Table of Contents

• REPL
  • repl.start([prompt], [stream], [eval], [useGlobal], [ignoreUndefined])
  • REPL Features

REPL#

A Read-Eval-Print-Loop (REPL) is available both as a standalone program and easily includable in other programs. 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.

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

For advanced line-editors, start node with the environmental variable NODE_NO_READLINE=1. This will start the 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"

repl.start([prompt], [stream], [eval], [useGlobal], [ignoreUndefined])#

Starts a REPL with prompt as the prompt and stream for all I/O. prompt is optional and defaults to > . stream is optional and defaults to process.stdin. eval is optional too and defaults to async wrapper for eval().

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

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

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

function eval(cmd, callback) {
  callback(null, result);
}

Multiple REPLs may be started against the same running instance of node. 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;

repl.start("node via stdin> ");

net.createServer(function (socket) {
  connections += 1;
  repl.start("node via Unix socket> ", socket);
}).listen("/tmp/node-repl-sock");

net.createServer(function (socket) {
  connections += 1;
  repl.start("node via TCP socket> ", socket);
}).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 process without restarting it.

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.

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
var repl = require("repl"),
    msg = "message";

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

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

mjr:~$ 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.