Class Sign

The Sign class is a utility for generating signatures. It can be used in one of two ways:

  • As a writable stream, where data to be signed is written and the sign.sign() method is used to generate and return the signature, or
  • Using the sign.update() and sign.sign() methods to produce the signature.

The createSign method is used to create Sign instances. The argument is the string name of the hash function to use. Sign objects are not to be created directly using the new keyword.

Example: Using Sign and Verify objects as streams:

const {
generateKeyPairSync,
createSign,
createVerify
} = await import('crypto');

const { privateKey, publicKey } = generateKeyPairSync('ec', {
namedCurve: 'sect239k1'
});

const sign = createSign('SHA256');
sign.write('some data to sign');
sign.end();
const signature = sign.sign(privateKey, 'hex');

const verify = createVerify('SHA256');
verify.write('some data to sign');
verify.end();
console.log(verify.verify(publicKey, signature, 'hex'));
// Prints: true

Example: Using the sign.update() and verify.update() methods:

const {
generateKeyPairSync,
createSign,
createVerify
} = await import('crypto');

const { privateKey, publicKey } = generateKeyPairSync('rsa', {
modulusLength: 2048,
});

const sign = createSign('SHA256');
sign.update('some data to sign');
sign.end();
const signature = sign.sign(privateKey);

const verify = createVerify('SHA256');
verify.update('some data to sign');
verify.end();
console.log(verify.verify(publicKey, signature));
// Prints: true

Hierarchy

Constructors

Properties

destroyed: boolean

Is true after writable.destroy() has been called.

locked: boolean
writable: boolean

Is true if it is safe to call writable.write(), which means the stream has not been destroyed, errored or ended.

writableCorked: number

Number of times writable.uncork() needs to be called in order to fully uncork the stream.

writableEnded: boolean

Is true after writable.end() has been called. This property does not indicate whether the data has been flushed, for this use writable.writableFinished instead.

writableFinished: boolean

Is set to true immediately before the 'finish' event is emitted.

writableHighWaterMark: number

Return the value of highWaterMark passed when creating this Writable.

writableLength: number

This property contains the number of bytes (or objects) in the queue ready to be written. The value provides introspection data regarding the status of the highWaterMark.

writableObjectMode: boolean

Getter for the property objectMode of a given Writable stream.

captureRejectionSymbol: typeof captureRejectionSymbol
captureRejections: boolean

Sets or gets the default captureRejection value for all emitters.

defaultMaxListeners: number
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.

Methods

  • Parameters

    • callback: ((error?: Error) => void)
        • (error?: Error): void
        • Parameters

          • Optional error: Error

          Returns void

    Returns void

  • Parameters

    • error: Error
    • callback: ((error?: Error) => void)
        • (error?: Error): void
        • Parameters

          • Optional error: Error

          Returns void

    Returns void

  • Parameters

    • callback: ((error?: Error) => void)
        • (error?: Error): void
        • Parameters

          • Optional error: Error

          Returns void

    Returns void

  • Parameters

    • chunk: any
    • encoding: BufferEncoding
    • callback: ((error?: Error) => void)
        • (error?: Error): void
        • Parameters

          • Optional error: Error

          Returns void

    Returns void

  • Parameters

    • chunks: { chunk: any; encoding: BufferEncoding }[]
    • callback: ((error?: Error) => void)
        • (error?: Error): void
        • Parameters

          • Optional error: Error

          Returns void

    Returns void

  • Parameters

    • Optional reason: any

    Returns Promise<void>

  • Returns Promise<void>

  • The writable.cork() method forces all written data to be buffered in memory. The buffered data will be flushed when either the uncork or end methods are called.

    The primary intent of writable.cork() is to accommodate a situation in which several small chunks are written to the stream in rapid succession. Instead of immediately forwarding them to the underlying destination, writable.cork()buffers all the chunks until writable.uncork() is called, which will pass them all to writable._writev(), if present. This prevents a head-of-line blocking situation where data is being buffered while waiting for the first small chunk to be processed. However, use of writable.cork() without implementingwritable._writev() may have an adverse effect on throughput.

    See also: writable.uncork(), writable._writev().

    Returns void

  • Destroy the stream. Optionally emit an 'error' event, and emit a 'close'event (unless emitClose is set to false). After this call, the writable stream has ended and subsequent calls to write() or end() will result in an ERR_STREAM_DESTROYED error. This is a destructive and immediate way to destroy a stream. Previous calls towrite() may not have drained, and may trigger an ERR_STREAM_DESTROYED error. Use end() instead of destroy if data should flush before close, or wait for the 'drain' event before destroying the stream.

