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

emitter.prependOnceListener(eventName, listener)#

Added in: v6.0.0

Adds a one-time listener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.

server.prependOnceListener('connection', (stream) => {
  console.log('Ah, we have our first user!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

emitter.removeAllListeners([eventName])#

Added in: v0.1.26

Removes all listeners, or those of the specified eventName.

Note that it is bad practice to remove listeners added elsewhere in the code, particularly when the EventEmitter instance was created by some other component or module (e.g. sockets or file streams).

Returns a reference to the EventEmitter, so that calls can be chained.

emitter.removeListener(eventName, listener)#

Added in: v0.1.26

Removes the specified listener from the listener array for the event named eventName.

const callback = (stream) => {
  console.log('someone connected!');
};
server.on('connection', callback);
// ...
server.removeListener('connection', callback);

removeListener will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified eventName, then removeListener must be called multiple times to remove each instance.

Note that once an event has been emitted, all listeners attached to it at the time of emitting will be called in order. This implies that any removeListener() or removeAllListeners() calls after emitting and before the last listener finishes execution will not remove them from emit() in progress. Subsequent events will behave as expected.

const myEmitter = new MyEmitter();

const callbackA = () => {
  console.log('A');
  myEmitter.removeListener('event', callbackB);
};

const callbackB = () => {
  console.log('B');
};

myEmitter.on('event', callbackA);

myEmitter.on('event', callbackB);

// callbackA removes listener callbackB but it will still be called.
// Internal listener array at time of emit [callbackA, callbackB]
myEmitter.emit('event');
// Prints:
//   A
//   B

// callbackB is now removed.
// Internal listener array [callbackA]
myEmitter.emit('event');
// Prints:
//   A

Because listeners are managed using an internal array, calling this will change the position indices of any listener registered after the listener being removed. This will not impact the order in which listeners are called, but it means that any copies of the listener array as returned by the emitter.listeners() method will need to be recreated.

Returns a reference to the EventEmitter, so that calls can be chained.

emitter.setMaxListeners(n)#

Added in: v0.3.5

By default EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks. Obviously, not all events should be limited to just 10 listeners. The emitter.setMaxListeners() method allows the limit to be modified for this specific EventEmitter instance. The value can be set to Infinity (or 0) to indicate an unlimited number of listeners.

Returns a reference to the EventEmitter, so that calls can be chained.

File System#

Stability: 2 - Stable

File I/O is provided by simple wrappers around standard POSIX functions. To use this module do require('fs'). All the methods have asynchronous and synchronous forms.

The asynchronous form always takes a completion callback as its last argument. The arguments passed to the completion callback depend on the method, but the first argument is always reserved for an exception. If the operation was completed successfully, then the first argument will be null or undefined.

When using the synchronous form any exceptions are immediately thrown. You can use try/catch to handle exceptions or allow them to bubble up.

Here is an example of the asynchronous version:

const fs = require('fs');

fs.unlink('/tmp/hello', (err) => {
  if (err) throw err;
  console.log('successfully deleted /tmp/hello');
});

Here is the synchronous version:

const fs = require('fs');

fs.unlinkSync('/tmp/hello');
console.log('successfully deleted /tmp/hello');

With the asynchronous methods there is no guaranteed ordering. So the following is prone to error:

fs.rename('/tmp/hello', '/tmp/world', (err) => {
  if (err) throw err;
  console.log('renamed complete');
});
fs.stat('/tmp/world', (err, stats) => {
  if (err) throw err;
  console.log(`stats: ${JSON.stringify(stats)}`);
});

It could be that fs.stat is executed before fs.rename. The correct way to do this is to chain the callbacks.

fs.rename('/tmp/hello', '/tmp/world', (err) => {
  if (err) throw err;
  fs.stat('/tmp/world', (err, stats) => {
    if (err) throw err;
    console.log(`stats: ${JSON.stringify(stats)}`);
  });
});

In busy processes, the programmer is strongly encouraged to use the asynchronous versions of these calls. The synchronous versions will block the entire process until they complete--halting all connections.

The relative path to a filename can be used. Remember, however, that this path will be relative to process.cwd().

Most fs functions let you omit the callback argument. If you do, a default callback is used that rethrows errors. To get a trace to the original call site, set the NODE_DEBUG environment variable:

$ cat script.js
function bad() {
  require('fs').readFile('/');
}
bad();

$ env NODE_DEBUG=fs node script.js
fs.js:88
        throw backtrace;
        ^
Error: EISDIR: illegal operation on a directory, read
    <stack trace.>

Buffer API#

Added in: v6.0.0

fs functions support passing and receiving paths as both strings and Buffers. The latter is intended to make it possible to work with filesystems that allow for non-UTF-8 filenames. For most typical uses, working with paths as Buffers will be unnecessary, as the string API converts to and from UTF-8 automatically.

Note that on certain file systems (such as NTFS and HFS+) filenames will always be encoded as UTF-8. On such file systems, passing non-UTF-8 encoded Buffers to fs functions will not work as expected.

Class: fs.FSWatcher#

Added in: v0.5.8

Objects returned from fs.watch() are of this type.

The listener callback provided to fs.watch() receives the returned FSWatcher's change events.

The object itself emits these events:

Event: 'change'#

Added in: v0.5.8
  • eventType <string> The type of fs change
  • filename <string> | <Buffer> The filename that changed (if relevant/available)

Emitted when something changes in a watched directory or file. See more details in fs.watch().

The filename argument may not be provided depending on operating system support. If filename is provided, it will be provided as a Buffer if fs.watch() is called with its encoding option set to 'buffer', otherwise filename will be a string.

// Example when handled through fs.watch listener
fs.watch('./tmp', {encoding: 'buffer'}, (eventType, filename) => {
  if (filename)
    console.log(filename);
  // Prints: <Buffer ...>
});

Event: 'error'#

Added in: v0.5.8

Emitted when an error occurs.

watcher.close()#

Added in: v0.5.8

Stop watching for changes on the given fs.FSWatcher.

Class: fs.ReadStream#

Added in: v0.1.93

ReadStream is a Readable Stream.

Event: 'open'#

Added in: v0.1.93
  • fd <Integer> Integer file descriptor used by the ReadStream.

Emitted when the ReadStream's file is opened.

Event: 'close'#

Added in: v0.1.93
  • fd <integer> Integer file descriptor used by the ReadStream.

Emitted when the ReadStream's underlying file descriptor has been closed.

readStream.bytesRead#

Added in: 6.4.0

The number of bytes read so far.

readStream.path#

Added in: v0.1.93

The path to the file the stream is reading from as specified in the first argument to fs.createReadStream(). If path is passed as a string, then readStream.path will be a string. If path is passed as a Buffer, then readStream.path will be a Buffer.

Class: fs.Stats#

Added in: v0.1.21

Objects returned from fs.stat(), fs.lstat() and fs.fstat() and their synchronous counterparts are of this type.

  • stats.isFile()
  • stats.isDirectory()
  • stats.isBlockDevice()
  • stats.isCharacterDevice()
  • stats.isSymbolicLink() (only valid with fs.lstat())
  • stats.isFIFO()
  • stats.isSocket()

For a regular file util.inspect(stats) would return a string very similar to this:

Stats {
  dev: 2114,
  ino: 48064969,
  mode: 33188,
  nlink: 1,
  uid: 85,
  gid: 100,
  rdev: 0,
  size: 527,
  blksize: 4096,
  blocks: 8,
  atime: Mon, 10 Oct 2011 23:24:11 GMT,
  mtime: Mon, 10 Oct 2011 23:24:11 GMT,
  ctime: Mon, 10 Oct 2011 23:24:11 GMT,
  birthtime: Mon, 10 Oct 2011 23:24:11 GMT }

Please note that atime, mtime, birthtime, and ctime are instances of Date object and to compare the values of these objects you should use appropriate methods. For most general uses getTime() will return the number of milliseconds elapsed since 1 January 1970 00:00:00 UTC and this integer should be sufficient for any comparison, however there are additional methods which can be used for displaying fuzzy information. More details can be found in the MDN JavaScript Reference page.

Stat Time Values#

The times in the stat object have the following semantics:

  • atime "Access Time" - Time when file data last accessed. Changed by the mknod(2), utimes(2), and read(2) system calls.
  • mtime "Modified Time" - Time when file data last modified. Changed by the mknod(2), utimes(2), and write(2) system calls.
  • ctime "Change Time" - Time when file status was last changed (inode data modification). Changed by the chmod(2), chown(2), link(2), mknod(2), rename(2), unlink(2), utimes(2), read(2), and write(2) system calls.
  • birthtime "Birth Time" - Time of file creation. Set once when the file is created. On filesystems where birthtime is not available, this field may instead hold either the ctime or 1970-01-01T00:00Z (ie, unix epoch timestamp 0). Note that this value may be greater than atime or mtime in this case. On Darwin and other FreeBSD variants, also set if the atime is explicitly set to an earlier value than the current birthtime using the utimes(2) system call.

Prior to Node v0.12, the ctime held the birthtime on Windows systems. Note that as of v0.12, ctime is not "creation time", and on Unix systems, it never was.

Class: fs.WriteStream#

Added in: v0.1.93

WriteStream is a Writable Stream.

Event: 'open'#

Added in: v0.1.93
  • fd <Integer> Integer file descriptor used by the WriteStream.

Emitted when the WriteStream's file is opened.

Event: 'close'#

Added in: v0.1.93
  • fd <integer> Integer file descriptor used by the WriteStream.

Emitted when the WriteStream's underlying file descriptor has been closed.

writeStream.bytesWritten#

Added in: v0.4.7

The number of bytes written so far. Does not include data that is still queued for writing.

writeStream.path#

Added in: v0.1.93

The path to the file the stream is writing to as specified in the first argument to fs.createWriteStream(). If path is passed as a string, then writeStream.path will be a string. If path is passed as a Buffer, then writeStream.path will be a Buffer.

fs.access(path[, mode], callback)#

Added in: v0.11.15

Tests a user's permissions for the file or directory specified by path. The mode argument is an optional integer that specifies the accessibility checks to be performed. The following constants define the possible values of mode. It is possible to create a mask consisting of the bitwise OR of two or more values.

  • fs.constants.F_OK - path is visible to the calling process. This is useful for determining if a file exists, but says nothing about rwx permissions. Default if no mode is specified.
  • fs.constants.R_OK - path can be read by the calling process.
  • fs.constants.W_OK - path can be written by the calling process.
  • fs.constants.X_OK - path can be executed by the calling process. This has no effect on Windows (will behave like fs.constants.F_OK).

The final argument, callback, is a callback function that is invoked with a possible error argument. If any of the accessibility checks fail, the error argument will be populated. The following example checks if the file /etc/passwd can be read and written by the current process.

fs.access('/etc/passwd', fs.constants.R_OK | fs.constants.W_OK, (err) => {
  console.log(err ? 'no access!' : 'can read/write');
});

Using fs.access() to check for the accessibility of a file before calling fs.open(), fs.readFile() or fs.writeFile() is not recommended. Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should open/read/write the file directly and handle the error raised if the file is not accessible.

For example:

write (NOT RECOMMENDED)

fs.access('myfile', (err) => {
  if (!err) {
    console.error('myfile already exists');
    return;
  }

  fs.open('myfile', 'wx', (err, fd) => {
    if (err) throw err;
    writeMyData(fd);
  });
});

write (RECOMMENDED)

fs.open('myfile', 'wx', (err, fd) => {
  if (err) {
    if (err.code === 'EEXIST') {
      console.error('myfile already exists');
      return;
    }

    throw err;
  }

  writeMyData(fd);
});

read (NOT RECOMMENDED)

fs.access('myfile', (err) => {
  if (err) {
    if (err.code === 'ENOENT') {
      console.error('myfile does not exist');
      return;
    }

    throw err;
  }

  fs.open('myfile', 'r', (err, fd) => {
    if (err) throw err;
    readMyData(fd);
  });
});

read (RECOMMENDED)

fs.open('myfile', 'r', (err, fd) => {
  if (err) {
    if (err.code === 'ENOENT') {
      console.error('myfile does not exist');
      return;
    }

    throw err;
  }

  readMyData(fd);
});

The "not recommended" examples above check for accessibility and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any.

In general, check for the accessibility of a file only if the file won’t be used directly, for example when its accessibility is a signal from another process.

fs.accessSync(path[, mode])#

Added in: v0.11.15

Synchronous version of fs.access(). This throws if any accessibility checks fail, and does nothing otherwise.

fs.appendFile(file, data[, options], callback)#

Added in: v0.6.7

Asynchronously append data to a file, creating the file if it does not yet exist. data can be a string or a buffer.

