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Bun

Node.js module

readline

The 'node:readline' module provides an interface for reading data from a Readable stream (such as process.stdin) one line at a time. It includes the readline.Interface class with question, on('line'), and close events.

Use cases include building command-line tools, interactive prompts, and REPL-style applications.

Works in Bun

Fully implemented.

  • 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.

    • readonly cursor: number

      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.

    • readonly 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();
});
    • readonly terminal: boolean
    • static captureRejections: boolean

      Value: boolean

      Change the default captureRejections option on all new EventEmitter objects.

    • readonly static captureRejectionSymbol: typeof captureRejectionSymbol

      Value: Symbol.for('nodejs.rejection')

      See how to write a custom rejection handler.

    • static defaultMaxListeners: number

      By default, a maximum of 10 listeners can be registered for any single event. This limit can be changed for individual EventEmitter instances using the emitter.setMaxListeners(n) method. To change the default for allEventEmitter instances, the events.defaultMaxListeners property can be used. If this value is not a positive number, a RangeError is thrown.

      Take caution when setting the events.defaultMaxListeners because the change affects all EventEmitter instances, including those created before the change is made. However, calling emitter.setMaxListeners(n) still has precedence over events.defaultMaxListeners.

      This is not a hard limit. The EventEmitter instance will allow more listeners to be added but will output a trace warning to stderr indicating that a "possible EventEmitter memory leak" has been detected. For any single EventEmitter, the emitter.getMaxListeners() and emitter.setMaxListeners() methods can be used to temporarily avoid this warning:

      import { EventEmitter } from 'node:events';
const emitter = new EventEmitter();
emitter.setMaxListeners(emitter.getMaxListeners() + 1);
emitter.once('event', () => {
  // do stuff
  emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));
});

      The --trace-warnings command-line flag can be used to display the stack trace for such warnings.

      The emitted warning can be inspected with process.on('warning') and will have the additional emitter, type, and count properties, referring to the event emitter instance, the event's name and the number of attached listeners, respectively. Its name property is set to 'MaxListenersExceededWarning'.

    • readonly static errorMonitor: typeof errorMonitor

      This symbol shall be used to install a listener for only monitoring 'error' events. Listeners installed using this symbol are called before the regular 'error' listeners are called.

      Installing a listener using this symbol does not change the behavior once an 'error' event is emitted. Therefore, the process will still crash if no regular 'error' listener is installed.

    • [Symbol.asyncIterator](): AsyncIterator<string>;
    • error: Error,
      event: string | symbol,
      ...args: AnyRest
      ): void;

    • Alias for rl.close().

    • event: string,
      listener: (...args: any[]) => void
      ): this;

      events.EventEmitter

      1. close
      2. line
      3. pause
      4. resume
      5. SIGCONT
      6. SIGINT
      7. SIGTSTP
      8. history
      event: 'close',
      listener: () => void
      ): this;

      events.EventEmitter

      1. close
      2. line
      3. pause
      4. resume
      5. SIGCONT
      6. SIGINT
      7. SIGTSTP
      8. history
      event: 'line',
      listener: (input: string) => void
      ): this;

      events.EventEmitter

      1. close
      2. line
      3. pause
      4. resume
      5. SIGCONT
      6. SIGINT
      7. SIGTSTP
      8. history
      event: 'pause',
      listener: () => void
      ): this;

      events.EventEmitter

      1. close
      2. line
      3. pause
      4. resume
      5. SIGCONT
      6. SIGINT
      7. SIGTSTP
      8. history
      event: 'resume',
      listener: () => void
      ): this;

      events.EventEmitter

      1. close
      2. line
      3. pause
      4. resume
      5. SIGCONT
      6. SIGINT
      7. SIGTSTP
      8. history
      event: 'SIGCONT',
      listener: () => void
      ): this;

      events.EventEmitter

      1. close
      2. line
      3. pause
      4. resume
      5. SIGCONT
      6. SIGINT
      7. SIGTSTP
      8. history
      event: 'SIGINT',
      listener: () => void
      ): this;

      events.EventEmitter

      1. close
      2. line
      3. pause
      4. resume
      5. SIGCONT
      6. SIGINT
      7. SIGTSTP
      8. history
      event: 'SIGTSTP',
      listener: () => void
      ): this;

      events.EventEmitter

      1. close
      2. line
      3. pause
      4. resume
      5. SIGCONT
      6. SIGINT
      7. SIGTSTP
      8. history
      event: 'history',
      listener: (history: string[]) => void
      ): this;

      events.EventEmitter

      1. close
      2. line
      3. pause
      4. resume
      5. SIGCONT
      6. SIGINT
      7. SIGTSTP
      8. history
    • close(): void;

      The rl.close() method closes the Interface 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 Interface instance.

