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Node.js v6.13.0-rc.1 Documentation
Table of Contents
REPL#
The repl
module provides a Read-Eval-Print-Loop (REPL) implementation that
is available both as a standalone program or includible in other applications.
It can be accessed using:
const repl = require('repl');
Design and Features#
The repl
module exports the repl.REPLServer
class. While running, instances
of repl.REPLServer
will accept individual lines of user input, evaluate those
according to a user-defined evaluation function, then output the result. Input
and output may be from stdin
and stdout
, respectively, or may be connected
to any Node.js stream.
Instances of repl.REPLServer
support automatic completion of inputs,
simplistic Emacs-style line editing, multi-line inputs, ANSI-styled output,
saving and restoring current REPL session state, error recovery, and
customizable evaluation functions.
Commands and Special Keys#
The following special commands are supported by all REPL instances:
.break
- When in the process of inputting a multi-line expression, entering the.break
command (or pressing the<ctrl>-C
key combination) will abort further input or processing of that expression..clear
- Resets the REPLcontext
to an empty object and clears any multi-line expression currently being input..exit
- Close the I/O stream, causing the REPL to exit..help
- Show this list of special commands..save
- Save the current REPL session to a file:> .save ./file/to/save.js
.load
- Load a file into the current REPL session.> .load ./file/to/load.js
.editor
- Enter editor mode (<ctrl>-D
to finish,<ctrl>-C
to cancel)
> .editor
// Entering editor mode (^D to finish, ^C to cancel)
function welcome(name) {
return `Hello ${name}!`;
}
welcome('Node.js User');
// ^D
'Hello Node.js User!'
>
The following key combinations in the REPL have these special effects:
<ctrl>-C
- When pressed once, has the same effect as the.break
command. When pressed twice on a blank line, has the same effect as the.exit
command.<ctrl>-D
- Has the same effect as the.exit
command.<tab>
- When pressed on a blank line, displays global and local(scope) variables. When pressed while entering other input, displays relevant autocompletion options.
Default Evaluation#
By default, all instances of repl.REPLServer
use an evaluation function that
evaluates JavaScript expressions and provides access to Node.js' built-in
modules. This default behavior can be overridden by passing in an alternative
evaluation function when the repl.REPLServer
instance is created.
JavaScript Expressions#
The default evaluator supports direct evaluation of JavaScript expressions:
> 1 + 1
2
> var m = 2
undefined
> m + 1
3
Unless otherwise scoped within blocks (e.g. { ... }
) or functions, variables
declared either implicitly or using the var
keyword are declared at the
global
scope.
Global and Local Scope#
The default evaluator provides access to any variables that exist in the global
scope. It is possible to expose a variable to the REPL explicitly by assigning
it to the context
object associated with each REPLServer
. For example:
const repl = require('repl');
const msg = 'message';
repl.start('> ').context.m = msg;
Properties in the context
object appear as local within the REPL:
$ node repl_test.js
> m
'message'
It is important to note that context properties are not read-only by default.
To specify read-only globals, context properties must be defined using
Object.defineProperty()
:
const repl = require('repl');
const msg = 'message';
const r = repl.start('> ');
Object.defineProperty(r.context, 'm', {
configurable: false,
enumerable: true,
value: msg
});
Accessing Core Node.js Modules#
The default evaluator will automatically load Node.js core modules into the
REPL environment when used. For instance, unless otherwise declared as a
global or scoped variable, the input fs
will be evaluated on-demand as
global.fs = require('fs')
.
> fs.createReadStream('./some/file');
Assignment of the _
(underscore) variable#
The default evaluator will, by default, assign the result of the most recently
evaluated expression to the special variable _
(underscore).
> [ 'a', 'b', 'c' ]
[ 'a', 'b', 'c' ]
> _.length
3
> _ += 1
4
Explicitly setting _
to a value will disable this behavior.
Custom Evaluation Functions#
When a new repl.REPLServer
is created, a custom evaluation function may be
provided. This can be used, for instance, to implement fully customized REPL
applications.
The following illustrates a hypothetical example of a REPL that performs translation of text from one language to another:
const repl = require('repl');
const Translator = require('translator').Translator;
const myTranslator = new Translator('en', 'fr');
function myEval(cmd, context, filename, callback) {
callback(null, myTranslator.translate(cmd));
}
repl.start({prompt: '> ', eval: myEval});
Recoverable Errors#
As a user is typing input into the REPL prompt, pressing the <enter>
key will
send the current line of input to the eval
function. In order to support
multi-line input, the eval function can return an instance of repl.Recoverable
to the provided callback function:
function eval(cmd, context, filename, callback) {
let result;
try {
result = vm.runInThisContext(cmd);
} catch (e) {
if (isRecoverableError(e)) {
return callback(new repl.Recoverable(e));
}
}
callback(null, result);
}
function isRecoverableError(error) {
if (error.name === 'SyntaxError') {
return /^(Unexpected end of input|Unexpected token)/.test(error.message);
}
return false;
}
Customizing REPL Output#
By default, repl.REPLServer
instances format output using the
util.inspect()
method before writing the output to the provided Writable
stream (process.stdout
by default). The useColors
boolean option can be
specified at construction to instruct the default writer to use ANSI style
codes to colorize the output from the util.inspect()
method.
