Node.js v21.0.0-nightly2023100285c09f178c documentation

Class: InterfaceConstructor#

• Extends: <EventEmitter>

Instances of the InterfaceConstructor class are constructed using the readlinePromises.createInterface() or 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 InterfaceConstructor 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 InterfaceConstructor instance.

The listener function is called without passing any arguments.

The InterfaceConstructor instance is 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 Enter or Return.

The 'line' event is also emitted if new data has been read from a stream and that stream ends without a final end-of-line marker.

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

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

Event: 'history'#

The 'history' event is emitted whenever the history array has changed.

The listener function is called with an array containing the history array. It will reflect all changes, added lines and removed lines due to historySize and removeHistoryDuplicates.

The primary purpose is to allow a listener to persist the history. It is also possible for the listener to change the history object. This could be useful to prevent certain lines to be added to the history, like a password.

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

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.

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

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.');
}); copy

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(1p).

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.

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

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.

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

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(1p), 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.

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

The 'SIGTSTP' event is not supported on Windows.

rl.close()#

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

Calling rl.close() does not immediately stop other events (including 'line') from being emitted by the InterfaceConstructor instance.

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 InterfaceConstructor instance.

rl.prompt([preserveCursor])#

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

The rl.prompt() method writes the InterfaceConstructor 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 InterfaceConstructor was created with output set to null or undefined the prompt is not written.

rl.resume()#

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

rl.setPrompt(prompt)#

• prompt <string>

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

rl.getPrompt()#

• Returns: <string> the current prompt string

The rl.getPrompt() method returns the current prompt used by rl.prompt().

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. See TTY keybindings for a list of key combinations.

If key is specified, data is ignored.

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

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

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

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

rl[Symbol.asyncIterator]()#

• Returns: <AsyncIterator>

Create an AsyncIterator object that iterates through each line in the input stream as a string. This method allows asynchronous iteration of InterfaceConstructor objects through for await...of loops.

Errors in the input stream are not forwarded.

If the loop is terminated with break, throw, or return, rl.close() will be called. In other words, iterating over a InterfaceConstructor will always consume the input stream fully.

Performance is not on par with the traditional 'line' event API. Use 'line' instead for performance-sensitive applications.

async function processLineByLine() {
  const rl = readline.createInterface({
    // ...
  });

  for await (const line of rl) {
    // Each line in the readline input will be successively available here as
    // `line`.
  }
} copy

readline.createInterface() will start to consume the input stream once invoked. Having asynchronous operations between interface creation and asynchronous iteration may result in missed lines.

rl.line#

• <string>

The current input data being processed by node.

This can be used when collecting input from a TTY stream to retrieve the current value that has been processed thus far, prior to the line event being emitted. Once the line event has been emitted, this property will be an empty string.

Be aware that modifying the value during the instance runtime may have unintended consequences if rl.cursor is not also controlled.

If not using a TTY stream for input, use the 'line' event.

One possible use case would be as follows:

const values = ['lorem ipsum', 'dolor sit amet'];
const rl = readline.createInterface(process.stdin);
const showResults = debounce(() => {
  console.log(
    '\n',
    values.filter((val) => val.startsWith(rl.line)).join(' '),
  );
}, 300);
process.stdin.on('keypress', (c, k) => {
  showResults();
}); copy

rl.cursor#

• <number> | <undefined>

The cursor position relative to rl.line.

This will track where the current cursor lands in the input string, when reading input from a TTY stream. The position of cursor determines the portion of the input string that will be modified as input is processed, as well as the column where the terminal caret will be rendered.

rl.getCursorPos()#

• Returns: <Object>
  • rows <number> the row of the prompt the cursor currently lands on
  • cols <number> the screen column the cursor currently lands on

Returns the real position of the cursor in relation to the input prompt + string. Long input (wrapping) strings, as well as multiple line prompts are included in the calculations.

Promises API#

Class: readlinePromises.Interface#

• Extends: <readline.InterfaceConstructor>

Instances of the readlinePromises.Interface class are constructed using the readlinePromises.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.

rl.question(query[, options])#

• query <string> A statement or query to write to output, prepended to the prompt.
• options <Object>
  • signal <AbortSignal> Optionally allows the question() to be canceled using an AbortSignal.
• Returns: <Promise> A promise that is fulfilled 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 readlinePromises.Interface was created with output set to null or undefined the query is not written.