Example:

fs.appendFile('message.txt', 'data to append', (err) => {
  if (err) throw err;
  console.log('The "data to append" was appended to file!');
});

If options is a string, then it specifies the encoding. Example:

fs.appendFile('message.txt', 'data to append', 'utf8', callback);

Any specified file descriptor has to have been opened for appending.

Note: If a file descriptor is specified as the file, it will not be closed automatically.

fs.appendFileSync(file, data[, options])#

Added in: v0.6.7

The synchronous version of fs.appendFile(). Returns undefined.

fs.chmod(path, mode, callback)#

Added in: v0.1.30

Asynchronously changes the permissions of a file. No arguments other than a possible exception are given to the completion callback.

See also: chmod(2)

fs.chmodSync(path, mode)#

Added in: v0.6.7

Synchronously changes the permissions of a file. Returns undefined. This is the synchronous version of fs.chmod().

See also: chmod(2)

fs.chown(path, uid, gid, callback)#

Added in: v0.1.97

Asynchronously changes owner and group of a file. No arguments other than a possible exception are given to the completion callback.

See also: chown(2)

fs.chownSync(path, uid, gid)#

Added in: v0.1.97

Synchronously changes owner and group of a file. Returns undefined. This is the synchronous version of fs.chown().

See also: chown(2)

fs.close(fd, callback)#

Added in: v0.0.2

Asynchronous close(2). No arguments other than a possible exception are given to the completion callback.

fs.closeSync(fd)#

Added in: v0.1.21

Synchronous close(2). Returns undefined.

fs.constants#

Returns an object containing commonly used constants for file system operations. The specific constants currently defined are described in FS Constants.

fs.createReadStream(path[, options])#

Added in: v0.1.31

Returns a new ReadStream object. (See Readable Stream).

Be aware that, unlike the default value set for highWaterMark on a readable stream (16 kb), the stream returned by this method has a default value of 64 kb for the same parameter.

options is an object or string with the following defaults:

const defaults = {
  flags: 'r',
  encoding: null,
  fd: null,
  mode: 0o666,
  autoClose: true,
  highWaterMark: 64 * 1024
};

options can include start and end values to read a range of bytes from the file instead of the entire file. Both start and end are inclusive and start counting at 0. If fd is specified and start is omitted or undefined, fs.createReadStream() reads sequentially from the current file position. The encoding can be any one of those accepted by Buffer.

If fd is specified, ReadStream will ignore the path argument and will use the specified file descriptor. This means that no 'open' event will be emitted. Note that fd should be blocking; non-blocking fds should be passed to net.Socket.

If autoClose is false, then the file descriptor won't be closed, even if there's an error. It is your responsibility to close it and make sure there's no file descriptor leak. If autoClose is set to true (default behavior), on error or end the file descriptor will be closed automatically.

mode sets the file mode (permission and sticky bits), but only if the file was created.

An example to read the last 10 bytes of a file which is 100 bytes long:

fs.createReadStream('sample.txt', {start: 90, end: 99});

If options is a string, then it specifies the encoding.

fs.createWriteStream(path[, options])#

Added in: v0.1.31

Returns a new WriteStream object. (See Writable Stream).

options is an object or string with the following defaults:

const defaults = {
  flags: 'w',
  defaultEncoding: 'utf8',
  fd: null,
  mode: 0o666,
  autoClose: true
};

options may also include a start option to allow writing data at some position past the beginning of the file. Modifying a file rather than replacing it may require a flags mode of r+ rather than the default mode w. The defaultEncoding can be any one of those accepted by Buffer.

If autoClose is set to true (default behavior) on error or end the file descriptor will be closed automatically. If autoClose is false, then the file descriptor won't be closed, even if there's an error. It is your responsibility to close it and make sure there's no file descriptor leak.

Like ReadStream, if fd is specified, WriteStream will ignore the path argument and will use the specified file descriptor. This means that no 'open' event will be emitted. Note that fd should be blocking; non-blocking fds should be passed to net.Socket.

If options is a string, then it specifies the encoding.

fs.exists(path, callback)#

Added in: v0.0.2 Deprecated since: v1.0.0
Stability: 0 - Deprecated: Use fs.stat() or fs.access() instead.

Test whether or not the given path exists by checking with the file system. Then call the callback argument with either true or false. Example:

fs.exists('/etc/passwd', (exists) => {
  console.log(exists ? 'it\'s there' : 'no passwd!');
});

Note that the parameter to this callback is not consistent with other Node.js callbacks. Normally, the first parameter to a Node.js callback is an err parameter, optionally followed by other parameters. The fs.exists() callback has only one boolean parameter. This is one reason fs.access() is recommended instead of fs.exists().

Using fs.exists() to check for the existence of a file before calling fs.open(), fs.readFile() or fs.writeFile() is not recommended. Doing so introduces a race condition, since other processes may change the file's state between the two calls. Instead, user code should open/read/write the file directly and handle the error raised if the file does not exist.

For example:

write (NOT RECOMMENDED)

fs.exists('myfile', (exists) => {
  if (exists) {
    console.error('myfile already exists');
  } else {
    fs.open('myfile', 'wx', (err, fd) => {
      if (err) throw err;
      writeMyData(fd);
    });
  }
});

write (RECOMMENDED)

fs.open('myfile', 'wx', (err, fd) => {
  if (err) {
    if (err.code === 'EEXIST') {
      console.error('myfile already exists');
      return;
    }

    throw err;
  }

  writeMyData(fd);
});

read (NOT RECOMMENDED)

fs.exists('myfile', (exists) => {
  if (exists) {
    fs.open('myfile', 'r', (err, fd) => {
      readMyData(fd);
    });
  } else {
    console.error('myfile does not exist');
  }
});

read (RECOMMENDED)

fs.open('myfile', 'r', (err, fd) => {
  if (err) {
    if (err.code === 'ENOENT') {
      console.error('myfile does not exist');
      return;
    }

    throw err;
  }

  readMyData(fd);
});

The "not recommended" examples above check for existence and then use the file; the "recommended" examples are better because they use the file directly and handle the error, if any.

In general, check for the existence of a file only if the file won’t be used directly, for example when its existence is a signal from another process.

fs.existsSync(path)#

Added in: v0.1.21

Synchronous version of fs.exists(). Returns true if the path exists, false otherwise.

Note that fs.exists() is deprecated, but fs.existsSync() is not. (The callback parameter to fs.exists() accepts parameters that are inconsistent with other Node.js callbacks. fs.existsSync() does not use a callback.)

fs.fchmod(fd, mode, callback)#

Added in: v0.4.7

Asynchronous fchmod(2). No arguments other than a possible exception are given to the completion callback.

fs.fchmodSync(fd, mode)#

Added in: v0.4.7

Synchronous fchmod(2). Returns undefined.

fs.fchown(fd, uid, gid, callback)#

Added in: v0.4.7

Asynchronous fchown(2). No arguments other than a possible exception are given to the completion callback.

fs.fchownSync(fd, uid, gid)#

Added in: v0.4.7

Synchronous fchown(2). Returns undefined.

fs.fdatasync(fd, callback)#

Added in: v0.1.96

Asynchronous fdatasync(2). No arguments other than a possible exception are given to the completion callback.

fs.fdatasyncSync(fd)#

Added in: v0.1.96

Synchronous fdatasync(2). Returns undefined.

fs.fstat(fd, callback)#

Added in: v0.1.95

Asynchronous fstat(2). The callback gets two arguments (err, stats) where stats is an fs.Stats object. fstat() is identical to stat(), except that the file to be stat-ed is specified by the file descriptor fd.

fs.fstatSync(fd)#

Added in: v0.1.95

Synchronous fstat(2). Returns an instance of fs.Stats.

fs.fsync(fd, callback)#

Added in: v0.1.96

Asynchronous fsync(2). No arguments other than a possible exception are given to the completion callback.

fs.fsyncSync(fd)#

Added in: v0.1.96

Synchronous fsync(2). Returns undefined.

fs.ftruncate(fd, len, callback)#

Added in: v0.8.6

Asynchronous ftruncate(2). No arguments other than a possible exception are given to the completion callback.

If the file referred to by the file descriptor was larger than len bytes, only the first len bytes will be retained in the file.

For example, the following program retains only the first four bytes of the file

console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js

// get the file descriptor of the file to be truncated
const fd = fs.openSync('temp.txt', 'r+');

// truncate the file to first four bytes
fs.ftruncate(fd, 4, (err) => {
  assert.ifError(err);
  console.log(fs.readFileSync('temp.txt', 'utf8'));
});
// Prints: Node

If the file previously was shorter than len bytes, it is extended, and the extended part is filled with null bytes ('\0'). For example,

console.log(fs.readFileSync('temp.txt', 'utf8'));
// Prints: Node.js

// get the file descriptor of the file to be truncated
const fd = fs.openSync('temp.txt', 'r+');

// truncate the file to 10 bytes, whereas the actual size is 7 bytes
fs.ftruncate(fd, 10, (err) => {
  assert.ifError(err);
  console.log(fs.readFileSync('temp.txt'));
});
// Prints: <Buffer 4e 6f 64 65 2e 6a 73 00 00 00>
// ('Node.js\0\0\0' in UTF8)

The last three bytes are null bytes ('\0'), to compensate the over-truncation.

fs.ftruncateSync(fd, len)#

Added in: v0.8.6

Synchronous ftruncate(2). Returns undefined.

fs.futimes(fd, atime, mtime, callback)#

Added in: v0.4.2

Change the file timestamps of a file referenced by the supplied file descriptor.

Note: This function does not work on AIX versions before 7.1, it will return the error UV_ENOSYS.

fs.futimesSync(fd, atime, mtime)#

Added in: v0.4.2

Synchronous version of fs.futimes(). Returns undefined.

fs.lchmod(path, mode, callback)#

Deprecated since: v0.4.7

Asynchronous lchmod(2). No arguments other than a possible exception are given to the completion callback.

Only available on macOS.

fs.lchmodSync(path, mode)#

Deprecated since: v0.4.7

Synchronous lchmod(2). Returns undefined.

fs.lchown(path, uid, gid, callback)#

Deprecated since: v0.4.7

Asynchronous lchown(2). No arguments other than a possible exception are given to the completion callback.

fs.lchownSync(path, uid, gid)#

Deprecated since: v0.4.7

Synchronous lchown(2). Returns undefined.

fs.link(existingPath, newPath, callback)#

Added in: v0.1.31

Asynchronous link(2). No arguments other than a possible exception are given to the completion callback.

fs.linkSync(existingPath, newPath)#

Added in: v0.1.31

Synchronous link(2). Returns undefined.

fs.lstat(path, callback)#

Added in: v0.1.30

Asynchronous lstat(2). The callback gets two arguments (err, stats) where stats is a fs.Stats object. lstat() is identical to stat(), except that if path is a symbolic link, then the link itself is stat-ed, not the file that it refers to.

fs.lstatSync(path)#

Added in: v0.1.30

Synchronous lstat(2). Returns an instance of fs.Stats.

fs.mkdir(path[, mode], callback)#

Added in: v0.1.8

Asynchronously creates a directory. No arguments other than a possible exception are given to the completion callback. mode defaults to 0o777.

See also: mkdir(2)

fs.mkdirSync(path[, mode])#

Added in: v0.1.21

Synchronously creates a directory. Returns undefined. This is the synchronous version of fs.mkdir().

See also: mkdir(2)

fs.mkdtemp(prefix[, options], callback)#

Added in: v5.10.0

Creates a unique temporary directory.

Generates six random characters to be appended behind a required prefix to create a unique temporary directory.

The created folder path is passed as a string to the callback's second parameter.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use.

Example:

fs.mkdtemp(path.join(os.tmpdir(), 'foo-'), (err, folder) => {
  if (err) throw err;
  console.log(folder);
  // Prints: /tmp/foo-itXde2 or C:\Users\...\AppData\Local\Temp\foo-itXde2
});

Note: The fs.mkdtemp() method will append the six randomly selected characters directly to the prefix string. For instance, given a directory /tmp, if the intention is to create a temporary directory within /tmp, the prefix must end with a trailing platform-specific path separator (require('path').sep).

// The parent directory for the new temporary directory
const tmpDir = os.tmpdir();

// This method is *INCORRECT*:
fs.mkdtemp(tmpDir, (err, folder) => {
  if (err) throw err;
  console.log(folder);
  // Will print something similar to `/tmpabc123`.
  // Note that a new temporary directory is created
  // at the file system root rather than *within*
  // the /tmp directory.
});

// This method is *CORRECT*:
const { sep } = require('path');
fs.mkdtemp(`${tmpDir}${sep}`, (err, folder) => {
  if (err) throw err;
  console.log(folder);
  // Will print something similar to `/tmp/abc123`.
  // A new temporary directory is created within
  // the /tmp directory.
});

fs.mkdtempSync(prefix[, options])#

Added in: v5.10.0

The synchronous version of fs.mkdtemp(). Returns the created folder path.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use.

fs.open(path, flags[, mode], callback)#

Added in: v0.0.2

Asynchronous file open. See open(2). flags can be:

  • 'r' - Open file for reading. An exception occurs if the file does not exist.

