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Readline

Stability: 2 - Stable

The readline module provides an interface for reading data from a Readable stream (such as process.stdin) one line at a time. It can be accessed using:

const readline = require('readline');

The following simple example illustrates the basic use of the readline module.

const readline = require('readline');

const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});

rl.question('What do you think of Node.js? ', (answer) => {
  // TODO: Log the answer in a database
  console.log(`Thank you for your valuable feedback: ${answer}`);

  rl.close();
});

Note Once this code is invoked, the Node.js application will not terminate until the readline.Interface is closed because the interface waits for data to be received on the input stream.

Class: Interface

Instances of the readline.Interface class are constructed using the readline.createInterface() method. Every instance is associated with a single input Readable stream and a single output Writable stream. The output stream is used to print prompts for user input that arrives on, and is read from, the input stream.

Event: 'close'

The 'close' event is emitted when one of the following occur:

  • The rl.close() method is called and the readline.Interface instance has relinquished control over the input and output streams;
  • The input stream receives its 'end' event;
  • The input stream receives <ctrl>-D to signal end-of-transmission (EOT);
  • The input stream receives <ctrl>-C to signal SIGINT and there is no SIGINT event listener registered on the readline.Interface instance.

The listener function is called without passing any arguments.

The readline.Interface instance should be considered to be "finished" once the 'close' event is emitted.

Event: 'line'

The 'line' event is emitted whenever the input stream receives an end-of-line input (\n, \r, or \r\n). This usually occurs when the user presses the <Enter>, or <Return> keys.

The listener function is called with a string containing the single line of received input.

For example:

rl.on('line', (input) => {
  console.log(`Received: ${input}`);
});

Event: 'pause'

The 'pause' event is emitted when one of the following occur:

  • The input stream is paused.
  • The input stream is not paused and receives the SIGCONT event. (See events SIGTSTP and SIGCONT)

The listener function is called without passing any arguments.

For example:

rl.on('pause', () => {
  console.log('Readline paused.');
});

Event: 'resume'

The 'resume' event is emitted whenever the input stream is resumed.

The listener function is called without passing any arguments.

rl.on('resume', () => {
  console.log('Readline resumed.');
});

Event: 'SIGCONT'

The 'SIGCONT' event is emitted when a Node.js process previously moved into the background using <ctrl>-Z (i.e. SIGTSTP) is then brought back to the foreground using fg(1).

If the input stream was paused before the SIGTSTP request, this event will not be emitted.

The listener function is invoked without passing any arguments.

For example:

rl.on('SIGCONT', () => {
  // `prompt` will automatically resume the stream
  rl.prompt();
});

Note: The 'SIGCONT' event is not supported on Windows.

Event: 'SIGINT'

The 'SIGINT' event is emitted whenever the input stream receives a <ctrl>-C input, known typically as SIGINT. If there are no 'SIGINT' event listeners registered when the input stream receives a SIGINT, the 'pause' event will be emitted.

The listener function is invoked without passing any arguments.

For example:

rl.on('SIGINT', () => {
  rl.question('Are you sure you want to exit?', (answer) => {
    if (answer.match(/^y(es)?$/i)) rl.pause();
  });
});

Event: 'SIGTSTP'

The 'SIGTSTP' event is emitted when the input stream receives a <ctrl>-Z input, typically known as SIGTSTP. If there are no SIGTSTP event listeners registered when the input stream receives a SIGTSTP, the Node.js process will be sent to the background.

When the program is resumed using fg(1), the 'pause' and SIGCONT events will be emitted. These can be used to resume the input stream.

The 'pause' and 'SIGCONT' events will not be emitted if the input was paused before the process was sent to the background.

The listener function is invoked without passing any arguments.

For example:

rl.on('SIGTSTP', () => {
  // This will override SIGTSTP and prevent the program from going to the
  // background.
  console.log('Caught SIGTSTP.');
});

Note: The 'SIGTSTP' event is not supported on Windows.

rl.close()

The rl.close() method closes the readline.Interface instance and relinquishes control over the input and output streams. When called, the 'close' event will be emitted.

rl.pause()

The rl.pause() method pauses the input stream, allowing it to be resumed later if necessary.

Calling rl.pause() does not immediately pause other events (including 'line') from being emitted by the readline.Interface instance.

rl.prompt([preserveCursor])

  • preserveCursor <boolean> If true, prevents the cursor placement from being reset to 0.

The rl.prompt() method writes the readline.Interface instances configured prompt to a new line in output in order to provide a user with a new location at which to provide input.

When called, rl.prompt() will resume the input stream if it has been paused.

If the readline.Interface was created with output set to null or undefined the prompt is not written.

rl.question(query, callback)

  • query <String> A statement or query to write to output, prepended to the prompt.
  • callback <Function> A callback function that is invoked with the user's input in response to the query.

The rl.question() method displays the query by writing it to the output, waits for user input to be provided on input, then invokes the callback function passing the provided input as the first argument.

When called, rl.question() will resume the input stream if it has been paused.

If the readline.Interface was created with output set to null or undefined the query is not written.

