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# REPL#

Stability: 2 - Stable

The `repl` module provides a Read-Eval-Print-Loop (REPL) implementation that is available both as a standalone program or includible in other applications. It can be accessed using:

    const repl = require('repl');

## Design and Features#

The `repl` module exports the `repl.REPLServer` class. While running, instances of `repl.REPLServer` will accept individual lines of user input, evaluate those according to a user-defined evaluation function, then output the result. Input and output may be from `stdin` and `stdout`, respectively, or may be connected to any Node.js stream.

Instances of `repl.REPLServer` support automatic completion of inputs, simplistic Emacs-style line editing, multi-line inputs, ANSI-styled output, saving and restoring current REPL session state, error recovery, and customizable evaluation functions.

### Commands and Special Keys#

The following special commands are supported by all REPL instances:

- `.break` - When in the process of inputting a multi-line expression, entering the `.break` command (or pressing the `<ctrl>-C` key combination) will abort further input or processing of that expression.
- `.clear` - Resets the REPL `context` to an empty object and clears any multi-line expression currently being input.
- `.exit` - Close the I/O stream, causing 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`
- `.editor` - Enter editor mode (`<ctrl>-D` to finish, `<ctrl>-C` to cancel).

    > .editor
    // Entering editor mode (^D to finish, ^C to cancel)
    function welcome(name) {
      return `Hello ${name}!`;
    }

    welcome('Node.js User');

    // ^D
    'Hello Node.js User!'
    >

The following key combinations in the REPL have these special effects:

- `<ctrl>-C` - When pressed once, has the same effect as the `.break` command. When pressed twice on a blank line, has the same effect as the `.exit` command.
- `<ctrl>-D` - Has the same effect as the `.exit` command.
- `<tab>` - When pressed on a blank line, displays global and local (scope) variables. When pressed while entering other input, displays relevant autocompletion options.

### Default Evaluation#

By default, all instances of `repl.REPLServer` use an evaluation function that evaluates JavaScript expressions and provides access to Node.js' built-in modules. This default behavior can be overridden by passing in an alternative evaluation function when the `repl.REPLServer` instance is created.

#### JavaScript Expressions#

The default evaluator supports direct evaluation of JavaScript expressions:

    > 1 + 1
    2
    > const m = 2
    undefined
    > m + 1
    3

Unless otherwise scoped within blocks or functions, variables declared either implicitly or using the `const`, `let`, or `var` keywords are declared at the global scope.

#### Global and Local Scope#

The default evaluator provides access to any variables that exist in the global scope. It is possible to expose a variable to the REPL explicitly by assigning it to the `context` object associated with each `REPLServer`:

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

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

Properties in the `context` object appear as local within the REPL:

    $ node repl_test.js
    > m
    'message'

Context properties are not read-only by default. To specify read-only globals, context properties must be defined using `Object.defineProperty()`:

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

    const r = repl.start('> ');
    Object.defineProperty(r.context, 'm', {
      configurable: false,
      enumerable: true,
      value: msg
    });

#### Accessing Core Node.js Modules#

The default evaluator will automatically load Node.js core modules into the REPL environment when used. For instance, unless otherwise declared as a global or scoped variable, the input `fs` will be evaluated on-demand as `global.fs = require('fs')`.

    > fs.createReadStream('./some/file');

#### Global Uncaught Exceptions#

The REPL uses the `domain` module to catch all uncaught exceptions for that REPL session.

This use of the `domain` module in the REPL has these side effects:

- Uncaught exceptions do not emit the `'uncaughtException'` event.
- Trying to use `process.setUncaughtExceptionCaptureCallback()` throws an `ERR_DOMAIN_CANNOT_SET_UNCAUGHT_EXCEPTION_CAPTURE` error.

#### Assignment of the `_` (underscore) variable#

History

Version | Changes
--- | ---
v9.8.0 | Added `_error` support.

The default evaluator will, by default, assign the result of the most recently evaluated expression to the special variable `_` (underscore). Explicitly setting `_` to a value will disable this behavior.

    > [ 'a', 'b', 'c' ]
    [ 'a', 'b', 'c' ]
    > _.length
    3
    > _ += 1
    Expression assignment to _ now disabled.
    4
    > 1 + 1
    2
    > _
    4

Similarly, `_error` will refer to the last seen error, if there was any. Explicitly setting `_error` to a value will disable this behavior.

    > throw new Error('foo');
    Error: foo
    > _error.message
    'foo'

#### `await` keyword#

With the `--experimental-repl-await` command line option specified, experimental support for the `await` keyword is enabled.

    > await Promise.resolve(123)
    123
    > await Promise.reject(new Error('REPL await'))
    Error: REPL await
        at repl:1:45
    > const timeout = util.promisify(setTimeout);
    undefined
    > const old = Date.now(); await timeout(1000); console.log(Date.now() - old);
    1002
    undefined

### Custom Evaluation Functions#

When a new `repl.REPLServer` is created, a custom evaluation function may be provided. This can be used, for instance, to implement fully customized REPL applications.