  • 'r+' - Open file for reading and writing. An exception occurs if the file does not exist.

  • 'rs+' - Open file for reading and writing in synchronous mode. Instructs the operating system to bypass the local file system cache.

    This is primarily useful for opening files on NFS mounts as it allows you to skip the potentially stale local cache. It has a very real impact on I/O performance so don't use this flag unless you need it.

    Note that this doesn't turn fs.open() into a synchronous blocking call. If that's what you want then you should be using fs.openSync()

  • 'w' - Open file for writing. The file is created (if it does not exist) or truncated (if it exists).

  • 'wx' - Like 'w' but fails if path exists.

  • 'w+' - Open file for reading and writing. The file is created (if it does not exist) or truncated (if it exists).

  • 'wx+' - Like 'w+' but fails if path exists.

  • 'a' - Open file for appending. The file is created if it does not exist.

  • 'ax' - Like 'a' but fails if path exists.

  • 'a+' - Open file for reading and appending. The file is created if it does not exist.

  • 'ax+' - Like 'a+' but fails if path exists.

mode sets the file mode (permission and sticky bits), but only if the file was created. It defaults to 0o666 (readable and writable).

The callback gets two arguments (err, fd).

The exclusive flag 'x' (O_EXCL flag in open(2)) ensures that path is newly created. On POSIX systems, path is considered to exist even if it is a symlink to a non-existent file. The exclusive flag may or may not work with network file systems.

flags can also be a number as documented by open(2); commonly used constants are available from fs.constants. On Windows, flags are translated to their equivalent ones where applicable, e.g. O_WRONLY to FILE_GENERIC_WRITE, or O_EXCL|O_CREAT to CREATE_NEW, as accepted by CreateFileW.

On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

Note: The behavior of fs.open() is platform-specific for some flags. As such, opening a directory on macOS and Linux with the 'a+' flag - see example below - will return an error. In contrast, on Windows and FreeBSD, a file descriptor will be returned.

// macOS and Linux
fs.open('<directory>', 'a+', (err, fd) => {
  // => [Error: EISDIR: illegal operation on a directory, open <directory>]
});

// Windows and FreeBSD
fs.open('<directory>', 'a+', (err, fd) => {
  // => null, <fd>
});

Some characters (< > : " / \ | ? *) are reserved under Windows as documented by Naming Files, Paths, and Namespaces. Under NTFS, if the filename contains a colon, Node.js will open a file system stream, as described by this MSDN page.

Functions based on fs.open() exhibit this behavior as well. eg. fs.writeFile(), fs.readFile(), etc.

fs.openSync(path, flags[, mode])#

Added in: v0.1.21

Synchronous version of fs.open(). Returns an integer representing the file descriptor.

fs.read(fd, buffer, offset, length, position, callback)#

Added in: v0.0.2

Read data from the file specified by fd.

buffer is the buffer that the data will be written to.

offset is the offset in the buffer to start writing at.

length is an integer specifying the number of bytes to read.

position is an argument specifying where to begin reading from in the file. If position is null, data will be read from the current file position, and the file position will be updated. If position is an integer, the file position will remain unchanged.

The callback is given the three arguments, (err, bytesRead, buffer).

fs.readdir(path[, options], callback)#

Added in: v0.1.8

Asynchronous readdir(3). Reads the contents of a directory. The callback gets two arguments (err, files) where files is an array of the names of the files in the directory excluding '.' and '..'.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the filenames passed to the callback. If the encoding is set to 'buffer', the filenames returned will be passed as Buffer objects.

fs.readdirSync(path[, options])#

Added in: v0.1.21

Synchronous readdir(3). Returns an array of filenames excluding '.' and '..'.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the filenames passed to the callback. If the encoding is set to 'buffer', the filenames returned will be passed as Buffer objects.

fs.readFile(file[, options], callback)#

Added in: v0.1.29

Asynchronously reads the entire contents of a file. Example:

fs.readFile('/etc/passwd', (err, data) => {
  if (err) throw err;
  console.log(data);
});

The callback is passed two arguments (err, data), where data is the contents of the file.

If no encoding is specified, then the raw buffer is returned.

If options is a string, then it specifies the encoding. Example:

fs.readFile('/etc/passwd', 'utf8', callback);

Note: When the path is a directory, the behavior of fs.readFile() and [fs.readFileSync()][] is platform-specific. On macOS, Linux, and Windows, an error will be returned. On FreeBSD, a representation of the directory's contents will be returned.

// macOS, Linux and Windows
fs.readFile('<directory>', (err, data) => {
  // => [Error: EISDIR: illegal operation on a directory, read <directory>]
});

//  FreeBSD
fs.readFile('<directory>', (err, data) => {
  // => null, <data>
});

Any specified file descriptor has to support reading.

Note: If a file descriptor is specified as the path, it will not be closed automatically.

fs.readFileSync(file[, options])#

Added in: v0.1.8

Synchronous version of fs.readFile. Returns the contents of the file.

If the encoding option is specified then this function returns a string. Otherwise it returns a buffer.

Note: Similar to [fs.readFile()][], when the path is a directory, the behavior of fs.readFileSync() is platform-specific.

// macOS, Linux and Windows
fs.readFileSync('<directory>');
// => [Error: EISDIR: illegal operation on a directory, read <directory>]

//  FreeBSD
fs.readFileSync('<directory>'); // => null, <data>

fs.readlink(path[, options], callback)#

Added in: v0.1.31

Asynchronous readlink(2). The callback gets two arguments (err, linkString).

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the link path passed to the callback. If the encoding is set to 'buffer', the link path returned will be passed as a Buffer object.

fs.readlinkSync(path[, options])#

Added in: v0.1.31

Synchronous readlink(2). Returns the symbolic link's string value.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the link path passed to the callback. If the encoding is set to 'buffer', the link path returned will be passed as a Buffer object.

fs.readSync(fd, buffer, offset, length, position)#

Added in: v0.1.21

Synchronous version of fs.read(). Returns the number of bytesRead.

fs.realpath(path[, options], callback)#

Added in: v0.1.31

Asynchronous realpath(3). The callback gets two arguments (err, resolvedPath). May use process.cwd to resolve relative paths.

Only paths that can be converted to UTF8 strings are supported.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the path passed to the callback. If the encoding is set to 'buffer', the path returned will be passed as a Buffer object.

fs.realpathSync(path[, options])#

Added in: v0.1.31

Synchronous realpath(3). Returns the resolved path.

Only paths that can be converted to UTF8 strings are supported.

The optional options argument can be a string specifying an encoding, or an object with an encoding property specifying the character encoding to use for the returned value. If the encoding is set to 'buffer', the path returned will be passed as a Buffer object.

fs.rename(oldPath, newPath, callback)#

Added in: v0.0.2

Asynchronous rename(2). No arguments other than a possible exception are given to the completion callback.

fs.renameSync(oldPath, newPath)#

Added in: v0.1.21

Synchronous rename(2). Returns undefined.

fs.rmdir(path, callback)#

Added in: v0.0.2

Asynchronous rmdir(2). No arguments other than a possible exception are given to the completion callback.

Note: Using fs.rmdir() on a file (not a directory) results in an ENOENT error on Windows and an ENOTDIR error on POSIX.

fs.rmdirSync(path)#

Added in: v0.1.21

Synchronous rmdir(2). Returns undefined.

Note: Using fs.rmdirSync() on a file (not a directory) results in an ENOENT error on Windows and an ENOTDIR error on POSIX.

fs.stat(path, callback)#

Added in: v0.0.2

Asynchronous stat(2). The callback gets two arguments (err, stats) where stats is an fs.Stats object.

In case of an error, the err.code will be one of Common System Errors.

Using fs.stat() to check for the existence of a file before calling fs.open(), fs.readFile() or fs.writeFile() is not recommended. Instead, user code should open/read/write the file directly and handle the error raised if the file is not available.

To check if a file exists without manipulating it afterwards, fs.access() is recommended.

fs.statSync(path)#

Added in: v0.1.21

Synchronous stat(2). Returns an instance of fs.Stats.

fs.symlink(target, path[, type], callback)#

Added in: v0.1.31

Asynchronous symlink(2). No arguments other than a possible exception are given to the completion callback. The type argument can be set to 'dir', 'file', or 'junction' (default is 'file') and is only available on Windows (ignored on other platforms). Note that Windows junction points require the destination path to be absolute. When using 'junction', the target argument will automatically be normalized to absolute path.

Here is an example below:

fs.symlink('./foo', './new-port', callback);

It creates a symbolic link named "new-port" that points to "foo".

fs.symlinkSync(target, path[, type])#

Added in: v0.1.31

Synchronous symlink(2). Returns undefined.

fs.truncate(path, len, callback)#

Added in: v0.8.6

Asynchronous truncate(2). No arguments other than a possible exception are given to the completion callback. A file descriptor can also be passed as the first argument. In this case, fs.ftruncate() is called.

fs.truncateSync(path, len)#

Added in: v0.8.6

Synchronous truncate(2). Returns undefined. A file descriptor can also be passed as the first argument. In this case, fs.ftruncateSync() is called.

fs.unlink(path, callback)#

Added in: v0.0.2

Asynchronous unlink(2). No arguments other than a possible exception are given to the completion callback.

fs.unlinkSync(path)#

Added in: v0.1.21

Synchronous unlink(2). Returns undefined.

fs.unwatchFile(filename[, listener])#

Added in: v0.1.31

Stop watching for changes on filename. If listener is specified, only that particular listener is removed. Otherwise, all listeners are removed and you have effectively stopped watching filename.

Calling fs.unwatchFile() with a filename that is not being watched is a no-op, not an error.

Note: fs.watch() is more efficient than fs.watchFile() and fs.unwatchFile(). fs.watch() should be used instead of fs.watchFile() and fs.unwatchFile() when possible.

fs.utimes(path, atime, mtime, callback)#

Added in: v0.4.2

Change file timestamps of the file referenced by the supplied path.

Note: the arguments atime and mtime of the following related functions follow these rules:

  • The value should be a Unix timestamp in seconds. For example, Date.now() returns milliseconds, so it should be divided by 1000 before passing it in.
  • If the value is a numeric string like '123456789', the value will get converted to the corresponding number.
  • If the value is NaN or Infinity, the value will get converted to Date.now() / 1000.

fs.utimesSync(path, atime, mtime)#

Added in: v0.4.2

Synchronous version of fs.utimes(). Returns undefined.

fs.watch(filename[, options][, listener])#

Added in: v0.5.10
  • filename <string> | <Buffer>
  • options <string> | <Object>
    • persistent <boolean> Indicates whether the process should continue to run as long as files are being watched. default = true
    • recursive <boolean> Indicates whether all subdirectories should be watched, or only the current directory. The applies when a directory is specified, and only on supported platforms (See Caveats). default = false
    • encoding <string> Specifies the character encoding to be used for the filename passed to the listener. default = 'utf8'
  • listener <Function>

Watch for changes on filename, where filename is either a file or a directory. The returned object is a fs.FSWatcher.

The second argument is optional. If options is provided as a string, it specifies the encoding. Otherwise options should be passed as an object.

The listener callback gets two arguments (eventType, filename). eventType is either 'rename' or 'change', and filename is the name of the file which triggered the event.

Note that on most platforms, 'rename' is emitted whenever a filename appears or disappears in the directory.

Also note the listener callback is attached to the 'change' event fired by fs.FSWatcher, but it is not the same thing as the 'change' value of eventType.

Caveats#

The fs.watch API is not 100% consistent across platforms, and is unavailable in some situations.

The recursive option is only supported on macOS and Windows.

Availability#

This feature depends on the underlying operating system providing a way to be notified of filesystem changes.

  • On Linux systems, this uses inotify
  • On BSD systems, this uses kqueue
  • On macOS, this uses kqueue for files and FSEvents for directories.
  • On SunOS systems (including Solaris and SmartOS), this uses event ports.
  • On Windows systems, this feature depends on ReadDirectoryChangesW.
  • On Aix systems, this feature depends on AHAFS, which must be enabled.

If the underlying functionality is not available for some reason, then fs.watch will not be able to function. For example, watching files or directories can be unreliable, and in some cases impossible, on network file systems (NFS, SMB, etc), or host file systems when using virtualization software such as Vagrant, Docker, etc.

You can still use fs.watchFile, which uses stat polling, but it is slower and less reliable.

