5 A Read-Eval-Print-Loop (REPL) is available both as a standalone program and
6 easily includable in other programs. The REPL provides a way to interactively
7 run JavaScript and see the results. It can be used for debugging, testing, or
8 just trying things out.
10 By executing `node` without any arguments from the command-line you will be
11 dropped into the REPL. It has simplistic emacs line-editing.
14 Type '.help' for options.
17 > a.forEach(function (v) {
24 For advanced line-editors, start Node.js with the environmental variable
25 `NODE_NO_READLINE=1`. This will start the main and debugger REPL in canonical
26 terminal settings which will allow you to use with `rlwrap`.
28 For example, you could add this to your bashrc file:
30 alias node="env NODE_NO_READLINE=1 rlwrap node"
34 By default, the REPL will persist history between `node` REPL sessions by saving
35 to a `.node_repl_history` file in the user's home directory. This can be
36 disabled by setting the environment variable `NODE_REPL_HISTORY=""`.
38 ### NODE_REPL_HISTORY_FILE
40 Stability: 0 - Deprecated: Use `NODE_REPL_HISTORY` instead.
42 Previously in Node.js/io.js v2.x, REPL history was controlled by using a
43 `NODE_REPL_HISTORY_FILE` environment variable, and the history was saved in JSON
44 format. This variable has now been deprecated, and your REPL history will
45 automatically be converted to using plain text. The new file will be saved to
46 either your home directory, or a directory defined by the `NODE_REPL_HISTORY`
47 variable, as documented below.
49 ## Environment Variable Options
51 The built-in repl (invoked by running `node` or `node -i`) may be controlled
52 via the following environment variables:
54 - `NODE_REPL_HISTORY` - When a valid path is given, persistent REPL history
55 will be saved to the specified file rather than `.node_repl_history` in the
56 user's home directory. Setting this value to `""` will disable persistent
58 - `NODE_REPL_HISTORY_SIZE` - defaults to `1000`. Controls how many lines of
59 history will be persisted if history is available. Must be a positive number.
60 - `NODE_REPL_MODE` - may be any of `sloppy`, `strict`, or `magic`. Defaults
61 to `magic`, which will automatically run "strict mode only" statements in
64 ## repl.start(options)
66 Returns and starts a `REPLServer` instance, that inherits from
67 [Readline Interface][]. Accepts an "options" Object that takes
70 - `prompt` - the prompt and `stream` for all I/O. Defaults to `> `.
72 - `input` - the readable stream to listen to. Defaults to `process.stdin`.
74 - `output` - the writable stream to write readline data to. Defaults to
77 - `terminal` - pass `true` if the `stream` should be treated like a TTY, and
78 have ANSI/VT100 escape codes written to it. Defaults to checking `isTTY`
79 on the `output` stream upon instantiation.
81 - `eval` - function that will be used to eval each given line. Defaults to
82 an async wrapper for `eval()`. See below for an example of a custom `eval`.
84 - `useColors` - a boolean which specifies whether or not the `writer` function
85 should output colors. If a different `writer` function is set then this does
86 nothing. Defaults to the repl's `terminal` value.
88 - `useGlobal` - if set to `true`, then the repl will use the `global` object,
89 instead of running scripts in a separate context. Defaults to `false`.
91 - `ignoreUndefined` - if set to `true`, then the repl will not output the
92 return value of command if it's `undefined`. Defaults to `false`.
94 - `writer` - the function to invoke for each command that gets evaluated which
95 returns the formatting (including coloring) to display. Defaults to
98 - `replMode` - controls whether the repl runs all commands in strict mode,
99 default mode, or a hybrid mode ("magic" mode.) Acceptable values are:
100 * `repl.REPL_MODE_SLOPPY` - run commands in sloppy mode.
101 * `repl.REPL_MODE_STRICT` - run commands in strict mode. This is equivalent to
102 prefacing every repl statement with `'use strict'`.
103 * `repl.REPL_MODE_MAGIC` - attempt to run commands in default mode. If they
104 fail to parse, re-try in strict mode.
106 You can use your own `eval` function if it has following signature:
108 function eval(cmd, context, filename, callback) {
109 callback(null, result);
112 On tab completion - `eval` will be called with `.scope` as an input string. It
113 is expected to return an array of scope names to be used for the auto-completion.