Example usage:

rl.question('What is your favorite food?', (answer) => {
  console.log(`Oh, so your favorite food is ${answer}`);
});

Note: The callback function passed to rl.question() does not follow the typical pattern of accepting an Error object or null as the first argument. The callback is called with the provided answer as the only argument.

rl.resume()

The rl.resume() method resumes the input stream if it has been paused.

rl.setPrompt(prompt)

The rl.setPrompt() method sets the prompt that will be written to output whenever rl.prompt() is called.

rl.write(data[, key])

  • data <String>
  • key <Object>
    • ctrl <boolean> true to indicate the <ctrl> key.
    • meta <boolean> true to indicate the <Meta> key.
    • shift <boolean> true to indicate the <Shift> key.
    • name <String> The name of the a key.

The rl.write() method will write either data or a key sequence identified by key to the output. The key argument is supported only if output is a TTY text terminal.

If key is specified, data is ignored.

When called, rl.write() will resume the input stream if it has been paused.

If the readline.Interface was created with output set to null or undefined the data and key are not written.

For example:

rl.write('Delete this!');
// Simulate Ctrl+u to delete the line written previously
rl.write(null, {ctrl: true, name: 'u'});

Note: The rl.write() method will write the data to the readline Interface's input as if it were provided by the user.

readline.clearLine(stream, dir)

  • stream <Writable>
  • dir <number>
    • -1 - to the left from cursor
    • 1 - to the right from cursor
    • 0 - the entire line

The readline.clearLine() method clears current line of given TTY stream in a specified direction identified by dir.

readline.clearScreenDown(stream)

  • stream <Writable>

The readline.clearScreenDown() method clears the given TTY stream from the current position of the cursor down.

readline.createInterface(options)

  • options <Object>
    • input <Readable> The Readable stream to listen to. This option is required.
    • output <Writable> The Writable stream to write readline data to.
    • completer <Function> An optional function used for Tab autocompletion.
    • terminal <boolean> true if the input and output streams 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.
    • historySize <number> maximum number of history lines retained. To disable the history set this value to 0. Defaults to 30. This option makes sense only if terminal is set to true by the user or by an internal output check, otherwise the history caching mechanism is not initialized at all.
    • prompt - the prompt string to use. Default: '> '
    • crlfDelay <number> If the delay between \r and \n exceeds crlfDelay milliseconds, both \r and \n will be treated as separate end-of-line input. Default to 100 milliseconds. crlfDelay will be coerced to [100, 2000] range.
    • removeHistoryDuplicates <boolean> If true, when a new input line added to the history list duplicates an older one, this removes the older line from the list. Defaults to false.

The readline.createInterface() method creates a new readline.Interface instance.

For example:

const readline = require('readline');
const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});

Once the readline.Interface instance is created, the most common case is to listen for the 'line' event:

rl.on('line', (line) => {
  console.log(`Received: ${line}`);
});

If terminal is true for this instance then the output stream will get the best compatibility if it defines an output.columns property and emits a 'resize' event on the output if or when the columns ever change (process.stdout does this automatically when it is a TTY).

Use of the completer Function

The completer function takes the current line entered by the user as an argument, and returns an Array with 2 entries:

  • An Array with matching entries for the completion.
  • The substring that was used for the matching.

For instance: [[substr1, substr2, ...], originalsubstring].

function completer(line) {
  const completions = '.help .error .exit .quit .q'.split(' ');
  const hits = completions.filter((c) => { return c.indexOf(line) === 0 });
  // show all completions if none found
  return [hits.length ? hits : completions, line];
}

The completer function can be called asynchronously if it accepts two arguments:

function completer(linePartial, callback) {
  callback(null, [['123'], linePartial]);
}

readline.cursorTo(stream, x, y)

  • stream <Writable>
  • x <number>
  • y <number>

The readline.cursorTo() method moves cursor to the specified position in a given TTY stream.

readline.emitKeypressEvents(stream[, interface])

  • stream <Readable>
  • interface <readline.Interface>

The readline.emitKeypressEvents() method causes the given Writable stream to begin emitting 'keypress' events corresponding to received input.

Optionally, interface specifies a readline.Interface instance for which autocompletion is disabled when copy-pasted input is detected.

If the stream is a TTY, then it must be in raw mode.

readline.emitKeypressEvents(process.stdin);
if (process.stdin.isTTY)
  process.stdin.setRawMode(true);

readline.moveCursor(stream, dx, dy)

  • stream <Writable>
  • dx <number>
  • dy <Number>

The readline.moveCursor() method moves the cursor relative to its current position in a given TTY stream.

Example: Tiny CLI

The following example illustrates the use of readline.Interface class to implement a small command-line interface:

const readline = require('readline');
const rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
  prompt: 'OHAI> '
});

rl.prompt();

rl.on('line', (line) => {
  switch(line.trim()) {
    case 'hello':
      console.log('world!');
      break;
    default:
      console.log(`Say what? I might have heard '${line.trim()}'`);
      break;
  }
  rl.prompt();
}).on('close', () => {
  console.log('Have a great day!');
  process.exit(0);
});

Example: Read File Stream Line-by-Line

A common use case for readline is to consume input from a filesystem Readable stream one line at a time, as illustrated in the following example:

const readline = require('readline');
const fs = require('fs');

const rl = readline.createInterface({
  input: fs.createReadStream('sample.txt')
});

rl.on('line', (line) => {
  console.log(`Line from file: ${line}`);
});

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https://nodejs.org/dist/latest-v6.x/docs/api/readline.html