Inodes#

On Linux and macOS systems, fs.watch() resolves the path to an inode and watches the inode. If the watched path is deleted and recreated, it is assigned a new inode. The watch will emit an event for the delete but will continue watching the original inode. Events for the new inode will not be emitted. This is expected behavior.

On AIX, save and close of a file being watched causes two notifications - one for adding new content, and one for truncation. Moreover, save and close operations on some platforms cause inode changes that force watch operations to become invalid and ineffective. AIX retains inode for the lifetime of a file, that way though this is different from Linux / OS X, this improves the usability of file watching. This is expected behavior.

Filename Argument#

Providing filename argument in the callback is only supported on Linux and Windows. Even on supported platforms, filename is not always guaranteed to be provided. Therefore, don't assume that filename argument is always provided in the callback, and have some fallback logic if it is null.

fs.watch('somedir', (eventType, filename) => {
  console.log(`event type is: ${eventType}`);
  if (filename) {
    console.log(`filename provided: ${filename}`);
  } else {
    console.log('filename not provided');
  }
});

fs.watchFile(filename[, options], listener)#

Added in: v0.1.31

Watch for changes on filename. The callback listener will be called each time the file is accessed.

The options argument may be omitted. If provided, it should be an object. The options object may contain a boolean named persistent that indicates whether the process should continue to run as long as files are being watched. The options object may specify an interval property indicating how often the target should be polled in milliseconds. The default is { persistent: true, interval: 5007 }.

The listener gets two arguments the current stat object and the previous stat object:

fs.watchFile('message.text', (curr, prev) => {
  console.log(`the current mtime is: ${curr.mtime}`);
  console.log(`the previous mtime was: ${prev.mtime}`);
});

These stat objects are instances of fs.Stat.

If you want to be notified when the file was modified, not just accessed, you need to compare curr.mtime and prev.mtime.

Note: when an fs.watchFile operation results in an ENOENT error, it will invoke the listener once, with all the fields zeroed (or, for dates, the Unix Epoch). In Windows, blksize and blocks fields will be undefined, instead of zero. If the file is created later on, the listener will be called again, with the latest stat objects. This is a change in functionality since v0.10.

Note: fs.watch() is more efficient than fs.watchFile and fs.unwatchFile. fs.watch should be used instead of fs.watchFile and fs.unwatchFile when possible.

Note: When a file being watched by fs.watchFile() disappears and reappears, then the previousStat reported in the second callback event (the file's reappearance) will be the same as the previousStat of the first callback event (its disappearance).

This happens when:

  • the file is deleted, followed by a restore
  • the file is renamed twice - the second time back to its original name

fs.write(fd, buffer[, offset[, length[, position]]], callback)#

Added in: v0.0.2

Write buffer to the file specified by fd.

offset determines the part of the buffer to be written, and length is an integer specifying the number of bytes to write.

position refers to the offset from the beginning of the file where this data should be written. If typeof position !== 'number', the data will be written at the current position. See pwrite(2).

The callback will be given three arguments (err, written, buffer) where written specifies how many bytes were written from buffer.

Note that it is unsafe to use fs.write multiple times on the same file without waiting for the callback. For this scenario, fs.createWriteStream is strongly recommended.

On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

fs.write(fd, string[, position[, encoding]], callback)#

Added in: v0.11.5

Write string to the file specified by fd. If string is not a string, then the value will be coerced to one.

position refers to the offset from the beginning of the file where this data should be written. If typeof position !== 'number' the data will be written at the current position. See pwrite(2).

encoding is the expected string encoding.

The callback will receive the arguments (err, written, string) where written specifies how many bytes the passed string required to be written. Note that bytes written is not the same as string characters. See Buffer.byteLength.

Unlike when writing buffer, the entire string must be written. No substring may be specified. This is because the byte offset of the resulting data may not be the same as the string offset.

Note that it is unsafe to use fs.write multiple times on the same file without waiting for the callback. For this scenario, fs.createWriteStream is strongly recommended.

On Linux, positional writes don't work when the file is opened in append mode. The kernel ignores the position argument and always appends the data to the end of the file.

fs.writeFile(file, data[, options], callback)#

Added in: v0.1.29

Asynchronously writes data to a file, replacing the file if it already exists. data can be a string or a buffer.

The encoding option is ignored if data is a buffer. It defaults to 'utf8'.

Example:

fs.writeFile('message.txt', 'Hello Node.js', (err) => {
  if (err) throw err;
  console.log('The file has been saved!');
});

If options is a string, then it specifies the encoding. Example:

fs.writeFile('message.txt', 'Hello Node.js', 'utf8', callback);

Any specified file descriptor has to support writing.

Note that it is unsafe to use fs.writeFile multiple times on the same file without waiting for the callback. For this scenario, fs.createWriteStream is strongly recommended.

Note: If a file descriptor is specified as the file, it will not be closed automatically.

fs.writeFileSync(file, data[, options])#

Added in: v0.1.29

The synchronous version of fs.writeFile(). Returns undefined.

fs.writeSync(fd, buffer[, offset[, length[, position]]])#

Added in: v0.1.21

fs.writeSync(fd, string[, position[, encoding]])#

Added in: v0.11.5

Synchronous versions of fs.write(). Returns the number of bytes written.

FS Constants#

The following constants are exported by fs.constants. Note: Not every constant will be available on every operating system.

File Access Constants#

The following constants are meant for use with fs.access().

Constant Description
F_OK Flag indicating that the file is visible to the calling process.
R_OK Flag indicating that the file can be read by the calling process.
W_OK Flag indicating that the file can be written by the calling process.
X_OK Flag indicating that the file can be executed by the calling process.

File Open Constants#

The following constants are meant for use with fs.open().

Constant Description
O_RDONLY Flag indicating to open a file for read-only access.
O_WRONLY Flag indicating to open a file for write-only access.
O_RDWR Flag indicating to open a file for read-write access.
O_CREAT Flag indicating to create the file if it does not already exist.
O_EXCL Flag indicating that opening a file should fail if the O_CREAT flag is set and the file already exists.
O_NOCTTY Flag indicating that if path identifies a terminal device, opening the path shall not cause that terminal to become the controlling terminal for the process (if the process does not already have one).
O_TRUNC Flag indicating that if the file exists and is a regular file, and the file is opened successfully for write access, its length shall be truncated to zero.
O_APPEND Flag indicating that data will be appended to the end of the file.
O_DIRECTORY Flag indicating that the open should fail if the path is not a directory.
O_NOATIME Flag indicating reading accesses to the file system will no longer result in an update to the atime information associated with the file. This flag is available on Linux operating systems only.
O_NOFOLLOW Flag indicating that the open should fail if the path is a symbolic link.
O_SYNC Flag indicating that the file is opened for synchronous I/O.
O_SYMLINK Flag indicating to open the symbolic link itself rather than the resource it is pointing to.
O_DIRECT When set, an attempt will be made to minimize caching effects of file I/O.
O_NONBLOCK Flag indicating to open the file in nonblocking mode when possible.

File Type Constants#

The following constants are meant for use with the fs.Stats object's mode property for determining a file's type.

Constant Description
S_IFMT Bit mask used to extract the file type code.
S_IFREG File type constant for a regular file.
S_IFDIR File type constant for a directory.
S_IFCHR File type constant for a character-oriented device file.
S_IFBLK File type constant for a block-oriented device file.
S_IFIFO File type constant for a FIFO/pipe.
S_IFLNK File type constant for a symbolic link.
S_IFSOCK File type constant for a socket.

File Mode Constants#

The following constants are meant for use with the fs.Stats object's mode property for determining the access permissions for a file.

Constant Description
S_IRWXU File mode indicating readable, writable and executable by owner.
S_IRUSR File mode indicating readable by owner.
S_IWUSR File mode indicating writable by owner.
S_IXUSR File mode indicating executable by owner.
S_IRWXG File mode indicating readable, writable and executable by group.
S_IRGRP File mode indicating readable by group.
S_IWGRP File mode indicating writable by group.
S_IXGRP File mode indicating executable by group.
S_IRWXO File mode indicating readable, writable and executable by others.
S_IROTH File mode indicating readable by others.
S_IWOTH File mode indicating writable by others.
S_IXOTH File mode indicating executable by others.

Global Objects#

These objects are available in all modules. Some of these objects aren't actually in the global scope but in the module scope - this will be noted.

The objects listed here are specific to Node.js. There are a number of built-in objects that are part of the JavaScript language itself, which are also globally accessible.

Class: Buffer#

Added in: v0.1.103

Used to handle binary data. See the buffer section.

__dirname#

Added in: v0.1.27

The directory name of the current module. This the same as the path.dirname() of the __filename.

__dirname is not actually a global but rather local to each module.

Example: running node example.js from /Users/mjr

console.log(__dirname);
// Prints: /Users/mjr
console.log(path.dirname(__filename));
// Prints: /Users/mjr

__filename#

Added in: v0.0.1

The file name of the current module. This is the resolved absolute path of the current module file.

For a main program this is not necessarily the same as the file name used in the command line.

See __dirname for the directory name of the current module.

__filename is not actually a global but rather local to each module.

Examples:

Running node example.js from /Users/mjr

console.log(__filename);
// Prints: /Users/mjr/example.js
console.log(__dirname);
// Prints: /Users/mjr

Given two modules: a and b, where b is a dependency of a and there is a directory structure of:

  • /Users/mjr/app/a.js
  • /Users/mjr/app/node_modules/b/b.js

References to __filename within b.js will return /Users/mjr/app/node_modules/b/b.js while references to __filename within a.js will return /Users/mjr/app/a.js.

clearImmediate(immediateObject)#

Added in: v0.9.1

clearImmediate is described in the timers section.

clearInterval(intervalObject)#

Added in: v0.0.1

clearInterval is described in the timers section.

clearTimeout(timeoutObject)#

Added in: v0.0.1

clearTimeout is described in the timers section.

console#

Added in: v0.1.100

Used to print to stdout and stderr. See the console section.

exports#

Added in: v0.1.12

A reference to the module.exports that is shorter to type. See module system documentation for details on when to use exports and when to use module.exports.

exports is not actually a global but rather local to each module.

See the module system documentation for more information.

global#

Added in: v0.1.27

In browsers, the top-level scope is the global scope. That means that in browsers if you're in the global scope var something will define a global variable. In Node.js this is different. The top-level scope is not the global scope; var something inside an Node.js module will be local to that module.

module#

Added in: v0.1.16

A reference to the current module. In particular module.exports is used for defining what a module exports and makes available through require().

module is not actually a global but rather local to each module.

See the module system documentation for more information.

process#

Added in: v0.1.7

The process object. See the process object section.

require()#

Added in: v0.1.13

To require modules. See the Modules section. require is not actually a global but rather local to each module.

require.cache#

Added in: v0.3.0

Modules are cached in this object when they are required. By deleting a key value from this object, the next require will reload the module. Note that this does not apply to native addons, for which reloading will result in an Error.

require.extensions#

Added in: v0.3.0 Deprecated since: v0.10.6
Stability: 0 - Deprecated

Instruct require on how to handle certain file extensions.

Process files with the extension .sjs as .js:

require.extensions['.sjs'] = require.extensions['.js'];

Deprecated In the past, this list has been used to load non-JavaScript modules into Node.js by compiling them on-demand. However, in practice, there are much better ways to do this, such as loading modules via some other Node.js program, or compiling them to JavaScript ahead of time.

Since the Module system is locked, this feature will probably never go away. However, it may have subtle bugs and complexities that are best left untouched.

require.resolve()#

Added in: v0.3.0

Use the internal require() machinery to look up the location of a module, but rather than loading the module, just return the resolved filename.

setImmediate(callback[, ...args])#

Added in: v0.9.1

setImmediate is described in the timers section.

setInterval(callback, delay[, ...args])#

Added in: v0.0.1

setInterval is described in the timers section.

setTimeout(callback, delay[, ...args])#

Added in: v0.0.1

setTimeout is described in the timers section.

HTTP#

Stability: 2 - Stable

To use the HTTP server and client one must require('http').

The HTTP interfaces in Node.js are designed to support many features of the protocol which have been traditionally difficult to use. In particular, large, possibly chunk-encoded, messages. The interface is careful to never buffer entire requests or responses--the user is able to stream data.

HTTP message headers are represented by an object like this:

{ 'content-length': '123',
  'content-type': 'text/plain',
  'connection': 'keep-alive',
  'host': 'mysite.com',
  'accept': '*/*' }

Keys are lowercased. Values are not modified.

In order to support the full spectrum of possible HTTP applications, Node.js's HTTP API is very low-level. It deals with stream handling and message parsing only. It parses a message into headers and body but it does not parse the actual headers or the body.

See message.headers for details on how duplicate headers are handled.

