Node.js v19.9.0 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