nodejs/doc/api/repl.markdown

# REPL

    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);
    ...   });
    1
    2
    3

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.

## 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 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;

    repl.start({
      prompt: "io.js via stdin> ",
      input: process.stdin,
      output: process.stdout
    });

    net.createServer(function (socket) {
      connections += 1;
      repl.start({
        prompt: "io.js via Unix socket> ",
        input: socket,
        output: socket
      }).on('exit', function() {
        socket.end();
      })
    }).listen("/tmp/iojs-repl-sock");

    net.createServer(function (socket) {
      connections += 1;
      repl.start({
        prompt: "io.js via TCP socket> ",
        input: socket,
        output: socket
      }).on('exit', function() {
        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 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: https://gist.github.com/2209310

For an example of running a REPL instance over `curl(1)`,
see: https://gist.github.com/2053342

### 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!');
      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.
    r = repl.start({ options ... });
    someExtension.extend(r.context);

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


## REPL Features

<!-- type=misc -->

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
    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
    '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 = function() {
    ...   return { bar: 'baz' };
    ... };
    [Function]

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

    > obj
    { bar: 'baz' }

[util.inspect()]: util.html#util_util_inspect_object_options
[here]: util.html#util_custom_inspect_function_on_objects