The raw headers as they were received are retained in the rawHeaders property, which is an array of [key, value, key2, value2, ...]. For example, the previous message header object might have a rawHeaders list like the following:

[ 'ConTent-Length', '123456',
  'content-LENGTH', '123',
  'content-type', 'text/plain',
  'CONNECTION', 'keep-alive',
  'Host', 'mysite.com',
  'accepT', '*/*' ]

Class: http.Agent#

Added in: v0.3.4

An Agent is responsible for managing connection persistence and reuse for HTTP clients. It maintains a queue of pending requests for a given host and port, reusing a single socket connection for each until the queue is empty, at which time the socket is either destroyed or put into a pool where it is kept to be used again for requests to the same host and port. Whether it is destroyed or pooled depends on the keepAlive option.

Pooled connections have TCP Keep-Alive enabled for them, but servers may still close idle connections, in which case they will be removed from the pool and a new connection will be made when a new HTTP request is made for that host and port. Servers may also refuse to allow multiple requests over the same connection, in which case the connection will have to be remade for every request and cannot be pooled. The Agent will still make the requests to that server, but each one will occur over a new connection.

When a connection is closed by the client or the server, it is removed from the pool. Any unused sockets in the pool will be unrefed so as not to keep the Node.js process running when there are no outstanding requests. (see socket.unref()).

It is good practice, to destroy() an Agent instance when it is no longer in use, because unused sockets consume OS resources.

Sockets are removed from an agent's pool when the socket emits either a 'close' event or an 'agentRemove' event. This means that if you intend to keep one HTTP request open for a long time and don't want it to stay in the pool you can do something along the lines of:

http.get(options, (res) => {
  // Do stuff
}).on('socket', (socket) => {
  socket.emit('agentRemove');
});

You may also use an agent for an individual request. By providing {agent: false} as an option to the http.get() or http.request() functions, a one-time use Agent with default options will be used for the client connection.

agent:false:

http.get({
  hostname: 'localhost',
  port: 80,
  path: '/',
  agent: false  // create a new agent just for this one request
}, (res) => {
  // Do stuff with response
});

new Agent(options)#

Added in: v0.3.4
  • options <Object> Set of configurable options to set on the agent. Can have the following fields:
    • keepAlive <boolean> Keep sockets around even when there are no outstanding requests, so they can be used for future requests without having to reestablish a TCP connection. Default = false
    • keepAliveMsecs <Integer> When using the keepAlive option, specifies the initial delay for TCP Keep-Alive packets. Ignored when the keepAlive option is false or undefined. Default = 1000.
    • maxSockets <number> Maximum number of sockets to allow per host. Default = Infinity.
    • maxFreeSockets <number> Maximum number of sockets to leave open in a free state. Only relevant if keepAlive is set to true. Default = 256.

The default http.globalAgent that is used by http.request() has all of these values set to their respective defaults.

To configure any of them, you must create your own http.Agent instance.

const http = require('http');
const keepAliveAgent = new http.Agent({ keepAlive: true });
options.agent = keepAliveAgent;
http.request(options, onResponseCallback);

agent.createConnection(options[, callback])#

Added in: v0.11.4

Produces a socket/stream to be used for HTTP requests.

By default, this function is the same as net.createConnection(). However, custom agents may override this method in case greater flexibility is desired.

A socket/stream can be supplied in one of two ways: by returning the socket/stream from this function, or by passing the socket/stream to callback.

callback has a signature of (err, stream).

agent.keepSocketAlive(socket)#

Added in: REPLACEME

Called when socket is detached from a request and could be persisted by the Agent. Default behavior is to:

socket.unref();
socket.setKeepAlive(agent.keepAliveMsecs);

This method can be overridden by a particular Agent subclass. If this method returns a falsy value, the socket will be destroyed instead of persisting it for use with the next request.

agent.reuseSocket(socket, request)#

Added in: REPLACEME

Called when socket is attached to request after being persisted because of the keep-alive options. Default behavior is to:

socket.ref();

This method can be overridden by a particular Agent subclass.

agent.destroy()#

Added in: v0.11.4

Destroy any sockets that are currently in use by the agent.

It is usually not necessary to do this. However, if you are using an agent with keepAlive enabled, then it is best to explicitly shut down the agent when you know that it will no longer be used. Otherwise, sockets may hang open for quite a long time before the server terminates them.

agent.freeSockets#

Added in: v0.11.4

An object which contains arrays of sockets currently awaiting use by the agent when keepAlive is enabled. Do not modify.

agent.getName(options)#

Added in: v0.11.4
  • options <Object> A set of options providing information for name generation
    • host <string> A domain name or IP address of the server to issue the request to
    • port <number> Port of remote server
    • localAddress <string> Local interface to bind for network connections when issuing the request
  • Returns: <String>

Get a unique name for a set of request options, to determine whether a connection can be reused. For an HTTP agent, this returns host:port:localAddress. For an HTTPS agent, the name includes the CA, cert, ciphers, and other HTTPS/TLS-specific options that determine socket reusability.

agent.maxFreeSockets#

Added in: v0.11.7

By default set to 256. For agents with keepAlive enabled, this sets the maximum number of sockets that will be left open in the free state.

agent.maxSockets#

Added in: v0.3.6

By default set to Infinity. Determines how many concurrent sockets the agent can have open per origin. Origin is either a 'host:port' or 'host:port:localAddress' combination.

agent.requests#

Added in: v0.5.9

An object which contains queues of requests that have not yet been assigned to sockets. Do not modify.

agent.sockets#

Added in: v0.3.6

An object which contains arrays of sockets currently in use by the agent. Do not modify.

Class: http.ClientRequest#

Added in: v0.1.17

This object is created internally and returned from http.request(). It represents an in-progress request whose header has already been queued. The header is still mutable using the setHeader(name, value), getHeader(name), removeHeader(name) API. The actual header will be sent along with the first data chunk or when closing the connection.

To get the response, add a listener for 'response' to the request object. 'response' will be emitted from the request object when the response headers have been received. The 'response' event is executed with one argument which is an instance of http.IncomingMessage.

During the 'response' event, one can add listeners to the response object; particularly to listen for the 'data' event.

If no 'response' handler is added, then the response will be entirely discarded. However, if you add a 'response' event handler, then you must consume the data from the response object, either by calling response.read() whenever there is a 'readable' event, or by adding a 'data' handler, or by calling the .resume() method. Until the data is consumed, the 'end' event will not fire. Also, until the data is read it will consume memory that can eventually lead to a 'process out of memory' error.

Note: Node.js does not check whether Content-Length and the length of the body which has been transmitted are equal or not.

The request implements the Writable Stream interface. This is an EventEmitter with the following events:

Event: 'abort'#

Added in: v1.4.1

Emitted when the request has been aborted by the client. This event is only emitted on the first call to abort().

Event: 'connect'#

Added in: v0.7.0

Emitted each time a server responds to a request with a CONNECT method. If this event is not being listened for, clients receiving a CONNECT method will have their connections closed.

A client and server pair that shows you how to listen for the 'connect' event:

const http = require('http');
const net = require('net');
const url = require('url');

// Create an HTTP tunneling proxy
const proxy = http.createServer((req, res) => {
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.end('okay');
});
proxy.on('connect', (req, cltSocket, head) => {
  // connect to an origin server
  const srvUrl = url.parse(`http://${req.url}`);
  const srvSocket = net.connect(srvUrl.port, srvUrl.hostname, () => {
    cltSocket.write('HTTP/1.1 200 Connection Established\r\n' +
                    'Proxy-agent: Node.js-Proxy\r\n' +
                    '\r\n');
    srvSocket.write(head);
    srvSocket.pipe(cltSocket);
    cltSocket.pipe(srvSocket);
  });
});

// now that proxy is running
proxy.listen(1337, '127.0.0.1', () => {

  // make a request to a tunneling proxy
  const options = {
    port: 1337,
    hostname: '127.0.0.1',
    method: 'CONNECT',
    path: 'www.google.com:80'
  };

  const req = http.request(options);
  req.end();

  req.on('connect', (res, socket, head) => {
    console.log('got connected!');

    // make a request over an HTTP tunnel
    socket.write('GET / HTTP/1.1\r\n' +
                 'Host: www.google.com:80\r\n' +
                 'Connection: close\r\n' +
                 '\r\n');
    socket.on('data', (chunk) => {
      console.log(chunk.toString());
    });
    socket.on('end', () => {
      proxy.close();
    });
  });
});

Event: 'continue'#

Added in: v0.3.2

Emitted when the server sends a '100 Continue' HTTP response, usually because the request contained 'Expect: 100-continue'. This is an instruction that the client should send the request body.

Event: 'response'#

Added in: v0.1.0

Emitted when a response is received to this request. This event is emitted only once.

Event: 'socket'#

Added in: v0.5.3

Emitted after a socket is assigned to this request.

Event: 'upgrade'#

Added in: v0.1.94

Emitted each time a server responds to a request with an upgrade. If this event is not being listened for, clients receiving an upgrade header will have their connections closed.

A client server pair that show you how to listen for the 'upgrade' event.

const http = require('http');

// Create an HTTP server
const srv = http.createServer((req, res) => {
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.end('okay');
});
srv.on('upgrade', (req, socket, head) => {
  socket.write('HTTP/1.1 101 Web Socket Protocol Handshake\r\n' +
               'Upgrade: WebSocket\r\n' +
               'Connection: Upgrade\r\n' +
               '\r\n');

  socket.pipe(socket); // echo back
});

// now that server is running
srv.listen(1337, '127.0.0.1', () => {

  // make a request
  const options = {
    port: 1337,
    hostname: '127.0.0.1',
    headers: {
      'Connection': 'Upgrade',
      'Upgrade': 'websocket'
    }
  };

  const req = http.request(options);
  req.end();

  req.on('upgrade', (res, socket, upgradeHead) => {
    console.log('got upgraded!');
    socket.end();
    process.exit(0);
  });
});

request.abort()#

Added in: v0.3.8

Marks the request as aborting. Calling this will cause remaining data in the response to be dropped and the socket to be destroyed.

request.aborted#

Added in: v0.11.14

If a request has been aborted, this value is the time when the request was aborted, in milliseconds since 1 January 1970 00:00:00 UTC.

request.end([data][, encoding][, callback])#

Added in: v0.1.90

Finishes sending the request. If any parts of the body are unsent, it will flush them to the stream. If the request is chunked, this will send the terminating '0\r\n\r\n'.

If data is specified, it is equivalent to calling request.write(data, encoding) followed by request.end(callback).

If callback is specified, it will be called when the request stream is finished.

request.flushHeaders()#

Added in: v1.6.0

Flush the request headers.

For efficiency reasons, Node.js normally buffers the request headers until you call request.end() or write the first chunk of request data. It then tries hard to pack the request headers and data into a single TCP packet.

That's usually what you want (it saves a TCP round-trip) but not when the first data is not sent until possibly much later. request.flushHeaders() lets you bypass the optimization and kickstart the request.

request.setNoDelay([noDelay])#

Added in: v0.5.9

Once a socket is assigned to this request and is connected socket.setNoDelay() will be called.

request.setSocketKeepAlive([enable][, initialDelay])#

Added in: v0.5.9

Once a socket is assigned to this request and is connected socket.setKeepAlive() will be called.

request.setTimeout(timeout[, callback])#

Added in: v0.5.9
  • timeout <number> Milliseconds before a request is considered to be timed out.
  • callback <Function> Optional function to be called when a timeout occurs. Same as binding to the timeout event.

Once a socket is assigned to this request and is connected socket.setTimeout() will be called.

Returns request.

request.write(chunk[, encoding][, callback])#

Added in: v0.1.29

Sends a chunk of the body. By calling this method many times, the user can stream a request body to a server--in that case it is suggested to use the ['Transfer-Encoding', 'chunked'] header line when creating the request.

The encoding argument is optional and only applies when chunk is a string. Defaults to 'utf8'.

The callback argument is optional and will be called when this chunk of data is flushed.

Returns request.

Class: http.Server#

Added in: v0.1.17

This class inherits from net.Server and has the following additional events:

Event: 'checkContinue'#

Added in: v0.3.0

Emitted each time a request with an HTTP Expect: 100-continue is received. If this event is not listened for, the server will automatically respond with a 100 Continue as appropriate.

Handling this event involves calling response.writeContinue() if the client should continue to send the request body, or generating an appropriate HTTP response (e.g. 400 Bad Request) if the client should not continue to send the request body.

Note that when this event is emitted and handled, the 'request' event will not be emitted.

Event: 'checkExpectation'#

Added in: v5.5.0

Emitted each time a request with an HTTP Expect header is received, where the value is not 100-continue. If this event is not listened for, the server will automatically respond with a 417 Expectation Failed as appropriate.