    • event: string | symbol,
      ...args: any[]
      ): boolean;

      Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.

      Returns true if the event had listeners, false otherwise.

      import { EventEmitter } from 'node:events';
const myEmitter = new EventEmitter();

// First listener
myEmitter.on('event', function firstListener() {
  console.log('Helloooo! first listener');
});
// Second listener
myEmitter.on('event', function secondListener(arg1, arg2) {
  console.log(`event with parameters ${arg1}, ${arg2} in second listener`);
});
// Third listener
myEmitter.on('event', function thirdListener(...args) {
  const parameters = args.join(', ');
  console.log(`event with parameters ${parameters} in third listener`);
});

console.log(myEmitter.listeners('event'));

myEmitter.emit('event', 1, 2, 3, 4, 5);

// Prints:
// [
//   [Function: firstListener],
//   [Function: secondListener],
//   [Function: thirdListener]
// ]
// Helloooo! first listener
// event with parameters 1, 2 in second listener
// event with parameters 1, 2, 3, 4, 5 in third listener
      event: 'close'
      ): boolean;
      event: 'line',
      input: string
      ): boolean;
      event: 'pause'
      ): boolean;
      event: 'resume'
      ): boolean;
      event: 'SIGCONT'
      ): boolean;
      event: 'SIGINT'
      ): boolean;
      event: 'SIGTSTP'
      ): boolean;
      event: 'history',
      history: string[]
      ): boolean;
    • eventNames(): string | symbol[];

      Returns an array listing the events for which the emitter has registered listeners. The values in the array are strings or Symbols.

      import { EventEmitter } from 'node:events';

const myEE = new EventEmitter();
myEE.on('foo', () => {});
myEE.on('bar', () => {});

const sym = Symbol('symbol');
myEE.on(sym, () => {});

console.log(myEE.eventNames());
// Prints: [ 'foo', 'bar', Symbol(symbol) ]

    • 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.

    • getMaxListeners(): number;

      Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to EventEmitter.defaultMaxListeners.

    • getPrompt(): string;

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

      @returns

      the current prompt string

    • eventName: string | symbol,
      listener?: Function
      ): number;

      Returns the number of listeners listening for the event named eventName. If listener is provided, it will return how many times the listener is found in the list of the listeners of the event.

      @param eventName

      The name of the event being listened for

      @param listener

      The event handler function

    • eventName: string | symbol
      ): Function[];

      Returns a copy of the array of listeners for the event named eventName.

      server.on('connection', (stream) => {
  console.log('someone connected!');
});
console.log(util.inspect(server.listeners('connection')));
// Prints: [ [Function] ]
    • off<K>(
      eventName: string | symbol,
      listener: (...args: any[]) => void
      ): this;

      Alias for emitter.removeListener().

    • event: string,
      listener: (...args: any[]) => void
      ): this;

      Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.

      server.on('connection', (stream) => {
  console.log('someone connected!');
});

      Returns a reference to the EventEmitter, so that calls can be chained.

      By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

      import { EventEmitter } from 'node:events';
const myEE = new EventEmitter();
myEE.on('foo', () => console.log('a'));
myEE.prependListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a
      @param listener

      The callback function

      event: 'close',
      listener: () => void
      ): this;
      event: 'line',
      listener: (input: string) => void
      ): this;
      event: 'pause',
      listener: () => void
      ): this;
      event: 'resume',
      listener: () => void
      ): this;
      event: 'SIGCONT',
      listener: () => void
      ): this;
      event: 'SIGINT',
      listener: () => void
      ): this;
      event: 'SIGTSTP',
      listener: () => void
      ): this;
      event: 'history',
      listener: (history: string[]) => void
      ): this;
    • event: string,
      listener: (...args: any[]) => void
      ): this;

      Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.

      server.once('connection', (stream) => {
  console.log('Ah, we have our first user!');
});

      Returns a reference to the EventEmitter, so that calls can be chained.