    Once destroy() has been called any further calls will be a no-op and no further errors except from _destroy() may be emitted as 'error'.

    Implementors should not override this method, but instead implement writable._destroy().

    Parameters

    • Optional error: Error

      Optional, an error to emit with 'error' event.

    Returns "crypto".Sign

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

    Returns true if the event had listeners, false otherwise.

    const EventEmitter = require('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

    Parameters

    • event: "close"

    Returns boolean

  • Parameters

    • event: "drain"

    Returns boolean

  • Parameters

    • event: "error"
    • err: Error

    Returns boolean

  • Parameters

    • event: "finish"

    Returns boolean

  • Parameters

    Returns boolean

  • Parameters

    Returns boolean

  • Parameters

    • event: string | symbol
    • Rest ...args: any[]

    Returns boolean

  • Calling the writable.end() method signals that no more data will be written to the Writable. The optional chunk and encoding arguments allow one final additional chunk of data to be written immediately before closing the stream.

    Calling the write method after calling end will raise an error.

    // Write 'hello, ' and then end with 'world!'.
    const fs = require('fs');
    const file = fs.createWriteStream('example.txt');
    file.write('hello, ');
    file.end('world!');
    // Writing more now is not allowed!

    Parameters

    • Optional cb: (() => void)
        • (): void
        • Returns void

    Returns "crypto".Sign

  • Parameters

    • chunk: any
    • Optional cb: (() => void)
        • (): void
        • Returns void

    Returns "crypto".Sign

  • Parameters

    • chunk: any
    • encoding: BufferEncoding
    • Optional cb: (() => void)
        • (): void
        • Returns void

    Returns "crypto".Sign

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

    const EventEmitter = require('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 (string | symbol)[]

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

    Returns number

  • Returns the number of listeners listening to the event named eventName.

    Parameters

    • eventName: string | symbol

      The name of the event being listened for

    Returns number

  • 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] ]

    Parameters

    • eventName: string | symbol

    Returns Function[]

  • Alias for emitter.removeListener().

    Parameters

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

          • Rest ...args: any[]

          Returns void

    Returns "crypto".Sign

  • 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 eventNameand 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. Theemitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

    const myEE = new EventEmitter();
    myEE.on('foo', () => console.log('a'));
    myEE.prependListener('foo', () => console.log('b'));
    myEE.emit('foo');
    // Prints:
    // b
    // a

    Parameters

    • event: "close"

      The name of the event.

    • listener: (() => void)

      The callback function

        • (): void
        • Returns void

    Returns "crypto".Sign

  • Parameters

    • event: "drain"
    • listener: (() => void)
        • (): void
        • Returns void

    Returns "crypto".Sign

  • Parameters

    • event: "error"
    • listener: ((err: Error) => void)
        • (err: Error): void
        • Parameters

          • err: Error

          Returns void

    Returns "crypto".Sign

  • Parameters

    • event: "finish"
    • listener: (() => void)
        • (): void
        • Returns void

    Returns "crypto".Sign

  • Parameters

    Returns "crypto".Sign

  • Parameters

    Returns "crypto".Sign

  • Parameters

    • event: string | symbol
    • listener: ((...args: any[]) => void)
        • (...args: any[]): void
        • Parameters

          • Rest ...args: any[]

          Returns void

    Returns "crypto".Sign

  • Adds a one-timelistener 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. Theemitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

    const myEE = new EventEmitter();
    myEE.once('foo', () => console.log('a'));
    myEE.prependOnceListener('foo', () => console.log('b'));
    myEE.emit('foo');
    // Prints:
    // b
    // a

    Parameters

    • event: "close"

      The name of the event.

    • listener: (() => void)

      The callback function

        • (): void
        • Returns void

    Returns "crypto".Sign

  • Parameters

    • event: "drain"
    • listener: (() => void)
        • (): void
        • Returns void

    Returns "crypto".Sign

  • Parameters

    • event: "error"
    • listener: ((err: Error) => void)
        • (err: Error): void
        • Parameters

          • err: Error

          Returns void

    Returns "crypto".Sign

  • Parameters

    • event: "finish"
    • listener: (() => void)
        • (): void
        • Returns void

    Returns "crypto".Sign

  • Parameters

    Returns "crypto".Sign

  • Parameters

    Returns "crypto".Sign

  • Parameters

    • event: string | symbol
    • listener: ((...args: any[]) => void)
        • (...args: any[]): void
        • Parameters

          • Rest ...args: any[]

          Returns void

    Returns "crypto".Sign

  • Type Parameters

    Parameters

    • destination: T
    • Optional options: { end?: boolean }
      • Optional end?: boolean

    Returns T

  • 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 eventNameand 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.