If the question is called after rl.close(), it returns a rejected promise.

Example usage:

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

Using an AbortSignal to cancel a question.

const signal = AbortSignal.timeout(10_000);

signal.addEventListener('abort', () => {
  console.log('The food question timed out');
}, { once: true });

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

Class: readlinePromises.Readline#

new readlinePromises.Readline(stream[, options])#

• stream <stream.Writable> A TTY stream.
• options <Object>
  • autoCommit <boolean> If true, no need to call rl.commit().

rl.clearLine(dir)#

• dir <integer>
  • -1: to the left from cursor
  • 1: to the right from cursor
  • 0: the entire line
• Returns: this

The rl.clearLine() method adds to the internal list of pending action an action that clears current line of the associated stream in a specified direction identified by dir. Call rl.commit() to see the effect of this method, unless autoCommit: true was passed to the constructor.

rl.clearScreenDown()#

• Returns: this

The rl.clearScreenDown() method adds to the internal list of pending action an action that clears the associated stream from the current position of the cursor down. Call rl.commit() to see the effect of this method, unless autoCommit: true was passed to the constructor.

rl.commit()#

• Returns: <Promise>

The rl.commit() method sends all the pending actions to the associated stream and clears the internal list of pending actions.

rl.cursorTo(x[, y])#

• x <integer>
• y <integer>
• Returns: this

The rl.cursorTo() method adds to the internal list of pending action an action that moves cursor to the specified position in the associated stream. Call rl.commit() to see the effect of this method, unless autoCommit: true was passed to the constructor.

rl.moveCursor(dx, dy)#

• dx <integer>
• dy <integer>
• Returns: this

The rl.moveCursor() method adds to the internal list of pending action an action that moves the cursor relative to its current position in the associated stream. Call rl.commit() to see the effect of this method, unless autoCommit: true was passed to the constructor.

rl.rollback()#

• Returns: this

The rl.rollback methods clears the internal list of pending actions without sending it to the associated stream.

readlinePromises.createInterface(options)#

• options <Object>
  • input <stream.Readable> The Readable stream to listen to. This option is required.
  • output <stream.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. Default: checking isTTY on the output stream upon instantiation.
  • history <string[]> Initial list of history lines. 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. Default: [].
  • historySize <number> Maximum number of history lines retained. To disable the history set this value to 0. 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. Default: 30.
  • 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. Default: false.
  • prompt <string> 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. crlfDelay will be coerced to a number no less than 100. It can be set to Infinity, in which case \r followed by \n will always be considered a single newline (which may be reasonable for reading files with \r\n line delimiter). Default: 100.
  • escapeCodeTimeout <number> The duration readlinePromises will wait for a character (when reading an ambiguous key sequence in milliseconds one that can both form a complete key sequence using the input read so far and can take additional input to complete a longer key sequence). Default: 500.
  • tabSize <integer> The number of spaces a tab is equal to (minimum 1). Default: 8.
• Returns: <readlinePromises.Interface>

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

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

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

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

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) => c.startsWith(line));
  // Show all completions if none found
  return [hits.length ? hits : completions, line];
} copy

The completer function can also return a <Promise>, or be asynchronous:

async function completer(linePartial) {
  await someAsyncWork();
  return [['123'], linePartial];
} copy

Callback API#

Class: readline.Interface#

• Extends: <readline.InterfaceConstructor>

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.

rl.question(query[, options], callback)#

• query <string> A statement or query to write to output, prepended to the prompt.
• options <Object>
  • signal <AbortSignal> Optionally allows the question() to be canceled using an AbortController.
• 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.

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.

An error will be thrown if calling rl.question() after rl.close().

Example usage:

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

Using an AbortController to cancel a question.

const ac = new AbortController();
const signal = ac.signal;

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

signal.addEventListener('abort', () => {
  console.log('The food question timed out');
}, { once: true });

setTimeout(() => ac.abort(), 10000); copy

readline.clearLine(stream, dir[, callback])#

• stream <stream.Writable>
• dir <number>
  • -1: to the left from cursor
  • 1: to the right from cursor
  • 0: the entire line
• callback <Function> Invoked once the operation completes.
• Returns: <boolean> false if stream wishes for the calling code to wait for the 'drain' event to be emitted before continuing to write additional data; otherwise true.