      By default, event listeners are invoked in the order they are added. The emitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

      import { EventEmitter } from 'node:events';
const myEE = new EventEmitter();
myEE.once('foo', () => console.log('a'));
myEE.prependOnceListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a
      @param listener

      The callback function

      event: 'close',
      listener: () => void
      ): this;
      event: 'line',
      listener: (input: string) => void
      ): this;
      event: 'pause',
      listener: () => void
      ): this;
      event: 'resume',
      listener: () => void
      ): this;
      event: 'SIGCONT',
      listener: () => void
      ): this;
      event: 'SIGINT',
      listener: () => void
      ): this;
      event: 'SIGTSTP',
      listener: () => void
      ): this;
      event: 'history',
      listener: (history: string[]) => void
      ): this;
    • pause(): this;

      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 Interface instance.

    • event: string,
      listener: (...args: any[]) => void
      ): this;

      Adds the listener function to the beginning of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.

      server.prependListener('connection', (stream) => {
  console.log('someone connected!');
});

      Returns a reference to the EventEmitter, so that calls can be chained.

      @param listener

      The callback function

      event: 'close',
      listener: () => void
      ): this;
      event: 'line',
      listener: (input: string) => void
      ): this;
      event: 'pause',
      listener: () => void
      ): this;
      event: 'resume',
      listener: () => void
      ): this;
      event: 'SIGCONT',
      listener: () => void
      ): this;
      event: 'SIGINT',
      listener: () => void
      ): this;
      event: 'SIGTSTP',
      listener: () => void
      ): this;
      event: 'history',
      listener: (history: string[]) => void
      ): this;
    • event: string,
      listener: (...args: any[]) => void
      ): this;

      Adds a one-timelistener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.

      server.prependOnceListener('connection', (stream) => {
  console.log('Ah, we have our first user!');
});

      Returns a reference to the EventEmitter, so that calls can be chained.

      @param listener

      The callback function

      event: 'close',
      listener: () => void
      ): this;
      event: 'line',
      listener: (input: string) => void
      ): this;
      event: 'pause',
      listener: () => void
      ): this;
      event: 'resume',
      listener: () => void
      ): this;
      event: 'SIGCONT',
      listener: () => void
      ): this;
      event: 'SIGINT',
      listener: () => void
      ): this;
      event: 'SIGTSTP',
      listener: () => void
      ): this;
      event: 'history',
      listener: (history: string[]) => void
      ): this;
    • preserveCursor?: boolean
      ): void;

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

      @param preserveCursor

      If true, prevents the cursor placement from being reset to 0.

    • query: string,
      callback: (answer: string) => void
      ): void;

      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 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}`);
});

      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);
      @param query

      A statement or query to write to output, prepended to the prompt.

      @param callback

      A callback function that is invoked with the user's input in response to the query.

      query: string,
      options: Abortable,
      callback: (answer: string) => void
      ): void;

      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 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}`);
});

      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);
      @param query

      A statement or query to write to output, prepended to the prompt.

      @param callback

      A callback function that is invoked with the user's input in response to the query.

    • eventName: string | symbol
      ): Function[];

      Returns a copy of the array of listeners for the event named eventName, including any wrappers (such as those created by .once()).

      import { EventEmitter } from 'node:events';
const emitter = new EventEmitter();
emitter.once('log', () => console.log('log once'));

// Returns a new Array with a function `onceWrapper` which has a property
// `listener` which contains the original listener bound above
const listeners = emitter.rawListeners('log');
const logFnWrapper = listeners[0];

// Logs "log once" to the console and does not unbind the `once` event
logFnWrapper.listener();

// Logs "log once" to the console and removes the listener
logFnWrapper();

emitter.on('log', () => console.log('log persistently'));
// Will return a new Array with a single function bound by `.on()` above
const newListeners = emitter.rawListeners('log');

// Logs "log persistently" twice
newListeners[0]();
emitter.emit('log');
    • eventName?: string | symbol
      ): this;

      Removes all listeners, or those of the specified eventName.