The following illustrates a hypothetical example of a REPL that performs translation of text from one language to another:

    const repl = require('repl');
    const { Translator } = require('translator');

    const myTranslator = new Translator('en', 'fr');

    function myEval(cmd, context, filename, callback) {
      callback(null, myTranslator.translate(cmd));
    }

    repl.start({ prompt: '> ', eval: myEval });

#### Recoverable Errors#

As a user is typing input into the REPL prompt, pressing the `<enter>` key will send the current line of input to the `eval` function. In order to support multi-line input, the eval function can return an instance of `repl.Recoverable` to the provided callback function:

    function myEval(cmd, context, filename, callback) {
      let result;
      try {
        result = vm.runInThisContext(cmd);
      } catch (e) {
        if (isRecoverableError(e)) {
          return callback(new repl.Recoverable(e));
        }
      }
      callback(null, result);
    }

    function isRecoverableError(error) {
      if (error.name === 'SyntaxError') {
        return /^(Unexpected end of input|Unexpected token)/.test(error.message);
      }
      return false;
    }

### Customizing REPL Output#

By default, `repl.REPLServer` instances format output using the `util.inspect()` method before writing the output to the provided `Writable` stream (`process.stdout` by default). The `useColors` boolean option can be specified at construction to instruct the default writer to use ANSI style codes to colorize the output from the `util.inspect()` method.

It is possible to fully customize the output of a `repl.REPLServer` instance by passing a new function in using the `writer` option on construction. The following example, for instance, simply converts any input text to upper case:

    const repl = require('repl');

    const r = repl.start({ prompt: '> ', eval: myEval, writer: myWriter });

    function myEval(cmd, context, filename, callback) {
      callback(null, cmd);
    }

    function myWriter(output) {
      return output.toUpperCase();
    }

## Class: REPLServer#

Added in: v0.1.91

The `repl.REPLServer` class inherits from the `readline.Interface` class. Instances of `repl.REPLServer` are created using the `repl.start()` method and should not be created directly using the JavaScript `new` keyword.

### Event: 'exit'#

Added in: v0.7.7

The `'exit'` event is emitted when the REPL is exited either by receiving the `.exit` command as input, the user pressing `<ctrl>-C` twice to signal `SIGINT`, or by pressing `<ctrl>-D` to signal `'end'` on the input stream. The listener callback is invoked without any arguments.

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

### Event: 'reset'#

Added in: v0.11.0

The `'reset'` event is emitted when the REPL's context is reset. This occurs whenever the `.clear` command is received as input unless the REPL is using the default evaluator and the `repl.REPLServer` instance was created with the `useGlobal` option set to `true`. The listener callback will be called with a reference to the `context` object as the only argument.

This can be used primarily to re-initialize REPL context to some pre-defined state:

    const repl = require('repl');

    function initializeContext(context) {
      context.m = 'test';
    }

    const r = repl.start({ prompt: '> ' });
    initializeContext(r.context);

    r.on('reset', initializeContext);

When this code is executed, the global `'m'` variable can be modified but then reset to its initial value using the `.clear` command:

    $ ./node example.js
    > m
    'test'
    > m = 1
    1
    > m
    1
    > .clear
    Clearing context...
    > m
    'test'
    >

### replServer.defineCommand(keyword, cmd)#

Added in: v0.3.0

- `keyword` <string> The command keyword (without a leading `.` character).
- `cmd` <Object> | <Function> The function to invoke when the command is processed.

The `replServer.defineCommand()` method is used to add new `.`-prefixed commands to the REPL instance. Such commands are invoked by typing a `.` followed by the `keyword`. The `cmd` is either a `Function` or an `Object` with the following properties:

- `help` <string> Help text to be displayed when `.help` is entered (Optional).
- `action` <Function> The function to execute, optionally accepting a single string argument.

The following example shows two new commands added to the REPL instance:

    const repl = require('repl');

    const replServer = repl.start({ prompt: '> ' });
    replServer.defineCommand('sayhello', {
      help: 'Say hello',
      action(name) {
        this.clearBufferedCommand();
        console.log(`Hello, ${name}!`);
        this.displayPrompt();
      }
    });
    replServer.defineCommand('saybye', function saybye() {
      console.log('Goodbye!');
      this.close();
    });

The new commands can then be used from within the REPL instance:

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

### replServer.displayPrompt([preserveCursor])#

Added in: v0.1.91

- `preserveCursor` <boolean>

The `replServer.displayPrompt()` method readies the REPL instance for input from the user, printing the configured `prompt` to a new line in the `output` and resuming the `input` to accept new input.

When multi-line input is being entered, an ellipsis is printed rather than the 'prompt'.

When `preserveCursor` is `true`, the cursor placement will not be reset to `0`.

The `replServer.displayPrompt` method is primarily intended to be called from within the action function for commands registered using the `replServer.defineCommand()` method.