It is possible to fully customize the output of a repl.REPLServer
instance
by passing a new function in using the writer
option on construction. The
following example, for instance, simply converts any input text to upper case:
const repl = require('repl');
const r = repl.start({prompt: '>', eval: myEval, writer: myWriter});
function myEval(cmd, context, filename, callback) {
callback(null, cmd);
}
function myWriter(output) {
return output.toUpperCase();
}
Class: REPLServer#
The repl.REPLServer
class inherits from the readline.Interface
class.
Instances of repl.REPLServer
are created using the repl.start()
method and
should not be created directly using the JavaScript new
keyword.
Event: 'exit'#
The 'exit'
event is emitted when the REPL is exited either by receiving the
.exit
command as input, the user pressing <ctrl>-C
twice to signal SIGINT
,
or by pressing <ctrl>-D
to signal 'end'
on the input stream. The listener
callback is invoked without any arguments.
replServer.on('exit', () => {
console.log('Received "exit" event from repl!');
process.exit();
});
Event: 'reset'#
The 'reset'
event is emitted when the REPL's context is reset. This occurs
whenever the .clear
command is received as input unless the REPL is using
the default evaluator and the repl.REPLServer
instance was created with the
useGlobal
option set to true
. The listener callback will be called with a
reference to the context
object as the only argument.
This can be used primarily to re-initialize REPL context to some pre-defined state as illustrated in the following simple example:
const repl = require('repl');
function initializeContext(context) {
context.m = 'test';
}
const r = repl.start({prompt: '>'});
initializeContext(r.context);
r.on('reset', initializeContext);
When this code is executed, the global 'm'
variable can be modified but then
reset to its initial value using the .clear
command:
$ ./node example.js
>m
'test'
>m = 1
1
>m
1
>.clear
Clearing context...
>m
'test'
>
replServer.defineCommand(keyword, cmd)#
keyword
<string> The command keyword (without a leading.
character).cmd
<Object> | <Function> The function to invoke when the command is processed.
The replServer.defineCommand()
method is used to add new .
-prefixed commands
to the REPL instance. Such commands are invoked by typing a .
followed by the
keyword
. The cmd
is either a Function or an object with the following
properties:
help
<string> Help text to be displayed when.help
is entered (Optional).action
<Function> The function to execute, optionally accepting a single string argument.
The following example shows two new commands added to the REPL instance:
const repl = require('repl');
const replServer = repl.start({prompt: '> '});
replServer.defineCommand('sayhello', {
help: 'Say hello',
action: function(name) {
this.lineParser.reset();
this.bufferedCommand = '';
console.log(`Hello, ${name}!`);
this.displayPrompt();
}
});
replServer.defineCommand('saybye', function() {
console.log('Goodbye!');
this.close();
});
The new commands can then be used from within the REPL instance:
> .sayhello Node.js User
Hello, Node.js User!
> .saybye
Goodbye!
replServer.displayPrompt([preserveCursor])#
preserveCursor
<boolean>
The replServer.displayPrompt()
method readies the REPL instance for input
from the user, printing the configured prompt
to a new line in the output
and resuming the input
to accept new input.
When multi-line input is being entered, an ellipsis is printed rather than the 'prompt'.
When preserveCursor
is true
, the cursor placement will not be reset to 0
.
The replServer.displayPrompt
method is primarily intended to be called from
within the action function for commands registered using the
replServer.defineCommand()
method.
repl.start([options])#
options
<Object> | <string>prompt
<string> The input prompt to display. Defaults to>
.input
<Readable> The Readable stream from which REPL input will be read. Defaults toprocess.stdin
.output
<Writable> The Writable stream to which REPL output will be written. Defaults toprocess.stdout
.terminal
<boolean> Iftrue
, specifies that theoutput
should be treated as a TTY terminal, and have ANSI/VT100 escape codes written to it. Defaults to checking the value of theisTTY
property on theoutput
stream upon instantiation.eval
<Function> The function to be used when evaluating each given line of input. Defaults to an async wrapper for the JavaScripteval()
function. Aneval
function can error withrepl.Recoverable
to indicate the input was incomplete and prompt for additional lines.useColors
<boolean> Iftrue
, specifies that the defaultwriter
function should include ANSI color styling to REPL output. If a customwriter
function is provided then this has no effect. Defaults to the REPL instancesterminal
value.useGlobal
<boolean> Iftrue
, specifies that the default evaluation function will use the JavaScriptglobal
as the context as opposed to creating a new separate context for the REPL instance. Defaults tofalse
.ignoreUndefined
<boolean> Iftrue
, specifies that the default writer will not output the return value of a command if it evaluates toundefined
. Defaults tofalse
.writer
<Function> The function to invoke to format the output of each command before writing tooutput
. Defaults toutil.inspect()
.completer
<Function> An optional function used for custom Tab auto completion. Seereadline.InterfaceCompleter
for an example.replMode
- A flag that specifies whether the default evaluator executes all JavaScript commands in strict mode, default mode, or a hybrid mode ("magic" mode.) Acceptable values are:repl.REPL_MODE_SLOPPY
- evaluates expressions in sloppy mode.repl.REPL_MODE_STRICT
- evaluates expressions in strict mode. This is equivalent to prefacing every repl statement with'use strict'
.repl.REPL_MODE_MAGIC
- attempt to evaluates expressions in default mode. If expressions fail to parse, re-try in strict mode.