      It is bad practice to remove listeners added elsewhere in the code, particularly when the EventEmitter instance was created by some other component or module (e.g. sockets or file streams).

      Returns a reference to the EventEmitter, so that calls can be chained.

    • eventName: string | symbol,
      listener: (...args: any[]) => void
      ): this;

      Removes the specified listener from the listener array for the event named eventName.

      const callback = (stream) => {
  console.log('someone connected!');
};
server.on('connection', callback);
// ...
server.removeListener('connection', callback);

      removeListener() will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified eventName, then removeListener() must be called multiple times to remove each instance.

      Once an event is emitted, all listeners attached to it at the time of emitting are called in order. This implies that any removeListener() or removeAllListeners() calls after emitting and before the last listener finishes execution will not remove them fromemit() in progress. Subsequent events behave as expected.

      import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();

const callbackA = () => {
  console.log('A');
  myEmitter.removeListener('event', callbackB);
};

const callbackB = () => {
  console.log('B');
};

myEmitter.on('event', callbackA);

myEmitter.on('event', callbackB);

// callbackA removes listener callbackB but it will still be called.
// Internal listener array at time of emit [callbackA, callbackB]
myEmitter.emit('event');
// Prints:
//   A
//   B

// callbackB is now removed.
// Internal listener array [callbackA]
myEmitter.emit('event');
// Prints:
//   A

      Because listeners are managed using an internal array, calling this will change the position indices of any listener registered after the listener being removed. This will not impact the order in which listeners are called, but it means that any copies of the listener array as returned by the emitter.listeners() method will need to be recreated.

      When a single function has been added as a handler multiple times for a single event (as in the example below), removeListener() will remove the most recently added instance. In the example the once('ping') listener is removed:

      import { EventEmitter } from 'node:events';
const ee = new EventEmitter();

function pong() {
  console.log('pong');
}

ee.on('ping', pong);
ee.once('ping', pong);
ee.removeListener('ping', pong);

ee.emit('ping');
ee.emit('ping');

      Returns a reference to the EventEmitter, so that calls can be chained.

    • resume(): this;

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

    • n: number
      ): this;

      By default EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks. The emitter.setMaxListeners() method allows the limit to be modified for this specific EventEmitter instance. The value can be set to Infinity (or 0) to indicate an unlimited number of listeners.

      Returns a reference to the EventEmitter, so that calls can be chained.

    • prompt: string
      ): void;

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

    • data: string | Buffer<ArrayBufferLike>,
      key?: Key
      ): void;

      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 Interface 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' });

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

      data: undefined | null | string | Buffer<ArrayBufferLike>,
      key: Key
      ): void;

      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 Interface 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' });

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

    • signal: AbortSignal,
      resource: (event: Event) => void
      ): Disposable;

      Listens once to the abort event on the provided signal.

      Listening to the abort event on abort signals is unsafe and may lead to resource leaks since another third party with the signal can call e.stopImmediatePropagation(). Unfortunately Node.js cannot change this since it would violate the web standard. Additionally, the original API makes it easy to forget to remove listeners.

      This API allows safely using AbortSignals in Node.js APIs by solving these two issues by listening to the event such that stopImmediatePropagation does not prevent the listener from running.

      Returns a disposable so that it may be unsubscribed from more easily.

      import { addAbortListener } from 'node:events';

function example(signal) {
  let disposable;
  try {
    signal.addEventListener('abort', (e) => e.stopImmediatePropagation());
    disposable = addAbortListener(signal, (e) => {
      // Do something when signal is aborted.
    });
  } finally {
    disposable?.[Symbol.dispose]();
  }
}
      @returns

      Disposable that removes the abort listener.

    • emitter: EventEmitter<DefaultEventMap> | EventTarget,
      name: string | symbol
      ): Function[];

      Returns a copy of the array of listeners for the event named eventName.

      For EventEmitters this behaves exactly the same as calling .listeners on the emitter.