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

readline.clearScreenDown(stream[, callback])#

• stream <stream.Writable>
• callback <Function> Invoked once the operation completes.
• Returns: <boolean> false if stream wishes for the calling code to wait for the 'drain' event to be emitted before continuing to write additional data; otherwise true.

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

readline.createInterface(options)#

• options <Object>
  • input <stream.Readable> The Readable stream to listen to. This option is required.
  • output <stream.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. Default: checking isTTY on the output stream upon instantiation.
  • history <string[]> Initial list of history lines. 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. Default: [].
  • historySize <number> Maximum number of history lines retained. To disable the history set this value to 0. 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. Default: 30.
  • 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. Default: false.
  • prompt <string> 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. crlfDelay will be coerced to a number no less than 100. It can be set to Infinity, in which case \r followed by \n will always be considered a single newline (which may be reasonable for reading files with \r\n line delimiter). Default: 100.
  • escapeCodeTimeout <number> The duration readline will wait for a character (when reading an ambiguous key sequence in milliseconds one that can both form a complete key sequence using the input read so far and can take additional input to complete a longer key sequence). Default: 500.
  • tabSize <integer> The number of spaces a tab is equal to (minimum 1). Default: 8.
  • signal <AbortSignal> Allows closing the interface using an AbortSignal. Aborting the signal will internally call close on the interface.
• Returns: <readline.Interface>

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

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

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}`);
}); copy

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).

When creating a readline.Interface using stdin as input, the program will not terminate until it receives an EOF character. To exit without waiting for user input, call process.stdin.unref().

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) => c.startsWith(line));
  // Show all completions if none found
  return [hits.length ? hits : completions, line];
} copy

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

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

readline.cursorTo(stream, x[, y][, callback])#

• stream <stream.Writable>
• x <number>
• y <number>
• callback <Function> Invoked once the operation completes.
• Returns: <boolean> false if stream wishes for the calling code to wait for the 'drain' event to be emitted before continuing to write additional data; otherwise true.

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

readline.moveCursor(stream, dx, dy[, callback])#

• stream <stream.Writable>
• dx <number>
• dy <number>
• callback <Function> Invoked once the operation completes.
• Returns: <boolean> false if stream wishes for the calling code to wait for the 'drain' event to be emitted before continuing to write additional data; otherwise true.

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

readline.emitKeypressEvents(stream[, interface])#

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

The readline.emitKeypressEvents() method causes the given Readable 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.

This is automatically called by any readline instance on its input if the input is a terminal. Closing the readline instance does not stop the input from emitting 'keypress' events.

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

Example: Tiny CLI#

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

const readline = require('node: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);
}); copy

Example: Read file stream line-by-Line#

A common use case for readline is to consume an input file one line at a time. The easiest way to do so is leveraging the fs.ReadStream API as well as a for await...of loop:

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

async function processLineByLine() {
  const fileStream = fs.createReadStream('input.txt');

  const rl = readline.createInterface({
    input: fileStream,
    crlfDelay: Infinity,
  });
  // Note: we use the crlfDelay option to recognize all instances of CR LF
  // ('\r\n') in input.txt as a single line break.

  for await (const line of rl) {
    // Each line in input.txt will be successively available here as `line`.
    console.log(`Line from file: ${line}`);
  }
}

processLineByLine(); copy

Alternatively, one could use the 'line' event:

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

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

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

Currently, for await...of loop can be a bit slower. If async / await flow and speed are both essential, a mixed approach can be applied:

const { once } = require('node:events');
const { createReadStream } = require('node:fs');
const { createInterface } = require('node:readline');

(async function processLineByLine() {
  try {
    const rl = createInterface({
      input: createReadStream('big-file.txt'),
      crlfDelay: Infinity,
    });

    rl.on('line', (line) => {
      // Process the line.
    });

    await once(rl, 'close');

    console.log('File processed.');
  } catch (err) {
    console.error(err);
  }
})(); copy

TTY keybindings#