io.js v2.2.0 Documentation


Stability: 2 - Stable

A Read-Eval-Print-Loop (REPL) is available both as a standalone program and easily includable in other programs. The REPL provides a way to interactively run JavaScript and see the results. It can be used for debugging, testing, or just trying things out.

By executing iojs without any arguments from the command-line you will be dropped into the REPL. It has simplistic emacs line-editing.

mjr:~$ iojs
Type '.help' for options.
> a = [ 1, 2, 3];
[ 1, 2, 3 ]
> a.forEach(function (v) {
...   console.log(v);
...   });

For advanced line-editors, start io.js with the environmental 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 iojs="env NODE_NO_READLINE=1 rlwrap iojs"

The built-in repl (invoked by running iojs or iojs -i) may be controlled via the following environment variables:

  • NODE_REPL_HISTORY_FILE - if given, must be a path to a user-writable, user-readable file. When a valid path is given, persistent history support is enabled: REPL history will persist across iojs repl sessions.
  • NODE_REPL_HISTORY_SIZE - defaults to 1000. In conjunction with NODE_REPL_HISTORY_FILE, controls how many lines of history will be persisted. Must be a positive number.
  • NODE_REPL_MODE - may be any of sloppy, strict, or magic. Defaults to magic, which will automatically run "strict mode only" statements in strict mode.


Returns and starts a REPLServer instance, that inherits from [Readline Interface][]. Accepts an "options" Object that takes the following values:

  • prompt - the prompt and stream for all I/O. Defaults to > .

  • input - the readable stream to listen to. Defaults to process.stdin.

  • output - the writable stream to write readline data to. Defaults to process.stdout.

  • terminal - pass true if the stream should be treated like a TTY, and have ANSI/VT100 escape codes written to it. Defaults to checking isTTY on the output stream upon instantiation.

  • eval - function that will be used to eval each given line. Defaults to an async wrapper for eval(). See below for an example of a custom eval.

  • useColors - a boolean which specifies whether or not the writer function should output colors. If a different writer function is set then this does nothing. Defaults to the repl's terminal value.

  • useGlobal - if set to true, then the repl will use the global object, instead of running scripts in a separate context. Defaults to false.

  • ignoreUndefined - if set to true, then the repl will not output the return value of command if it's undefined. Defaults to false.

  • writer - the function to invoke for each command that gets evaluated which returns the formatting (including coloring) to display. Defaults to util.inspect.

  • replMode - controls whether the repl runs all commands in strict mode, default mode, or a hybrid mode ("magic" mode.) Acceptable values are:

    • repl.REPL_MODE_SLOPPY - run commands in sloppy mode.
    • repl.REPL_MODE_STRICT - run commands in strict mode. This is equivalent to prefacing every repl statement with 'use strict'.
    • repl.REPL_MODE_MAGIC - attempt to run commands in default mode. If they fail to parse, re-try in strict mode.

You can use your own eval function if it has following signature:

function eval(cmd, context, filename, callback) {
  callback(null, result);

On tab completion - eval will be called with .scope as an input string. It is expected to return an array of scope names to be used for the auto-completion.

Multiple REPLs may be started against the same running instance of io.js. Each will share the same global object but will have unique I/O.

Here is an example that starts a REPL on stdin, a Unix socket, and a TCP socket:

var net = require("net"),
    repl = require("repl");

connections = 0;

  prompt: "io.js via stdin> ",
  input: process.stdin,
  output: process.stdout

net.createServer(function (socket) {
  connections += 1;
    prompt: "io.js via Unix socket> ",
    input: socket,
    output: socket
  }).on('exit', function() {

net.createServer(function (socket) {
  connections += 1;
    prompt: "io.js via TCP socket> ",
    input: socket,
    output: socket
  }).on('exit', function() {

Running this program from the command line will start a REPL on stdin. Other REPL clients may connect through the Unix socket or TCP socket. telnet is useful for connecting to TCP sockets, and 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, you can connect to a long-running io.js process without restarting it.

For an example of running a "full-featured" (terminal) REPL over a net.Server and net.Socket instance, see:

For an example of running a REPL instance over curl(1), see:

Event: 'exit'#

function () {}

Emitted when the user exits the REPL in any of the defined ways. Namely, typing .exit at the repl, pressing Ctrl+C twice to signal SIGINT, or pressing Ctrl+D to signal "end" on the input stream.

Example of listening for exit:

r.on('exit', function () {
  console.log('Got "exit" event from repl!');

Event: 'reset'#

function (context) {}

Emitted when the REPL's context is reset. This happens when you type .clear. If you start the repl with { useGlobal: true } then this event will never be emitted.

Example of listening for reset:

// Extend the initial repl context.
r = repl.start({ options ... });

// When a new context is created extend it as well.
r.on('reset', function (context) {
  console.log('repl has a new context');

REPL Features#

Inside the REPL, Control+D will exit. Multi-line expressions can be input. Tab completion is supported for both global and local variables.

Core modules will be loaded on-demand into the environment. For example, accessing fs will require() the fs module as global.fs.

The special variable _ (underscore) contains the result of the last expression.

> [ "a", "b", "c" ]
[ 'a', 'b', 'c' ]
> _.length
> _ += 1

The REPL provides access to any variables in the global scope. You can expose a variable to the REPL explicitly by assigning it to the context object associated with each REPLServer. For example:

// repl_test.js
var repl = require("repl"),
    msg = "message";

repl.start("> ").context.m = msg;

Things in the context object appear as local within the REPL:

mjr:~$ iojs repl_test.js
> m

There are a few special REPL commands:

  • .break - While inputting a multi-line expression, sometimes you get lost or just don't care about completing it. .break will start over.
  • .clear - Resets the context object to an empty object and clears any multi-line expression.
  • .exit - Close the I/O stream, which will cause 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

The following key combinations in the REPL have these special effects:

  • <ctrl>C - Similar to the .break keyword. Terminates the current command. Press twice on a blank line to forcibly exit.
  • <ctrl>D - Similar to the .exit keyword.
  • <tab> - Show both global and local(scope) variables