Node.js v0.10.42 Manual & Documentation


Stability: 4 - API Frozen

These functions are in the module 'util'. Use require('util') to access them.

util.format(format, [...])#

Returns a formatted string using the first argument as a printf-like format.

The first argument is a string that contains zero or more placeholders. Each placeholder is replaced with the converted value from its corresponding argument. Supported placeholders are:

  • %s - String.
  • %d - Number (both integer and float).
  • %j - JSON.
  • % - single percent sign ('%'). This does not consume an argument.

If the placeholder does not have a corresponding argument, the placeholder is not replaced.

util.format('%s:%s', 'foo'); // 'foo:%s'

If there are more arguments than placeholders, the extra arguments are converted to strings with util.inspect() and these strings are concatenated, delimited by a space.

util.format('%s:%s', 'foo', 'bar', 'baz'); // 'foo:bar baz'

If the first argument is not a format string then util.format() returns a string that is the concatenation of all its arguments separated by spaces. Each argument is converted to a string with util.inspect().

util.format(1, 2, 3); // '1 2 3'


A synchronous output function. Will block the process and output string immediately to stderr.

require('util').debug('message on stderr');


Same as util.debug() except this will output all arguments immediately to stderr.


A synchronous output function. Will block the process and output all arguments to stdout with newlines after each argument.


A synchronous output function. Will block the process, cast each argument to a string then output to stdout. Does not place newlines after each argument.


Output with timestamp on stdout.

require('util').log('Timestamped message.');

util.inspect(object, [options])#

Return a string representation of object, which is useful for debugging.

An optional options object may be passed that alters certain aspects of the formatted string:

  • showHidden - if true then the object's non-enumerable properties will be shown too. Defaults to false.

  • depth - tells 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 below.

  • customInspect - if false, then custom inspect() functions defined on the objects being inspected won't be called. Defaults to true.

Example of inspecting all properties of the util object:

var util = require('util');

console.log(util.inspect(util, { showHidden: true, depth: null }));

Customizing util.inspect colors#

Color output (if enabled) of util.inspect is customizable globally via util.inspect.styles and util.inspect.colors objects.

util.inspect.styles is a map assigning each style a color from util.inspect.colors. Highlighted styles and their default values are: number (yellow) boolean (yellow) string (green) date (magenta) regexp (red) null (bold) undefined (grey) special - only function at this time (cyan) * name (intentionally no styling)

Predefined color codes are: white, grey, black, blue, cyan, green, magenta, red and yellow. There are also bold, italic, underline and inverse codes.

Objects also may define their own inspect(depth) function which util.inspect() will invoke and use the result of when inspecting the object:

var util = require('util');

var obj = { name: 'nate' };
obj.inspect = function(depth) {
  return '{' + + '}';

  // "{nate}"


Returns true if the given "object" is an Array. false otherwise.

var util = require('util');

  // true
util.isArray(new Array)
  // true
  // false


Returns true if the given "object" is a RegExp. false otherwise.

var util = require('util');

util.isRegExp(/some regexp/)
  // true
util.isRegExp(new RegExp('another regexp'))
  // true
  // false


Returns true if the given "object" is a Date. false otherwise.

var util = require('util');

util.isDate(new Date())
  // true
  // false (without 'new' returns a String)
  // false


Returns true if the given "object" is an Error. false otherwise.

var util = require('util');

util.isError(new Error())
  // true
util.isError(new TypeError())
  // true
util.isError({ name: 'Error', message: 'an error occurred' })
  // false

util.pump(readableStream, writableStream, [callback])#

Stability: 0 - Deprecated: Use readableStream.pipe(writableStream)

Read the data from readableStream and send it to the writableStream. When writableStream.write(data) returns false readableStream will be paused until the drain event occurs on the writableStream. callback gets an error as its only argument and is called when writableStream is closed or when an error occurs.

util.inherits(constructor, superConstructor)#

Inherit the prototype methods from one constructor into another. The prototype of constructor will be set to a new object created from superConstructor.

As an additional convenience, superConstructor will be accessible through the constructor.super_ property.

var util = require("util");
var events = require("events");

function MyStream() {;

util.inherits(MyStream, events.EventEmitter);

MyStream.prototype.write = function(data) {
    this.emit("data", data);

var stream = new MyStream();

console.log(stream instanceof events.EventEmitter); // true
console.log(MyStream.super_ === events.EventEmitter); // true

stream.on("data", function(data) {
    console.log('Received data: "' + data + '"');
stream.write("It works!"); // Received data: "It works!"