Note that when this event is emitted and handled, the 'request' event will not be emitted.

Event: 'clientError'#

Added in: v0.1.94

If a client connection emits an 'error' event, it will be forwarded here. Listener of this event is responsible for closing/destroying the underlying socket. For example, one may wish to more gracefully close the socket with an HTTP '400 Bad Request' response instead of abruptly severing the connection.

Default behavior is to destroy the socket immediately on malformed request.

socket is the net.Socket object that the error originated from.

const http = require('http');

const server = http.createServer((req, res) => {
  res.end();
});
server.on('clientError', (err, socket) => {
  socket.end('HTTP/1.1 400 Bad Request\r\n\r\n');
});
server.listen(8000);

When the 'clientError' event occurs, there is no request or response object, so any HTTP response sent, including response headers and payload, must be written directly to the socket object. Care must be taken to ensure the response is a properly formatted HTTP response message.

Event: 'close'#

Added in: v0.1.4

Emitted when the server closes.

Event: 'connect'#

Added in: v0.7.0

Emitted each time a client requests an HTTP CONNECT method. If this event is not listened for, then clients requesting a CONNECT method will have their connections closed.

After this event is emitted, the request's socket will not have a 'data' event listener, meaning you will need to bind to it in order to handle data sent to the server on that socket.

Event: 'connection'#

Added in: v0.1.0

When a new TCP stream is established. socket is an object of type net.Socket. Usually users will not want to access this event. In particular, the socket will not emit 'readable' events because of how the protocol parser attaches to the socket. The socket can also be accessed at request.connection.

Event: 'request'#

Added in: v0.1.0

Emitted each time there is a request. Note that there may be multiple requests per connection (in the case of HTTP Keep-Alive connections).

Event: 'upgrade'#

Added in: v0.1.94

Emitted each time a client requests an HTTP upgrade. If this event is not listened for, then clients requesting an upgrade will have their connections closed.

After this event is emitted, the request's socket will not have a 'data' event listener, meaning you will need to bind to it in order to handle data sent to the server on that socket.

server.close([callback])#

Added in: v0.1.90

Stops the server from accepting new connections. See net.Server.close().

server.listen(handle[, callback])#

Added in: v0.5.10

The handle object can be set to either a server or socket (anything with an underlying _handle member), or a {fd: <n>} object.

This will cause the server to accept connections on the specified handle, but it is presumed that the file descriptor or handle has already been bound to a port or domain socket.

Listening on a file descriptor is not supported on Windows.

This function is asynchronous. callback will be added as a listener for the 'listening' event. See also net.Server.listen().

Returns server.

Note: The server.listen() method may be called multiple times. Each subsequent call will re-open the server using the provided options.

server.listen(path[, callback])#

Added in: v0.1.90

Start a UNIX socket server listening for connections on the given path.

This function is asynchronous. callback will be added as a listener for the 'listening' event. See also net.Server.listen(path).

Note: The server.listen() method may be called multiple times. Each subsequent call will re-open the server using the provided options.

server.listen([port][, hostname][, backlog][, callback])#

Added in: v0.1.90

Begin accepting connections on the specified port and hostname. If the hostname is omitted, the server will accept connections on any IPv6 address (::) when IPv6 is available, or any IPv4 address (0.0.0.0) otherwise. Omit the port argument, or use a port value of 0, to have the operating system assign a random port, which can be retrieved by using server.address().port after the 'listening' event has been emitted.

To listen to a unix socket, supply a filename instead of port and hostname.

backlog is the maximum length of the queue of pending connections. The actual length will be determined by your OS through sysctl settings such as tcp_max_syn_backlog and somaxconn on linux. The default value of this parameter is 511 (not 512).

This function is asynchronous. callback will be added as a listener for the 'listening' event. See also net.Server.listen(port).

Note: The server.listen() method may be called multiple times. Each subsequent call will re-open the server using the provided options.

server.listening#

Added in: v5.7.0

A Boolean indicating whether or not the server is listening for connections.

server.maxHeadersCount#

Added in: v0.7.0

Limits maximum incoming headers count, equal to 1000 by default. If set to 0 - no limit will be applied.

server.setTimeout(msecs, callback)#

Added in: v0.9.12

Sets the timeout value for sockets, and emits a 'timeout' event on the Server object, passing the socket as an argument, if a timeout occurs.

If there is a 'timeout' event listener on the Server object, then it will be called with the timed-out socket as an argument.

By default, the Server's timeout value is 2 minutes, and sockets are destroyed automatically if they time out. However, if you assign a callback to the Server's 'timeout' event, then you are responsible for handling socket timeouts.

Returns server.

server.timeout#

Added in: v0.9.12

The number of milliseconds of inactivity before a socket is presumed to have timed out.

Note that the socket timeout logic is set up on connection, so changing this value only affects new connections to the server, not any existing connections.

Set to 0 to disable any kind of automatic timeout behavior on incoming connections.

Class: http.ServerResponse#

Added in: v0.1.17

This object is created internally by an HTTP server--not by the user. It is passed as the second parameter to the 'request' event.

The response implements, but does not inherit from, the Writable Stream interface. This is an EventEmitter with the following events:

Event: 'close'#

Added in: v0.6.7

Indicates that the underlying connection was terminated before response.end() was called or able to flush.

Event: 'finish'#

Added in: v0.3.6

Emitted when the response has been sent. More specifically, this event is emitted when the last segment of the response headers and body have been handed off to the operating system for transmission over the network. It does not imply that the client has received anything yet.

After this event, no more events will be emitted on the response object.

response.addTrailers(headers)#

Added in: v0.3.0

This method adds HTTP trailing headers (a header but at the end of the message) to the response.

Trailers will only be emitted if chunked encoding is used for the response; if it is not (e.g. if the request was HTTP/1.0), they will be silently discarded.

Note that HTTP requires the Trailer header to be sent if you intend to emit trailers, with a list of the header fields in its value. E.g.,

response.writeHead(200, { 'Content-Type': 'text/plain',
                          'Trailer': 'Content-MD5' });
response.write(fileData);
response.addTrailers({'Content-MD5': '7895bf4b8828b55ceaf47747b4bca667'});
response.end();

Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown.

response.end([data][, encoding][, callback])#

Added in: v0.1.90

This method signals to the server that all of the response headers and body have been sent; that server should consider this message complete. The method, response.end(), MUST be called on each response.

If data is specified, it is equivalent to calling response.write(data, encoding) followed by response.end(callback).

If callback is specified, it will be called when the response stream is finished.

response.finished#

Added in: v0.0.2

Boolean value that indicates whether the response has completed. Starts as false. After response.end() executes, the value will be true.

response.getHeader(name)#

Added in: v0.4.0

Reads out a header that's already been queued but not sent to the client. Note that the name is case insensitive.

Example:

const contentType = response.getHeader('content-type');

response.headersSent#

Added in: v0.9.3

Boolean (read-only). True if headers were sent, false otherwise.

response.removeHeader(name)#

Added in: v0.4.0

Removes a header that's queued for implicit sending.

Example:

response.removeHeader('Content-Encoding');

response.sendDate#

Added in: v0.7.5

When true, the Date header will be automatically generated and sent in the response if it is not already present in the headers. Defaults to true.

This should only be disabled for testing; HTTP requires the Date header in responses.

response.setHeader(name, value)#

Added in: v0.4.0

Sets a single header value for implicit headers. If this header already exists in the to-be-sent headers, its value will be replaced. Use an array of strings here if you need to send multiple headers with the same name.

Example:

response.setHeader('Content-Type', 'text/html');

or

response.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']);

Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown.

When headers have been set with response.setHeader(), they will be merged with any headers passed to response.writeHead(), with the headers passed to response.writeHead() given precedence.

// returns content-type = text/plain
const server = http.createServer((req, res) => {
  res.setHeader('Content-Type', 'text/html');
  res.setHeader('X-Foo', 'bar');
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.end('ok');
});

response.setTimeout(msecs, callback)#

Added in: v0.9.12

Sets the Socket's timeout value to msecs. If a callback is provided, then it is added as a listener on the 'timeout' event on the response object.

If no 'timeout' listener is added to the request, the response, or the server, then sockets are destroyed when they time out. If you assign a handler on the request, the response, or the server's 'timeout' events, then it is your responsibility to handle timed out sockets.

Returns response.

response.statusCode#

Added in: v0.4.0

When using implicit headers (not calling response.writeHead() explicitly), this property controls the status code that will be sent to the client when the headers get flushed.

Example:

response.statusCode = 404;

After response header was sent to the client, this property indicates the status code which was sent out.

response.statusMessage#

Added in: v0.11.8

When using implicit headers (not calling response.writeHead() explicitly), this property controls the status message that will be sent to the client when the headers get flushed. If this is left as undefined then the standard message for the status code will be used.

Example:

response.statusMessage = 'Not found';

After response header was sent to the client, this property indicates the status message which was sent out.

response.write(chunk[, encoding][, callback])#

Added in: v0.1.29

If this method is called and response.writeHead() has not been called, it will switch to implicit header mode and flush the implicit headers.

This sends a chunk of the response body. This method may be called multiple times to provide successive parts of the body.

Note that in the http module, the response body is omitted when the request is a HEAD request. Similarly, the 204 and 304 responses must not include a message body.

chunk can be a string or a buffer. If chunk is a string, the second parameter specifies how to encode it into a byte stream. By default the encoding is 'utf8'. callback will be called when this chunk of data is flushed.

Note: This is the raw HTTP body and has nothing to do with higher-level multi-part body encodings that may be used.

The first time response.write() is called, it will send the buffered header information and the first body to the client. The second time response.write() is called, Node.js assumes you're going to be streaming data, and sends that separately. That is, the response is buffered up to the first chunk of body.

Returns true if the entire data was flushed successfully to the kernel buffer. Returns false if all or part of the data was queued in user memory. 'drain' will be emitted when the buffer is free again.

response.writeContinue()#

Added in: v0.3.0

Sends a HTTP/1.1 100 Continue message to the client, indicating that the request body should be sent. See the 'checkContinue' event on Server.

response.writeHead(statusCode[, statusMessage][, headers])#

Added in: v0.1.30

Sends a response header to the request. The status code is a 3-digit HTTP status code, like 404. The last argument, headers, are the response headers. Optionally one can give a human-readable statusMessage as the second argument.

Example:

const body = 'hello world';
response.writeHead(200, {
  'Content-Length': Buffer.byteLength(body),
  'Content-Type': 'text/plain' });

This method must only be called once on a message and it must be called before response.end() is called.

If you call response.write() or response.end() before calling this, the implicit/mutable headers will be calculated and call this function for you.

When headers have been set with response.setHeader(), they will be merged with any headers passed to response.writeHead(), with the headers passed to response.writeHead() given precedence.

// returns content-type = text/plain
const server = http.createServer((req, res) => {
  res.setHeader('Content-Type', 'text/html');
  res.setHeader('X-Foo', 'bar');
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.end('ok');
});

Note that Content-Length is given in bytes not characters. The above example works because the string 'hello world' contains only single byte characters. If the body contains higher coded characters then Buffer.byteLength() should be used to determine the number of bytes in a given encoding. And Node.js does not check whether Content-Length and the length of the body which has been transmitted are equal or not.

Attempting to set a header field name or value that contains invalid characters will result in a TypeError being thrown.

Class: http.IncomingMessage#

Added in: v0.1.17

An IncomingMessage object is created by http.Server or http.ClientRequest and passed as the first argument to the 'request' and 'response' event respectively. It may be used to access response status, headers and data.

It implements the Readable Stream interface, as well as the following additional events, methods, and properties.

Event: 'aborted'#

Added in: v0.3.8

Emitted when the request has been aborted and the network socket has closed.

Event: 'close'#

Added in: v0.4.2

Indicates that the underlying connection was closed. Just like 'end', this event occurs only once per response.

message.destroy([error])#

Added in: v0.3.0

Calls destroy() on the socket that received the IncomingMessage. If error is provided, an 'error' event is emitted and error is passed as an argument to any listeners on the event.

message.headers#

Added in: v0.1.5

The request/response headers object.

Key-value pairs of header names and values. Header names are lower-cased. Example:

// Prints something like:
//
// { 'user-agent': 'curl/7.22.0',
//   host: '127.0.0.1:8000',
//   accept: '*/*' }
console.log(request.headers);

Duplicates in raw headers are handled in the following ways, depending on the header name:

  • Duplicates of age, authorization, content-length, content-type, etag, expires, from, host, if-modified-since, if-unmodified-since, last-modified, location, max-forwards, proxy-authorization, referer, retry-after, or user-agent are discarded.
  • set-cookie is always an array. Duplicates are added to the array.
  • For all other headers, the values are joined together with ', '.

message.httpVersion#

Added in: v0.1.1

In case of server request, the HTTP version sent by the client. In the case of client response, the HTTP version of the connected-to server. Probably either '1.1' or '1.0'.