    Parameters

    • event: "close"

      The name of the event.

    • listener: (() => void)

      The callback function

        • (): void
        • Returns void

    Returns "crypto".Sign

  • Parameters

    • event: "drain"
    • listener: (() => void)
        • (): void
        • Returns void

    Returns "crypto".Sign

  • Parameters

    • event: "error"
    • listener: ((err: Error) => void)
        • (err: Error): void
        • Parameters

          • err: Error

          Returns void

    Returns "crypto".Sign

  • Parameters

    • event: "finish"
    • listener: (() => void)
        • (): void
        • Returns void

    Returns "crypto".Sign

  • Parameters

    Returns "crypto".Sign

  • Parameters

    Returns "crypto".Sign

  • Parameters

    • event: string | symbol
    • listener: ((...args: any[]) => void)
        • (...args: any[]): void
        • Parameters

          • Rest ...args: any[]

          Returns void

    Returns "crypto".Sign

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

    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');

    Parameters

    • eventName: string | symbol

    Returns Function[]

  • 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.

    Parameters

    • Optional event: string | symbol

    Returns "crypto".Sign

  • Removes the specified listener from the listener array for the event namedeventName.

    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 anyremoveListener() or removeAllListeners() calls after emitting and_before_ the last listener finishes execution will not remove them fromemit() in progress. Subsequent events behave as expected.

    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:

    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.

    Parameters

    • event: "close"
    • listener: (() => void)
        • (): void
        • Returns void

    Returns "crypto".Sign

  • Parameters

    • event: "drain"
    • listener: (() => void)
        • (): void
        • Returns void

    Returns "crypto".Sign

  • Parameters

    • event: "error"
    • listener: ((err: Error) => void)
        • (err: Error): void
        • Parameters

          • err: Error

          Returns void

    Returns "crypto".Sign

  • Parameters

    • event: "finish"
    • listener: (() => void)
        • (): void
        • Returns void

    Returns "crypto".Sign

  • Parameters

    Returns "crypto".Sign

  • Parameters

    Returns "crypto".Sign

  • Parameters

    • event: string | symbol
    • listener: ((...args: any[]) => void)
        • (...args: any[]): void
        • Parameters

          • Rest ...args: any[]

          Returns void

    Returns "crypto".Sign

  • The writable.setDefaultEncoding() method sets the default encoding for a Writable stream.

    Parameters

    • encoding: BufferEncoding

      The new default encoding

    Returns "crypto".Sign

  • 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 toInfinity (or 0) to indicate an unlimited number of listeners.

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

    Parameters

    • n: number

    Returns "crypto".Sign

  • The writable.uncork() method flushes all data buffered since cork was called.

    When using writable.cork() and writable.uncork() to manage the buffering of writes to a stream, defer calls to writable.uncork() usingprocess.nextTick(). Doing so allows batching of allwritable.write() calls that occur within a given Node.js event loop phase.

    stream.cork();
    stream.write('some ');
    stream.write('data ');
    process.nextTick(() => stream.uncork());

    If the writable.cork() method is called multiple times on a stream, the same number of calls to writable.uncork() must be called to flush the buffered data.

    stream.cork();
    stream.write('some ');
    stream.cork();
    stream.write('data ');
    process.nextTick(() => {
    stream.uncork();
    // The data will not be flushed until uncork() is called a second time.
    stream.uncork();
    });

    See also: writable.cork().

    Returns void

  • Updates the Sign content with the given data, the encoding of which is given in inputEncoding. If encoding is not provided, and the data is a string, an encoding of 'utf8' is enforced. If data is a Buffer, TypedArray, orDataView, then inputEncoding is ignored.

    This can be called many times with new data as it is streamed.

    Parameters

    Returns "crypto".Sign

  • Parameters

    Returns "crypto".Sign

  • The writable.write() method writes some data to the stream, and calls the supplied callback once the data has been fully handled. If an error occurs, the callback will be called with the error as its first argument. The callback is called asynchronously and before 'error' is emitted.

    The return value is true if the internal buffer is less than thehighWaterMark configured when the stream was created after admitting chunk. If false is returned, further attempts to write data to the stream should stop until the 'drain' event is emitted.