breakEvalOnSigint
- Stop evaluating the current piece of code whenSIGINT
is received, i.e.Ctrl+C
is pressed. This cannot be used together with a customeval
function. Defaults tofalse
.
The repl.start()
method creates and starts a repl.REPLServer
instance.
If options
is a string, then it specifies the input prompt:
const repl = require('repl');
// a Unix style prompt
repl.start('$ ');
The Node.js REPL#
Node.js itself uses the repl
module to provide its own interactive interface
for executing JavaScript. This can be used by executing the Node.js binary
without passing any arguments (or by passing the -i
argument):
$ node
> a = [1, 2, 3];
[ 1, 2, 3 ]
> a.forEach((v) => {
... console.log(v);
... });
1
2
3
Environment Variable Options#
Various behaviors of the Node.js REPL can be customized using the following environment variables:
NODE_REPL_HISTORY
- When a valid path is given, persistent REPL history will be saved to the specified file rather than.node_repl_history
in the user's home directory. Setting this value to""
will disable persistent REPL history. Whitespace will be trimmed from the value.NODE_REPL_HISTORY_SIZE
- Defaults to1000
. Controls how many lines of history will be persisted if history is available. Must be a positive number.NODE_REPL_MODE
- May be any ofsloppy
,strict
, ormagic
. Defaults tomagic
, which will automatically run "strict mode only" statements in strict mode.
Persistent History#
By default, the Node.js REPL will persist history between node
REPL sessions
by saving inputs to a .node_repl_history
file located in the user's home
directory. This can be disabled by setting the environment variable
NODE_REPL_HISTORY=""
.
NODE_REPL_HISTORY_FILE#
NODE_REPL_HISTORY
instead.Previously in Node.js/io.js v2.x, REPL history was controlled by using a
NODE_REPL_HISTORY_FILE
environment variable, and the history was saved in JSON
format. This variable has now been deprecated, and the old JSON REPL history
file will be automatically converted to a simplified plain text format. This new
file will be saved to either the user's home directory, or a directory defined
by the NODE_REPL_HISTORY
variable, as documented in the
Environment Variable Options.
Using the Node.js REPL with advanced line-editors#
For advanced line-editors, start Node.js with the environment variable
NODE_NO_READLINE=1
. This will start the main and debugger REPL in canonical
terminal settings which will allow you to use with rlwrap
.
For example, you could add this to your bashrc file:
alias node="env NODE_NO_READLINE=1 rlwrap node"
Starting multiple REPL instances against a single running instance#
It is possible to create and run multiple REPL instances against a single
running instance of Node.js that share a single global
object but have
separate I/O interfaces.
The following example, for instance, provides separate REPLs on stdin
, a Unix
socket, and a TCP socket:
const net = require('net');
const repl = require('repl');
let connections = 0;
repl.start({
prompt: 'Node.js via stdin> ',
input: process.stdin,
output: process.stdout
});
net.createServer((socket) => {
connections += 1;
repl.start({
prompt: 'Node.js via Unix socket> ',
input: socket,
output: socket
}).on('exit', () => {
socket.end();
});
}).listen('/tmp/node-repl-sock');
net.createServer((socket) => {
connections += 1;
repl.start({
prompt: 'Node.js via TCP socket> ',
input: socket,
output: socket
}).on('exit', () => {
socket.end();
});
}).listen(5001);
Running this application from the command line will start a REPL on stdin.
Other REPL clients may connect through the Unix socket or TCP socket. telnet
,
for instance, is useful for connecting to TCP sockets, while socat
can be used
to connect to both Unix and TCP sockets.
By starting a REPL from a Unix socket-based server instead of stdin, it is possible to connect to a long-running Node.js process without restarting it.
For an example of running a "full-featured" (terminal
) REPL over
a net.Server
and net.Socket
instance, see: https://gist.github.com/2209310
For an example of running a REPL instance over curl(1), see: https://gist.github.com/2053342