Also message.httpVersionMajor is the first integer and message.httpVersionMinor is the second.

message.method#

Added in: v0.1.1

Only valid for request obtained from http.Server.

The request method as a string. Read only. Example: 'GET', 'DELETE'.

message.rawHeaders#

Added in: v0.11.6

The raw request/response headers list exactly as they were received.

Note that the keys and values are in the same list. It is not a list of tuples. So, the even-numbered offsets are key values, and the odd-numbered offsets are the associated values.

Header names are not lowercased, and duplicates are not merged.

// Prints something like:
//
// [ 'user-agent',
//   'this is invalid because there can be only one',
//   'User-Agent',
//   'curl/7.22.0',
//   'Host',
//   '127.0.0.1:8000',
//   'ACCEPT',
//   '*/*' ]
console.log(request.rawHeaders);

message.rawTrailers#

Added in: v0.11.6

The raw request/response trailer keys and values exactly as they were received. Only populated at the 'end' event.

message.setTimeout(msecs, callback)#

Added in: v0.5.9

Calls message.connection.setTimeout(msecs, callback).

Returns message.

message.statusCode#

Added in: v0.1.1

Only valid for response obtained from http.ClientRequest.

The 3-digit HTTP response status code. E.G. 404.

message.statusMessage#

Added in: v0.11.10

Only valid for response obtained from http.ClientRequest.

The HTTP response status message (reason phrase). E.G. OK or Internal Server Error.

message.socket#

Added in: v0.3.0

The net.Socket object associated with the connection.

With HTTPS support, use request.socket.getPeerCertificate() to obtain the client's authentication details.

message.trailers#

Added in: v0.3.0

The request/response trailers object. Only populated at the 'end' event.

message.url#

Added in: v0.1.90

Only valid for request obtained from http.Server.

Request URL string. This contains only the URL that is present in the actual HTTP request. If the request is:

GET /status?name=ryan HTTP/1.1\r\n
Accept: text/plain\r\n
\r\n

Then request.url will be:

'/status?name=ryan'

If you would like to parse the URL into its parts, you can use require('url').parse(request.url). Example:

$ node
> require('url').parse('/status?name=ryan')
{
  href: '/status?name=ryan',
  search: '?name=ryan',
  query: 'name=ryan',
  pathname: '/status'
}

If you would like to extract the parameters from the query string, you can use the require('querystring').parse function, or pass true as the second argument to require('url').parse. Example:

$ node
> require('url').parse('/status?name=ryan', true)
{
  href: '/status?name=ryan',
  search: '?name=ryan',
  query: {name: 'ryan'},
  pathname: '/status'
}

http.METHODS#

Added in: v0.11.8

A list of the HTTP methods that are supported by the parser.

http.STATUS_CODES#

Added in: v0.1.22

A collection of all the standard HTTP response status codes, and the short description of each. For example, http.STATUS_CODES[404] === 'Not Found'.

http.createClient([port][, host])#

Added in: v0.1.13 Deprecated since: v0.3.6
Stability: 0 - Deprecated: Use http.request() instead.

Constructs a new HTTP client. port and host refer to the server to be connected to.

http.createServer([requestListener])#

Added in: v0.1.13

Returns a new instance of http.Server.

The requestListener is a function which is automatically added to the 'request' event.

http.get(options[, callback])#

Added in: v0.3.6

Since most requests are GET requests without bodies, Node.js provides this convenience method. The only difference between this method and http.request() is that it sets the method to GET and calls req.end() automatically. Note that the callback must take care to consume the response data for reasons stated in http.ClientRequest section.

The callback is invoked with a single argument that is an instance of http.IncomingMessage

JSON Fetching Example:

http.get('http://nodejs.org/dist/index.json', (res) => {
  const statusCode = res.statusCode;
  const contentType = res.headers['content-type'];

  let error;
  if (statusCode !== 200) {
    error = new Error('Request Failed.\n' +
                      `Status Code: ${statusCode}`);
  } else if (!/^application\/json/.test(contentType)) {
    error = new Error('Invalid content-type.\n' +
                      `Expected application/json but received ${contentType}`);
  }
  if (error) {
    console.log(error.message);
    // consume response data to free up memory
    res.resume();
    return;
  }

  res.setEncoding('utf8');
  let rawData = '';
  res.on('data', (chunk) => rawData += chunk);
  res.on('end', () => {
    try {
      const parsedData = JSON.parse(rawData);
      console.log(parsedData);
    } catch (e) {
      console.log(e.message);
    }
  });
}).on('error', (e) => {
  console.log(`Got error: ${e.message}`);
});

http.globalAgent#

Added in: v0.5.9

Global instance of Agent which is used as the default for all HTTP client requests.

http.request(options[, callback])#

Added in: v0.3.6
  • options <Object>
    • protocol <string> Protocol to use. Defaults to 'http:'.
    • host <string> A domain name or IP address of the server to issue the request to. Defaults to 'localhost'.
    • hostname <string> Alias for host. To support url.parse(), hostname is preferred over host.
    • family <number> IP address family to use when resolving host and hostname. Valid values are 4 or 6. When unspecified, both IP v4 and v6 will be used.
    • port <number> Port of remote server. Defaults to 80.
    • localAddress <string> Local interface to bind for network connections.
    • socketPath <string> Unix Domain Socket (use one of host:port or socketPath).
    • method <string> A string specifying the HTTP request method. Defaults to 'GET'.
    • path <string> Request path. Defaults to '/'. Should include query string if any. E.G. '/index.html?page=12'. An exception is thrown when the request path contains illegal characters. Currently, only spaces are rejected but that may change in the future.
    • headers <Object> An object containing request headers.
    • auth <string> Basic authentication i.e. 'user:password' to compute an Authorization header.
    • agent <http.Agent> | <boolean> Controls Agent behavior. Possible values:
      • undefined (default): use http.globalAgent for this host and port.
      • Agent object: explicitly use the passed in Agent.
      • false: causes a new Agent with default values to be used.
    • createConnection <Function> A function that produces a socket/stream to use for the request when the agent option is not used. This can be used to avoid creating a custom Agent class just to override the default createConnection function. See agent.createConnection() for more details.
    • timeout <Integer>: A number specifying the socket timeout in milliseconds. This will set the timeout before the socket is connected.
  • callback <Function>
  • Returns: <http.ClientRequest>

Node.js maintains several connections per server to make HTTP requests. This function allows one to transparently issue requests.

options can be an object or a string. If options is a string, it is automatically parsed with url.parse().

The optional callback parameter will be added as a one-time listener for the 'response' event.

http.request() returns an instance of the http.ClientRequest class. The ClientRequest instance is a writable stream. If one needs to upload a file with a POST request, then write to the ClientRequest object.

Example:

const postData = querystring.stringify({
  'msg': 'Hello World!'
});

const options = {
  hostname: 'www.google.com',
  port: 80,
  path: '/upload',
  method: 'POST',
  headers: {
    'Content-Type': 'application/x-www-form-urlencoded',
    'Content-Length': Buffer.byteLength(postData)
  }
};

const req = http.request(options, (res) => {
  console.log(`STATUS: ${res.statusCode}`);
  console.log(`HEADERS: ${JSON.stringify(res.headers)}`);
  res.setEncoding('utf8');
  res.on('data', (chunk) => {
    console.log(`BODY: ${chunk}`);
  });
  res.on('end', () => {
    console.log('No more data in response.');
  });
});

req.on('error', (e) => {
  console.log(`problem with request: ${e.message}`);
});

// write data to request body
req.write(postData);
req.end();

Note that in the example req.end() was called. With http.request() one must always call req.end() to signify that you're done with the request - even if there is no data being written to the request body.

If any error is encountered during the request (be that with DNS resolution, TCP level errors, or actual HTTP parse errors) an 'error' event is emitted on the returned request object. As with all 'error' events, if no listeners are registered the error will be thrown.

There are a few special headers that should be noted.

  • Sending a 'Connection: keep-alive' will notify Node.js that the connection to the server should be persisted until the next request.

  • Sending a 'Content-Length' header will disable the default chunked encoding.

  • Sending an 'Expect' header will immediately send the request headers. Usually, when sending 'Expect: 100-continue', you should both set a timeout and listen for the 'continue' event. See RFC2616 Section 8.2.3 for more information.

  • Sending an Authorization header will override using the auth option to compute basic authentication.

HTTPS#

Stability: 2 - Stable

HTTPS is the HTTP protocol over TLS/SSL. In Node.js this is implemented as a separate module.

Class: https.Agent#

Added in: v0.4.5

An Agent object for HTTPS similar to http.Agent. See https.request() for more information.

Class: https.Server#

Added in: v0.3.4

This class is a subclass of tls.Server and emits events same as http.Server. See http.Server for more information.

server.setTimeout([msecs][, callback])#

Added in: v0.11.2

See http.Server#setTimeout().

server.timeout([msecs])#

Added in: v0.11.2
  • msecs <number> Defaults to 120000 (2 minutes).

See http.Server#timeout.

https.createServer(options[, requestListener])#

Added in: v0.3.4

Example:

// curl -k https://localhost:8000/
const https = require('https');
const fs = require('fs');

const options = {
  key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'),
  cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem')
};

https.createServer(options, (req, res) => {
  res.writeHead(200);
  res.end('hello world\n');
}).listen(8000);

Or

const https = require('https');
const fs = require('fs');

const options = {
  pfx: fs.readFileSync('test/fixtures/test_cert.pfx'),
  passphrase: 'sample'
};

https.createServer(options, (req, res) => {
  res.writeHead(200);
  res.end('hello world\n');
}).listen(8000);

server.close([callback])#

Added in: v0.1.90

See http.close() for details.

server.listen(handle[, callback])#

server.listen(path[, callback])#

server.listen([port][, host][, backlog][, callback])#

See http.listen() for details.

https.get(options[, callback])#

Added in: v0.3.6

Like http.get() but for HTTPS.

options can be an object or a string. If options is a string, it is automatically parsed with url.parse().

Example:

const https = require('https');

https.get('https://encrypted.google.com/', (res) => {
  console.log('statusCode:', res.statusCode);
  console.log('headers:', res.headers);

  res.on('data', (d) => {
    process.stdout.write(d);
  });

}).on('error', (e) => {
  console.error(e);
});

https.globalAgent#

Added in: v0.5.9

Global instance of https.Agent for all HTTPS client requests.

https.request(options[, callback])#

Added in: v0.3.6
  • options <Object> | <string> Accepts all options from http.request(), with some differences in default values:
    • protocol Defaults to https:
    • port Defaults to 443.
    • agent Defaults to https.globalAgent.
  • callback <Function>

Makes a request to a secure web server.

The following additional options from tls.connect() are also accepted when using a custom Agent: pfx, key, passphrase, cert, ca, ciphers, rejectUnauthorized, secureProtocol, servername

options can be an object or a string. If options is a string, it is automatically parsed with url.parse().

Example:

const https = require('https');

const options = {
  hostname: 'encrypted.google.com',
  port: 443,
  path: '/',
  method: 'GET'
};

const req = https.request(options, (res) => {
  console.log('statusCode:', res.statusCode);
  console.log('headers:', res.headers);

  res.on('data', (d) => {
    process.stdout.write(d);
  });
});

req.on('error', (e) => {
  console.error(e);
});
req.end();

Example using options from tls.connect():

const options = {
  hostname: 'encrypted.google.com',
  port: 443,
  path: '/',
  method: 'GET',
  key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'),
  cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem')
};
options.agent = new https.Agent(options);

const req = https.request(options, (res) => {
  // ...
});

Alternatively, opt out of connection pooling by not using an Agent.

Example:

const options = {
  hostname: 'encrypted.google.com',
  port: 443,
  path: '/',
  method: 'GET',
  key: fs.readFileSync('test/fixtures/keys/agent2-key.pem'),
  cert: fs.readFileSync('test/fixtures/keys/agent2-cert.pem'),
  agent: false
};

const req = https.request(options, (res) => {
  // ...
});

Internationalization Support#

Node.js has many features that make it easier to write internationalized programs. Some of them are:

Node.js (and its underlying V8 engine) uses ICU to implement these features in native C/C++ code. However, some of them require a very large ICU data file in order to support all locales of the world. Because it is expected that most Node.js users will make use of only a small portion of ICU functionality, only a subset of the full ICU data set is provided by Node.js by default. Several options are provided for customizing and expanding the ICU data set either when building or running Node.js.

Options for building Node.js#

To control how ICU is used in Node.js, four configure options are available during compilation. Additional details on how to compile Node.js are documented in BUILDING.md.