    While a stream is not draining, calls to write() will buffer chunk, and return false. Once all currently buffered chunks are drained (accepted for delivery by the operating system), the 'drain' event will be emitted. Once write() returns false, do not write more chunks until the 'drain' event is emitted. While calling write() on a stream that is not draining is allowed, Node.js will buffer all written chunks until maximum memory usage occurs, at which point it will abort unconditionally. Even before it aborts, high memory usage will cause poor garbage collector performance and high RSS (which is not typically released back to the system, even after the memory is no longer required). Since TCP sockets may never drain if the remote peer does not read the data, writing a socket that is not draining may lead to a remotely exploitable vulnerability.

    Writing data while the stream is not draining is particularly problematic for a Transform, because the Transform streams are paused by default until they are piped or a 'data' or 'readable' event handler is added.

    If the data to be written can be generated or fetched on demand, it is recommended to encapsulate the logic into a Readable and use pipe. However, if calling write() is preferred, it is possible to respect backpressure and avoid memory issues using the 'drain' event:

    function write(data, cb) {
    if (!stream.write(data)) {
    stream.once('drain', cb);
    } else {
    process.nextTick(cb);
    }
    }

    // Wait for cb to be called before doing any other write.
    write('hello', () => {
    console.log('Write completed, do more writes now.');
    });

    A Writable stream in object mode will always ignore the encoding argument.

    Returns

    false if the stream wishes for the calling code to wait for the 'drain' event to be emitted before continuing to write additional data; otherwise true.

    Parameters

    • chunk: any

      Optional data to write. For streams not operating in object mode, chunk must be a string, Buffer or Uint8Array. For object mode streams, chunk may be any JavaScript value other than null.

    • Optional callback: ((error: Error) => void)

      Callback for when this chunk of data is flushed.

        • (error: Error): void
        • Parameters

          • error: Error

          Returns void

    Returns boolean

  • Parameters

    • chunk: any
    • encoding: BufferEncoding
    • Optional callback: ((error: Error) => void)
        • (error: Error): void
        • Parameters

          • error: Error

          Returns void

    Returns boolean

  • 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.

    const { getEventListeners, EventEmitter } = require('events');

    {
    const ee = new EventEmitter();
    const listener = () => console.log('Events are fun');
    ee.on('foo', listener);
    getEventListeners(ee, 'foo'); // [listener]
    }
    {
    const et = new EventTarget();
    const listener = () => console.log('Events are fun');
    et.addEventListener('foo', listener);
    getEventListeners(et, 'foo'); // [listener]
    }

    Parameters

    • emitter: EventEmitter | DOMEventTarget
    • name: string | symbol

    Returns Function[]

  • A class method that returns the number of listeners for the given eventNameregistered on the given emitter.

    const { EventEmitter, listenerCount } = require('events');
    const myEmitter = new EventEmitter();
    myEmitter.on('event', () => {});
    myEmitter.on('event', () => {});
    console.log(listenerCount(myEmitter, 'event'));
    // Prints: 2

    Deprecated

    Since v3.2.0 - Use listenerCount instead.

    Parameters

    • emitter: EventEmitter

      The emitter to query

    • eventName: string | symbol

      The event name

    Returns number

  • const { on, EventEmitter } = require('events');

    (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')) {
    // 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:

    const { on, EventEmitter } = require('events');
    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());

    Returns

    that iterates eventName events emitted by the emitter

    Parameters

    • emitter: EventEmitter
    • eventName: string

      The name of the event being listened for

    • Optional options: StaticEventEmitterOptions

    Returns AsyncIterableIterator<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.

    const { once, EventEmitter } = require('events');

    async function run() {
    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.log('error happened', err);
    }
    }

    run();

    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:

    const { EventEmitter, once } = require('events');

    const ee = new EventEmitter();

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

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

    // Prints: ok boom

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

    const { EventEmitter, once } = require('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!

    Parameters

    • emitter: NodeEventTarget
    • eventName: string | symbol
    • Optional options: StaticEventEmitterOptions

    Returns Promise<any[]>

  • Parameters

    • emitter: DOMEventTarget
    • eventName: string
    • Optional options: StaticEventEmitterOptions

    Returns Promise<any[]>

  • const {
    setMaxListeners,
    EventEmitter
    } = require('events');

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

    setMaxListeners(5, target, emitter);

    Parameters

    • Optional n: number

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

    • Rest ...eventTargets: (EventEmitter | DOMEventTarget)[]

    Returns void

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