115 Multiple REPLs may be started against the same running instance of Node.js. Each
116 will share the same global object but will have unique I/O.
118 Here is an example that starts a REPL on stdin, a Unix socket, and a TCP socket:
120 var net = require('net'),
121 repl = require('repl'),
125 prompt: 'Node.js via stdin> ',
126 input: process.stdin,
127 output: process.stdout
130 net.createServer(function (socket) {
133 prompt: 'Node.js via Unix socket> ',
136 }).on('exit', function() {
139 }).listen('/tmp/node-repl-sock');
141 net.createServer(function (socket) {
144 prompt: 'Node.js via TCP socket> ',
147 }).on('exit', function() {
152 Running this program from the command line will start a REPL on stdin. Other
153 REPL clients may connect through the Unix socket or TCP socket. `telnet` is useful
154 for connecting to TCP sockets, and `socat` can be used to connect to both Unix and
157 By starting a REPL from a Unix socket-based server instead of stdin, you can
158 connect to a long-running Node.js process without restarting it.
160 For an example of running a "full-featured" (`terminal`) REPL over
161 a `net.Server` and `net.Socket` instance, see: https://gist.github.com/2209310
163 For an example of running a REPL instance over `curl(1)`,
164 see: https://gist.github.com/2053342
170 Emitted when the user exits the REPL in any of the defined ways. Namely, typing
171 `.exit` at the repl, pressing Ctrl+C twice to signal SIGINT, or pressing Ctrl+D
172 to signal "end" on the `input` stream.
174 Example of listening for `exit`:
176 r.on('exit', function () {
177 console.log('Got "exit" event from repl!');
184 `function (context) {}`
186 Emitted when the REPL's context is reset. This happens when you type `.clear`.
187 If you start the repl with `{ useGlobal: true }` then this event will never
190 Example of listening for `reset`:
192 // Extend the initial repl context.
193 var r = repl.start({ options ... });
194 someExtension.extend(r.context);
196 // When a new context is created extend it as well.
197 r.on('reset', function (context) {
198 console.log('repl has a new context');
199 someExtension.extend(context);
207 Inside the REPL, Control+D will exit. Multi-line expressions can be input.
208 Tab completion is supported for both global and local variables.
210 Core modules will be loaded on-demand into the environment. For example,
211 accessing `fs` will `require()` the `fs` module as `global.fs`.
213 The special variable `_` (underscore) contains the result of the last expression.
222 The REPL provides access to any variables in the global scope. You can expose
223 a variable to the REPL explicitly by assigning it to the `context` object
224 associated with each `REPLServer`. For example:
227 var repl = require('repl'),
230 repl.start('> ').context.m = msg;
232 Things in the `context` object appear as local within the REPL:
234 mjr:~$ node repl_test.js
238 There are a few special REPL commands:
240 - `.break` - While inputting a multi-line expression, sometimes you get lost
241 or just don't care about completing it. `.break` will start over.
242 - `.clear` - Resets the `context` object to an empty object and clears any
243 multi-line expression.
244 - `.exit` - Close the I/O stream, which will cause the REPL to exit.
245 - `.help` - Show this list of special commands.
246 - `.save` - Save the current REPL session to a file
247 >.save ./file/to/save.js
248 - `.load` - Load a file into the current REPL session.
249 >.load ./file/to/load.js
251 The following key combinations in the REPL have these special effects:
253 - `<ctrl>C` - Similar to the `.break` keyword. Terminates the current
254 command. Press twice on a blank line to forcibly exit.
255 - `<ctrl>D` - Similar to the `.exit` keyword.
256 - `<tab>` - Show both global and local(scope) variables
259 ### Customizing Object displays in the REPL
261 The REPL module internally uses
262 [util.inspect()][], when printing values. However, `util.inspect` delegates the
263 call to the object's `inspect()` function, if it has one. You can read more
264 about this delegation [here][].
266 For example, if you have defined an `inspect()` function on an object, like this:
268 > var obj = { foo: 'this will not show up in the inspect() output' };
270 > obj.inspect = function() {
271 ... return { bar: 'baz' };
275 and try to print `obj` in REPL, it will invoke the custom `inspect()` function:
280 [Readline Interface]: readline.html#readline_class_interface
281 [util.inspect()]: util.html#util_util_inspect_object_options
282 [here]: util.html#util_custom_inspect_function_on_objects