Table of Contents

• REPL
  • Design and Features
    • Commands and Special Keys
    • Default Evaluation
      • JavaScript Expressions
      • Global and Local Scope
      • Accessing Core Node.js Modules
      • Assignment of the _ (underscore) variable
    • Custom Evaluation Functions
      • Recoverable Errors
    • Customizing REPL Output
  • Class: REPLServer
    • Event: 'exit'
    • Event: 'reset'
    • replServer.defineCommand(keyword, cmd)
    • replServer.displayPrompt([preserveCursor])
    • replServer.clearBufferedCommand()
    • replServer.parseREPLKeyword(keyword, [rest])
  • repl.start([options])
  • The Node.js REPL
    • Environment Variable Options
    • Persistent History
      • NODE_REPL_HISTORY_FILE
    • Using the Node.js REPL with advanced line-editors
    • Starting multiple REPL instances against a single running instance

REPL#

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');

Assignment of the _ (underscore) variable#

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'

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#

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'#

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'#

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 as illustrated in the following simple example:

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)#

• 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])#

• 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()#

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])#

• keyword <string> the potential keyword to parse and execute
• rest <any> any parameters to the keyword command

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

repl.start([options])#

• options <Object> | <string>
  • prompt <string> The input prompt to display. Defaults to > (with a trailing space).
  • input <stream.Readable> The Readable stream from which REPL input will be read. Defaults to process.stdin.
  • output <stream.Writable> The Writable stream to which REPL output will be written. Defaults to process.stdout.