      For EventTargets this is the only way to get the event listeners for the event target. This is useful for debugging and diagnostic purposes.

      import { getEventListeners, EventEmitter } from 'node:events';

{
  const ee = new EventEmitter();
  const listener = () => console.log('Events are fun');
  ee.on('foo', listener);
  console.log(getEventListeners(ee, 'foo')); // [ [Function: listener] ]
}
{
  const et = new EventTarget();
  const listener = () => console.log('Events are fun');
  et.addEventListener('foo', listener);
  console.log(getEventListeners(et, 'foo')); // [ [Function: listener] ]
}
    • emitter: EventEmitter<DefaultEventMap> | EventTarget
      ): number;

      Returns the currently set max amount of listeners.

      For EventEmitters this behaves exactly the same as calling .getMaxListeners on the emitter.

      For EventTargets this is the only way to get the max event listeners for the event target. If the number of event handlers on a single EventTarget exceeds the max set, the EventTarget will print a warning.

      import { getMaxListeners, setMaxListeners, EventEmitter } from 'node:events';

{
  const ee = new EventEmitter();
  console.log(getMaxListeners(ee)); // 10
  setMaxListeners(11, ee);
  console.log(getMaxListeners(ee)); // 11
}
{
  const et = new EventTarget();
  console.log(getMaxListeners(et)); // 10
  setMaxListeners(11, et);
  console.log(getMaxListeners(et)); // 11
}
    • static on(
      emitter: EventEmitter,
      eventName: string | symbol,
      options?: StaticEventEmitterIteratorOptions
      ): AsyncIterator<any[]>;
      import { on, EventEmitter } from 'node:events';
import process from 'node:process';

const ee = new EventEmitter();

// Emit later on
process.nextTick(() => {
  ee.emit('foo', 'bar');
  ee.emit('foo', 42);
});

for await (const event of on(ee, 'foo')) {
  // The execution of this inner block is synchronous and it
  // processes one event at a time (even with await). Do not use
  // if concurrent execution is required.
  console.log(event); // prints ['bar'] [42]
}
// Unreachable here

      Returns an AsyncIterator that iterates eventName events. It will throw if the EventEmitter emits 'error'. It removes all listeners when exiting the loop. The value returned by each iteration is an array composed of the emitted event arguments.

      An AbortSignal can be used to cancel waiting on events:

      import { on, EventEmitter } from 'node:events';
import process from 'node:process';

const ac = new AbortController();

(async () => {
  const ee = new EventEmitter();

  // Emit later on
  process.nextTick(() => {
    ee.emit('foo', 'bar');
    ee.emit('foo', 42);
  });

  for await (const event of on(ee, 'foo', { signal: ac.signal })) {
    // The execution of this inner block is synchronous and it
    // processes one event at a time (even with await). Do not use
    // if concurrent execution is required.
    console.log(event); // prints ['bar'] [42]
  }
  // Unreachable here
})();

process.nextTick(() => ac.abort());

      Use the close option to specify an array of event names that will end the iteration:

      import { on, EventEmitter } from 'node:events';
import process from 'node:process';

const ee = new EventEmitter();

// Emit later on
process.nextTick(() => {
  ee.emit('foo', 'bar');
  ee.emit('foo', 42);
  ee.emit('close');
});

for await (const event of on(ee, 'foo', { close: ['close'] })) {
  console.log(event); // prints ['bar'] [42]
}
// the loop will exit after 'close' is emitted
console.log('done'); // prints 'done'
      @returns

      An AsyncIterator that iterates eventName events emitted by the emitter

      static on(
      emitter: EventTarget,
      eventName: string,
      options?: StaticEventEmitterIteratorOptions
      ): AsyncIterator<any[]>;
      import { on, EventEmitter } from 'node:events';
import process from 'node:process';

const ee = new EventEmitter();

// Emit later on
process.nextTick(() => {
  ee.emit('foo', 'bar');
  ee.emit('foo', 42);
});

for await (const event of on(ee, 'foo')) {
  // The execution of this inner block is synchronous and it
  // processes one event at a time (even with await). Do not use
  // if concurrent execution is required.
  console.log(event); // prints ['bar'] [42]
}
// Unreachable here

      Returns an AsyncIterator that iterates eventName events. It will throw if the EventEmitter emits 'error'. It removes all listeners when exiting the loop. The value returned by each iteration is an array composed of the emitted event arguments.