  • --with-intl=none / --without-intl
  • --with-intl=system-icu
  • --with-intl=small-icu (default)
  • --with-intl=full-icu

An overview of available Node.js and JavaScript features for each configure option:

none system-icu small-icu full-icu
String.prototype.normalize() none (function is no-op) full full full
String.prototype.to*Case() full full full full
Intl none (object does not exist) partial/full (depends on OS) partial (English-only) full
String.prototype.localeCompare() partial (not locale-aware) full full full
String.prototype.toLocale*Case() partial (not locale-aware) full full full
Number.prototype.toLocaleString() partial (not locale-aware) partial/full (depends on OS) partial (English-only) full
Date.prototype.toLocale*String() partial (not locale-aware) partial/full (depends on OS) partial (English-only) full

Note: The "(not locale-aware)" designation denotes that the function carries out its operation just like the non-Locale version of the function, if one exists. For example, under none mode, Date.prototype.toLocaleString()'s operation is identical to that of Date.prototype.toString().

Disable all internationalization features (none)#

If this option is chosen, most internationalization features mentioned above will be unavailable in the resulting node binary.

Build with a pre-installed ICU (system-icu)#

Node.js can link against an ICU build already installed on the system. In fact, most Linux distributions already come with ICU installed, and this option would make it possible to reuse the same set of data used by other components in the OS.

Functionalities that only require the ICU library itself, such as String.prototype.normalize(), are fully supported under system-icu. Features that require ICU locale data in addition, such as Intl.DateTimeFormat may be fully or partially supported, depending on the completeness of the ICU data installed on the system.

Embed a limited set of ICU data (small-icu)#

This option makes the resulting binary link against the ICU library statically, and includes a subset of ICU data (typically only the English locale) within the node executable.

Functionalities that only require the ICU library itself, such as String.prototype.normalize(), are fully supported under small-icu. Features that require ICU locale data in addition, such as Intl.DateTimeFormat, generally only work with the English locale:

const january = new Date(9e8);
const english = new Intl.DateTimeFormat('en', { month: 'long' });
const spanish = new Intl.DateTimeFormat('es', { month: 'long' });

console.log(english.format(january));
// Prints "January"
console.log(spanish.format(january));
// Prints "January" or "M01" on small-icu
// Should print "enero"

This mode provides a good balance between features and binary size, and it is the default behavior if no --with-intl flag is passed. The official binaries are also built in this mode.

Providing ICU data at runtime#

If the small-icu option is used, one can still provide additional locale data at runtime so that the JS methods would work for all ICU locales. Assuming the data file is stored at /some/directory, it can be made available to ICU through either:

  • The NODE_ICU_DATA environment variable:

    env NODE_ICU_DATA=/some/directory node

  • The --icu-data-dir CLI parameter:

    node --icu-data-dir=/some/directory

(If both are specified, the --icu-data-dir CLI parameter takes precedence.)

ICU is able to automatically find and load a variety of data formats, but the data must be appropriate for the ICU version, and the file correctly named. The most common name for the data file is icudt5X[bl].dat, where 5X denotes the intended ICU version, and b or l indicates the system's endianness. Check "ICU Data" article in the ICU User Guide for other supported formats and more details on ICU data in general.

The full-icu npm module can greatly simplify ICU data installation by detecting the ICU version of the running node executable and downloading the appropriate data file. After installing the module through npm i full-icu, the data file will be available at ./node_modules/full-icu. This path can be then passed either to NODE_ICU_DATA or --icu-data-dir as shown above to enable full Intl support.

Embed the entire ICU (full-icu)#

This option makes the resulting binary link against ICU statically and include a full set of ICU data. A binary created this way has no further external dependencies and supports all locales, but might be rather large. See BUILDING.md on how to compile a binary using this mode.

Detecting internationalization support#

To verify that ICU is enabled at all (system-icu, small-icu, or full-icu), simply checking the existence of Intl should suffice:

const hasICU = typeof Intl === 'object';

Alternatively, checking for process.versions.icu, a property defined only when ICU is enabled, works too:

const hasICU = typeof process.versions.icu === 'string';

To check for support for a non-English locale (i.e. full-icu or system-icu), Intl.DateTimeFormat can be a good distinguishing factor:

const hasFullICU = (() => {
  try {
    const january = new Date(9e8);
    const spanish = new Intl.DateTimeFormat('es', { month: 'long' });
    return spanish.format(january) === 'enero';
  } catch (err) {
    return false;
  }
})();

For more verbose tests for Intl support, the following resources may be found to be helpful:

  • btest402: Generally used to check whether Node.js with Intl support is built correctly.
  • Test262: ECMAScript's official conformance test suite includes a section dedicated to ECMA-402.

Modules#

Stability: 2 - Stable

In the Node.js module system, each file is treated as a separate module. For example, consider a file named foo.js:

const circle = require('./circle.js');
console.log(`The area of a circle of radius 4 is ${circle.area(4)}`);

On the first line, foo.js loads the module circle.js that is in the same directory as foo.js.

Here are the contents of circle.js:

const { PI } = Math;

exports.area = (r) => PI * r * r;

exports.circumference = (r) => 2 * PI * r;

The module circle.js has exported the functions area() and circumference(). To add functions and objects to the root of your module, you can add them to the special exports object.

Variables local to the module will be private, because the module is wrapped in a function by Node.js (see module wrapper). In this example, the variable PI is private to circle.js.

If you want the root of your module's export to be a function (such as a constructor) or if you want to export a complete object in one assignment instead of building it one property at a time, assign it to module.exports instead of exports.

Below, bar.js makes use of the square module, which exports a constructor:

const Square = require('./square.js');
const mySquare = new Square(2);
console.log(`The area of mySquare is ${mySquare.area()}`);

The square module is defined in square.js:

// assigning to exports will not modify module, must use module.exports
module.exports = (width) => {
  return {
    area: () => width * width
  };
};

The module system is implemented in the require('module') module.

Accessing the main module#

When a file is run directly from Node.js, require.main is set to its module. That means that you can determine whether a file has been run directly by testing require.main === module.

For a file foo.js, this will be true if run via node foo.js, but false if run by require('./foo').

Because module provides a filename property (normally equivalent to __filename), the entry point of the current application can be obtained by checking require.main.filename.

Addenda: Package Manager Tips#

The semantics of Node.js's require() function were designed to be general enough to support a number of reasonable directory structures. Package manager programs such as dpkg, rpm, and npm will hopefully find it possible to build native packages from Node.js modules without modification.

Below we give a suggested directory structure that could work:

Let's say that we wanted to have the folder at /usr/lib/node/<some-package>/<some-version> hold the contents of a specific version of a package.

Packages can depend on one another. In order to install package foo, you may have to install a specific version of package bar. The bar package may itself have dependencies, and in some cases, these dependencies may even collide or form cycles.

Since Node.js looks up the realpath of any modules it loads (that is, resolves symlinks), and then looks for their dependencies in the node_modules folders as described here, this situation is very simple to resolve with the following architecture:

  • /usr/lib/node/foo/1.2.3/ - Contents of the foo package, version 1.2.3.
  • /usr/lib/node/bar/4.3.2/ - Contents of the bar package that foo depends on.
  • /usr/lib/node/foo/1.2.3/node_modules/bar - Symbolic link to /usr/lib/node/bar/4.3.2/.
  • /usr/lib/node/bar/4.3.2/node_modules/* - Symbolic links to the packages that bar depends on.

Thus, even if a cycle is encountered, or if there are dependency conflicts, every module will be able to get a version of its dependency that it can use.

When the code in the foo package does require('bar'), it will get the version that is symlinked into /usr/lib/node/foo/1.2.3/node_modules/bar. Then, when the code in the bar package calls require('quux'), it'll get the version that is symlinked into /usr/lib/node/bar/4.3.2/node_modules/quux.

Furthermore, to make the module lookup process even more optimal, rather than putting packages directly in /usr/lib/node, we could put them in /usr/lib/node_modules/<name>/<version>. Then Node.js will not bother looking for missing dependencies in /usr/node_modules or /node_modules.

In order to make modules available to the Node.js REPL, it might be useful to also add the /usr/lib/node_modules folder to the $NODE_PATH environment variable. Since the module lookups using node_modules folders are all relative, and based on the real path of the files making the calls to require(), the packages themselves can be anywhere.

All Together...#

To get the exact filename that will be loaded when require() is called, use the require.resolve() function.

Putting together all of the above, here is the high-level algorithm in pseudocode of what require.resolve() does:

require(X) from module at path Y
1. If X is a core module,
   a. return the core module
   b. STOP
2. If X begins with '/'
   a. set Y to be the filesystem root
3. If X begins with './' or '/' or '../'
   a. LOAD_AS_FILE(Y + X)
   b. LOAD_AS_DIRECTORY(Y + X)
4. LOAD_NODE_MODULES(X, dirname(Y))
5. THROW "not found"

LOAD_AS_FILE(X)
1. If X is a file, load X as JavaScript text.  STOP
2. If X.js is a file, load X.js as JavaScript text.  STOP
3. If X.json is a file, parse X.json to a JavaScript Object.  STOP
4. If X.node is a file, load X.node as binary addon.  STOP

LOAD_INDEX(X)
1. If X/index.js is a file, load X/index.js as JavaScript text.  STOP
2. If X/index.json is a file, parse X/index.json to a JavaScript object. STOP
3. If X/index.node is a file, load X/index.node as binary addon.  STOP

LOAD_AS_DIRECTORY(X)
1. If X/package.json is a file,
   a. Parse X/package.json, and look for "main" field.
   b. let M = X + (json main field)
   c. LOAD_AS_FILE(M)
   d. LOAD_INDEX(M)
2. LOAD_INDEX(X)

LOAD_NODE_MODULES(X, START)
1. let DIRS=NODE_MODULES_PATHS(START)
2. for each DIR in DIRS:
   a. LOAD_AS_FILE(DIR/X)
   b. LOAD_AS_DIRECTORY(DIR/X)

NODE_MODULES_PATHS(START)
1. let PARTS = path split(START)
2. let I = count of PARTS - 1
3. let DIRS = []
4. while I >= 0,
   a. if PARTS[I] = "node_modules" CONTINUE
   b. DIR = path join(PARTS[0 .. I] + "node_modules")
   c. DIRS = DIRS + DIR
   d. let I = I - 1
5. return DIRS

Caching#

Modules are cached after the first time they are loaded. This means (among other things) that every call to require('foo') will get exactly the same object returned, if it would resolve to the same file.

Multiple calls to require('foo') may not cause the module code to be executed multiple times. This is an important feature. With it, "partially done" objects can be returned, thus allowing transitive dependencies to be loaded even when they would cause cycles.

If you want to have a module execute code multiple times, then export a function, and call that function.

Module Caching Caveats#

Modules are cached based on their resolved filename. Since modules may resolve to a different filename based on the location of the calling module (loading from node_modules folders), it is not a guarantee that require('foo') will always return the exact same object, if it would resolve to different files.

Additionally, on case-insensitive file systems or operating systems, different resolved filenames can point to the same file, but the cache will still treat them as different modules and will reload the file multiple times. For example, require('./foo') and require('./FOO') return two different objects, irrespective of whether or not ./foo and ./FOO are the same file.

Core Modules#

Node.js has several modules compiled into the binary. These modules are described in greater detail elsewhere in this documentation.

The core modules are defined within Node.js's source and are located in the lib/ folder.

Core modules are always preferentially loaded if their identifier is passed to require(). For instance, require('http') will always return the built in HTTP module, even if there is a file by that name.

Cycles#

When there are circular require() calls, a module might not have finished executing when it is returned.

Consider this situation:

a.js:

console.log('a starting');
exports.done = false;
const b = require('./b.js');
console.log('in a, b.done = %j', b.done);
exports.done = true;
console.log('a done');

b.js:

console.log('b starting');
exports.done = false;
const a = require('./a.js');
console.log('in b, a.done = %j', a.done);
exports.done = true;
console.log('b done');

main.js:

console.log('main starting');
const a = require('./a.js');
const b = require('./b.js');
console.log('in main, a.done=%j, b.done=%j', a.done, b.done);

When main.js loads a.js, then a.js in turn loads b.js. At that point, b.js tries to load a.js. In order to prevent an infinite loop, an unfinished copy of the a.js exports object is returned to the b.js module. b.js then finishes loading, and its exports object is provided to the a.js module.

By the time main.js has loaded both modules, they're both finished. The output of this program would thus be:

$ node main.js
main starting
a starting
b starting
in b, a.done = false
b done
in a, b.done = true
a done
in main, a.done=true, b.done=true

If you have cyclic module dependencies in your program, make sure to plan accordingly.

File Modules