REPL

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

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

For advanced line-editors, start Node.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 node="env NODE_NO_READLINE=1 rlwrap node"

Environment Variable Options

The built-in repl (invoked by running node or node -i) may be controlled via 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 to 1000. Controls how many lines of history will be persisted if history is available. 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.

Persistent History

By default, the REPL will persist history between node REPL sessions by saving to a .node_repl_history file in the user's home directory. This can be disabled by setting the environment variable NODE_REPL_HISTORY="".

NODE_REPL_HISTORY_FILE

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 your REPL history will automatically be converted to using plain text. The new file will be saved to either your home directory, or a directory defined by the NODE_REPL_HISTORY variable, as documented here.

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
3
> _ += 1
4

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
const repl = require('repl');
var msg = 'message';

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

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

$ node repl_test.js
> m
'message'

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

Customizing Object displays in the REPL

The REPL module internally uses util.inspect(), when printing values. However, util.inspect delegates the call to the object's inspect() function, if it has one. You can read more about this delegation here.

For example, if you have defined an inspect() function on an object, like this:

> var obj = {foo: 'this will not show up in the inspect() output'};
undefined
> obj.inspect = () => {
...   return {bar: 'baz'};
... };
[Function]

and try to print obj in REPL, it will invoke the custom inspect() function:

> obj
{bar: 'baz'}

Class: REPLServer

This inherits from Readline Interface with the following events:

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:

replServer.on('exit', () => {
  console.log('Got "exit" event from repl!');
  process.exit();
});

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.
var replServer = repl.start({ options ... });
someExtension.extend(r.context);

// When a new context is created extend it as well.
replServer.on('reset', (context) => {
  console.log('repl has a new context');
  someExtension.extend(context);
});

replServer.defineCommand(keyword, cmd)

• keyword <String>
• cmd <Object> | <Function>

Makes a command available in the REPL. The command is invoked by typing a . followed by the keyword. The cmd is an object with the following values:

• help - help text to be displayed when .help is entered (Optional).
• action - a function to execute, potentially taking in a string argument, when the command is invoked, bound to the REPLServer instance (Required).

If a function is provided instead of an object for cmd, it is treated as the action.

Example of defining a command:

// repl_test.js
const repl = require('repl');

var replServer = repl.start();
replServer.defineCommand('sayhello', {
  help: 'Say hello',
  action: function(name) {
    this.write(`Hello, ${name}!\n`);
    this.displayPrompt();
  }
});

Example of invoking that command from the REPL:

> .sayhello Node.js User
Hello, Node.js User!

replServer.displayPrompt([preserveCursor])

• preserveCursor <Boolean>

Like readline.prompt except also adding indents with ellipses when inside blocks. The preserveCursor argument is passed to readline.prompt. This is used primarily with defineCommand. It's also used internally to render each prompt line.

repl.start(options)

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 Node.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:

const net = require('net');
const repl = require('repl');
var 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 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 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