      An AbortSignal can be used to cancel waiting on events:

      import { on, EventEmitter } from 'node:events';
import process from 'node:process';

const ac = new AbortController();

(async () => {
  const ee = new EventEmitter();

  // Emit later on
  process.nextTick(() => {
    ee.emit('foo', 'bar');
    ee.emit('foo', 42);
  });

  for await (const event of on(ee, 'foo', { signal: ac.signal })) {
    // The execution of this inner block is synchronous and it
    // processes one event at a time (even with await). Do not use
    // if concurrent execution is required.
    console.log(event); // prints ['bar'] [42]
  }
  // Unreachable here
})();

process.nextTick(() => ac.abort());

      Use the close option to specify an array of event names that will end the iteration:

      import { on, EventEmitter } from 'node:events';
import process from 'node:process';

const ee = new EventEmitter();

// Emit later on
process.nextTick(() => {
  ee.emit('foo', 'bar');
  ee.emit('foo', 42);
  ee.emit('close');
});

for await (const event of on(ee, 'foo', { close: ['close'] })) {
  console.log(event); // prints ['bar'] [42]
}
// the loop will exit after 'close' is emitted
console.log('done'); // prints 'done'
      @returns

      An AsyncIterator that iterates eventName events emitted by the emitter

    • static once(
      emitter: EventEmitter,
      eventName: string | symbol,
      options?: StaticEventEmitterOptions
      ): Promise<any[]>;

      Creates a Promise that is fulfilled when the EventEmitter emits the given event or that is rejected if the EventEmitter emits 'error' while waiting. The Promise will resolve with an array of all the arguments emitted to the given event.

      This method is intentionally generic and works with the web platform EventTarget interface, which has no special'error' event semantics and does not listen to the 'error' event.

      import { once, EventEmitter } from 'node:events';
import process from 'node:process';

const ee = new EventEmitter();

process.nextTick(() => {
  ee.emit('myevent', 42);
});

const [value] = await once(ee, 'myevent');
console.log(value);

const err = new Error('kaboom');
process.nextTick(() => {
  ee.emit('error', err);
});

try {
  await once(ee, 'myevent');
} catch (err) {
  console.error('error happened', err);
}

      The special handling of the 'error' event is only used when events.once() is used to wait for another event. If events.once() is used to wait for the 'error' event itself, then it is treated as any other kind of event without special handling:

      import { EventEmitter, once } from 'node:events';

const ee = new EventEmitter();

once(ee, 'error')
  .then(([err]) => console.log('ok', err.message))
  .catch((err) => console.error('error', err.message));

ee.emit('error', new Error('boom'));

// Prints: ok boom

      An AbortSignal can be used to cancel waiting for the event:

      import { EventEmitter, once } from 'node:events';

const ee = new EventEmitter();
const ac = new AbortController();

async function foo(emitter, event, signal) {
  try {
    await once(emitter, event, { signal });
    console.log('event emitted!');
  } catch (error) {
    if (error.name === 'AbortError') {
      console.error('Waiting for the event was canceled!');
    } else {
      console.error('There was an error', error.message);
    }
  }
}

foo(ee, 'foo', ac.signal);
ac.abort(); // Abort waiting for the event
ee.emit('foo'); // Prints: Waiting for the event was canceled!
      static once(
      emitter: EventTarget,
      eventName: string,
      options?: StaticEventEmitterOptions
      ): Promise<any[]>;

      Creates a Promise that is fulfilled when the EventEmitter emits the given event or that is rejected if the EventEmitter emits 'error' while waiting. The Promise will resolve with an array of all the arguments emitted to the given event.