### replServer.clearBufferedCommand()#

Added in: v9.0.0

The `replServer.clearBufferedCommand()` method clears any command that has been buffered but not yet executed. This method is primarily intended to be called from within the action function for commands registered using the `replServer.defineCommand()` method.

### replServer.parseREPLKeyword(keyword[, rest])#

Added in: v0.8.9 Deprecated since: v9.0.0

- `keyword` <string> the potential keyword to parse and execute
- `rest` <any> any parameters to the keyword command
- Returns: <boolean>

Stability: 0 - Deprecated.

An internal method used to parse and execute `REPLServer` keywords. Returns `true` if `keyword` is a valid keyword, otherwise `false`.

## repl.start([options])[src]#

History

Version | Changes
--- | ---
v10.0.0 | The `REPL_MAGIC_MODE` `replMode` was removed.
v5.8.0 | The `options` parameter is optional now.
v0.1.91 | Added in: v0.1.91

- `options` <Object> | <string>

  - `prompt` <string> The input prompt to display. Default: `'> '` (with a trailing space).
  - `input` <stream.Readable> The `Readable` stream from which REPL input will be read. Default: `process.stdin`.
  - `output` <stream.Writable> The `Writable` stream to which REPL output will be written. Default: `process.stdout`.
  - `terminal` <boolean> If `true`, specifies that the `output` should be treated as a TTY terminal, and have ANSI/VT100 escape codes written to it. Default: checking the value of the `isTTY` property on the `output` stream upon instantiation.
  - `eval` <Function> The function to be used when evaluating each given line of input. Default: an async wrapper for the JavaScript `eval()` function. An `eval` function can error with `repl.Recoverable` to indicate the input was incomplete and prompt for additional lines.
  - `useColors` <boolean> If `true`, specifies that the default `writer` function should include ANSI color styling to REPL output. If a custom `writer` function is provided then this has no effect. Default: the REPL instances `terminal` value.
  - `useGlobal` <boolean> If `true`, specifies that the default evaluation function will use the JavaScript `global` as the context as opposed to creating a new separate context for the REPL instance. The node CLI REPL sets this value to `true`. Default: `false`.
  - `ignoreUndefined` <boolean> If `true`, specifies that the default writer will not output the return value of a command if it evaluates to `undefined`. Default: `false`.
  - `writer` <Function> The function to invoke to format the output of each command before writing to `output`. Default: `util.inspect()`.
  - `completer` <Function> An optional function used for custom Tab auto completion. See `readline.InterfaceCompleter` for an example.

  - `replMode` <symbol> A flag that specifies whether the default evaluator executes all JavaScript commands in strict mode or default (sloppy) mode. Acceptable values are:

    - `repl.REPL_MODE_SLOPPY` - evaluates expressions in sloppy mode.
    - `repl.REPL_MODE_STRICT` - evaluates expressions in strict mode. This is equivalent to prefacing every repl statement with `'use strict'`.
  - `breakEvalOnSigint` - Stop evaluating the current piece of code when `SIGINT` is received, i.e. `Ctrl+C` is pressed. This cannot be used together with a custom `eval` function. Default: `false`.
- Returns: <repl.REPLServer>

The `repl.start()` method creates and starts a `repl.REPLServer` instance.

If `options` is a string, then it specifies the input prompt:

    const repl = require('repl');

    // a Unix style prompt
    repl.start('$ ');

## The Node.js REPL#

Node.js itself uses the `repl` module to provide its own interactive interface for executing JavaScript. This can be used by executing the Node.js binary without passing any arguments (or by passing the `-i` argument):

    $ node
    > const a = [1, 2, 3];
    undefined
    > a
    [ 1, 2, 3 ]
    > a.forEach((v) => {
    ...   console.log(v);
    ...   });
    1
    2
    3

### Environment Variable Options#

Various behaviors of the Node.js REPL can be customized using 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` - Controls how many lines of history will be persisted if history is available. Must be a positive number. Default: `1000`.
- `NODE_REPL_MODE` - May be either `'sloppy'` or `'strict'`. Default: `'sloppy'`, which will allow non-strict mode code to be run.

### Persistent History#

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

### Using the Node.js REPL with advanced line-editors#

For advanced line-editors, start Node.js with the environment variable `NODE_NO_READLINE=1`. This will start the main and debugger REPL in canonical terminal settings, which will allow use with `rlwrap`.

For example, the following can be added to a `.bashrc` file:

    alias node="env NODE_NO_READLINE=1 rlwrap node"

### Starting multiple REPL instances against a single running instance#

It is possible to create and run multiple REPL instances against a single running instance of Node.js that share a single `global` object but have separate I/O interfaces.

The following example, for instance, provides separate REPLs on `stdin`, a Unix socket, and a TCP socket:

    const net = require('net');
    const repl = require('repl');
    let 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 application from the command line will start a REPL on stdin. Other REPL clients may connect through the Unix socket or TCP socket. `telnet`, for instance, is useful for connecting to TCP sockets, while `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, it is possible to 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/TooTallNate/2209310.

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