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@nodejs

# 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.