      This method is intentionally generic and works with the web platform EventTarget interface, which has no special'error' event semantics and does not listen to the 'error' event.

      import { once, EventEmitter } from 'node:events';
import process from 'node:process';

const ee = new EventEmitter();

process.nextTick(() => {
  ee.emit('myevent', 42);
});

const [value] = await once(ee, 'myevent');
console.log(value);

const err = new Error('kaboom');
process.nextTick(() => {
  ee.emit('error', err);
});

try {
  await once(ee, 'myevent');
} catch (err) {
  console.error('error happened', err);
}

      The special handling of the 'error' event is only used when events.once() is used to wait for another event. If events.once() is used to wait for the 'error' event itself, then it is treated as any other kind of event without special handling:

      import { EventEmitter, once } from 'node:events';

const ee = new EventEmitter();

once(ee, 'error')
  .then(([err]) => console.log('ok', err.message))
  .catch((err) => console.error('error', err.message));

ee.emit('error', new Error('boom'));

// Prints: ok boom

      An AbortSignal can be used to cancel waiting for the event:

      import { EventEmitter, once } from 'node:events';

const ee = new EventEmitter();
const ac = new AbortController();

async function foo(emitter, event, signal) {
  try {
    await once(emitter, event, { signal });
    console.log('event emitted!');
  } catch (error) {
    if (error.name === 'AbortError') {
      console.error('Waiting for the event was canceled!');
    } else {
      console.error('There was an error', error.message);
    }
  }
}

foo(ee, 'foo', ac.signal);
ac.abort(); // Abort waiting for the event
ee.emit('foo'); // Prints: Waiting for the event was canceled!
    • n?: number,
      ...eventTargets: EventEmitter<DefaultEventMap> | EventTarget[]
      ): void;
      import { setMaxListeners, EventEmitter } from 'node:events';

const target = new EventTarget();
const emitter = new EventEmitter();

setMaxListeners(5, target, emitter);
      @param n

      A non-negative number. The maximum number of listeners per EventTarget event.

      @param eventTargets

      Zero or more {EventTarget} or {EventEmitter} instances. If none are specified, n is set as the default max for all newly created {EventTarget} and {EventEmitter} objects.

  • function clearLine(
    stream: WritableStream,
    dir: Direction,
    callback?: () => void
    ): boolean;

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

    @param callback

    Invoked once the operation completes.

    @returns

    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.

  • function clearScreenDown(
    stream: WritableStream,
    callback?: () => void
    ): boolean;

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

    @param callback

    Invoked once the operation completes.

    @returns

    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.

  • function createInterface(
    input: ReadableStream,
    output?: WritableStream,
    completer?: Completer | AsyncCompleter,
    terminal?: boolean

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

    import readline from 'node: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).

    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().

    function createInterface(
    options: ReadLineOptions

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

    import readline from 'node: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).

    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().

  • function cursorTo(
    stream: WritableStream,
    x: number,
    y?: number,
    callback?: () => void
    ): boolean;

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

    @param callback

    Invoked once the operation completes.

    @returns

    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.

  • stream: ReadableStream,
    readlineInterface?: Interface
    ): void;

    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);

    Example: Tiny CLI

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

    import readline from '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);
});

    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:

    import fs from 'node:fs';
import readline from '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();

    Alternatively, one could use the 'line' event:

    import fs from 'node:fs';
import readline from 'node:readline';

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

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

    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:

    import { once } from 'node:events';
import { createReadStream } from 'node:fs';
import { createInterface } from '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);
  }
})();
  • function moveCursor(
    stream: WritableStream,
    dx: number,
    dy: number,
    callback?: () => void
    ): boolean;

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

    @param callback

    Invoked once the operation completes.

    @returns

    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.

Type definitions

  • interface CursorPos

  • interface Key

  • interface ReadLineOptions

    • completer?: Completer | AsyncCompleter

      An optional function used for Tab autocompletion.

    • crlfDelay?: number

      If the delay between \r and


      exceeds crlfDelay milliseconds, both \r and

      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

      will always be considered a single newline (which may be reasonable for reading files with 
      \r
      line delimiter).

    • 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).

    • 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.

    • 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.

    • input: ReadableStream
    • output?: WritableStream

      The Writable stream to write readline data to.

    • prompt?: string

      The prompt string to use.

    • 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.

    • signal?: AbortSignal

      Allows closing the interface using an AbortSignal. Aborting the signal will internally call close on the interface.

    • tabSize?: number

      The number of spaces a tab is equal to (minimum 1).

    • 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.

  • type AsyncCompleter = (line: string, callback: (err?: null | Error, result?: CompleterResult) => void) => void
  • type Completer = (line: string) => CompleterResult
  • type CompleterResult = [string[], string]
  • type Direction = -1 | 0 | 1