• Extends stream.Writable

Instances of fs.WriteStream are created and returned using the createWriteStream function.

Since

v0.1.93

Hierarchy

Constructors

Properties

bytesWritten: number

The number of bytes written so far. Does not include data that is still queued for writing.

Since

v0.4.7

closed: boolean

Is true after 'close' has been emitted.

Since

v18.0.0

destroyed: boolean

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

Since

v8.0.0

errored: null | Error

Returns error if the stream has been destroyed with an error.

Since

v18.0.0

path: string | Buffer

The path to the file the stream is writing to as specified in the first argument to createWriteStream. If path is passed as a string, thenwriteStream.path will be a string. If path is passed as a Buffer, thenwriteStream.path will be a Buffer.

Since

v0.1.93

pending: boolean

This property is true if the underlying file has not been opened yet, i.e. before the 'ready' event is emitted.

Since

v11.2.0

writable: boolean

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

Since

v11.4.0

writableCorked: number

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

Since

v13.2.0, v12.16.0

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.

Since

v12.9.0

writableFinished: boolean

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

Since

v12.6.0

writableHighWaterMark: number

Return the value of highWaterMark passed when creating this Writable.

Since

v9.3.0

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.

Since

v9.4.0

writableNeedDrain: boolean

Is true if the stream's buffer has been full and stream will emit 'drain'.

Since

v15.2.0, v14.17.0

writableObjectMode: boolean

Getter for the property objectMode of a given Writable stream.

Since

v12.3.0

captureRejectionSymbol: typeof captureRejectionSymbol
captureRejections: boolean

TODO: These should be described using static getter/setter pairs:

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?: null | Error) => void)
        • (error?: null | Error): void
        • Parameters

          • Optional error: null | Error

          Returns void

    Returns void

  • Parameters

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

          • Optional error: null | Error

          Returns void

    Returns void

  • Parameters

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

          • Optional error: null | Error

          Returns void

    Returns void

  • Parameters

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

          • Optional error: null | Error

          Returns void

    Returns void

  • Parameters

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

          • Optional error: null | Error

          Returns void

    Returns void

  • events.EventEmitter

    1. open
    2. close
    3. ready

    Parameters

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

    Returns WriteStream

  • Parameters

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

    Returns WriteStream

  • Parameters

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

          Returns void

    Returns WriteStream

  • Parameters

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

    Returns WriteStream

  • Parameters

    • event: "open"
    • listener: ((fd: number) => void)
        • (fd: number): void
        • Parameters

          • fd: number

          Returns void

    Returns WriteStream

  • Parameters

    Returns WriteStream

  • Parameters

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

    Returns WriteStream

  • Parameters

    Returns WriteStream

  • Parameters

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

          • Rest ...args: any[]

          Returns void

    Returns WriteStream

  • Closes writeStream. Optionally accepts a callback that will be executed once the writeStreamis closed.

    Since

    v0.9.4

    Parameters

    Returns 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().

    Since

    v0.11.2

    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().

    Since

    v8.0.0

    Parameters

    • Optional error: Error

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

    Returns WriteStream

  • 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

    Since

    v0.1.26

    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!

    Since

    v0.9.4

    Parameters

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

    Returns WriteStream

  • Parameters

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

    Returns WriteStream

  • Parameters

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

    Returns WriteStream

  • 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) ]

    Since

    v6.0.0

    Returns (string | symbol)[]

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

    Since

    v1.0.0

    Returns number

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

    Since

    v3.2.0

    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] ]

    Since

    v0.1.26

    Parameters

    • eventName: string | symbol

    Returns Function[]

  • Alias for emitter.removeListener().

    Since

    v10.0.0

    Parameters

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

          • Rest ...args: any[]

          Returns void

    Returns WriteStream

  • 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

    Since

    v0.1.101

    Parameters

    • event: "close"

      The name of the event.

    • listener: (() => void)

      The callback function

        • (): void
        • Returns void

    Returns WriteStream

  • Parameters

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

    Returns WriteStream

  • Parameters

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

          Returns void

    Returns WriteStream

  • Parameters

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

    Returns WriteStream

  • Parameters

    • event: "open"
    • listener: ((fd: number) => void)
        • (fd: number): void
        • Parameters

          • fd: number

          Returns void

    Returns WriteStream

  • Parameters

    Returns WriteStream

  • Parameters

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

    Returns WriteStream

  • Parameters

    Returns WriteStream

  • Parameters

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

          • Rest ...args: any[]

          Returns void

    Returns WriteStream

  • 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

    Since

    v0.3.0

    Parameters

    • event: "close"

      The name of the event.

    • listener: (() => void)

      The callback function

        • (): void
        • Returns void

    Returns WriteStream

  • Parameters

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

    Returns WriteStream

  • Parameters

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

          Returns void

    Returns WriteStream

  • Parameters

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

    Returns WriteStream

  • Parameters

    • event: "open"
    • listener: ((fd: number) => void)
        • (fd: number): void
        • Parameters

          • fd: number

          Returns void

    Returns WriteStream

  • Parameters

    Returns WriteStream

  • Parameters

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

    Returns WriteStream

  • Parameters

    Returns WriteStream

  • Parameters

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

          • Rest ...args: any[]

          Returns void

    Returns WriteStream

  • Type Parameters

    • T extends WritableStream<T>

    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.

    Since

    v6.0.0

    Parameters

    • event: "close"

      The name of the event.

    • listener: (() => void)

      The callback function

        • (): void
        • Returns void

    Returns WriteStream

  • Parameters

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

    Returns WriteStream

  • Parameters

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

          Returns void

    Returns WriteStream

  • Parameters

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

    Returns WriteStream

  • Parameters

    • event: "open"
    • listener: ((fd: number) => void)
        • (fd: number): void
        • Parameters

          • fd: number

          Returns void

    Returns WriteStream

  • Parameters

    Returns WriteStream

  • Parameters

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

    Returns WriteStream

  • Parameters

    Returns WriteStream

  • Parameters

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

          • Rest ...args: any[]

          Returns void

    Returns WriteStream

  • 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.

    Since

    v6.0.0

    Parameters

    • event: "close"

      The name of the event.

    • listener: (() => void)

      The callback function

        • (): void
        • Returns void

    Returns WriteStream

  • Parameters

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

    Returns WriteStream

  • Parameters

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

          Returns void

    Returns WriteStream

  • Parameters

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

    Returns WriteStream

  • Parameters

    • event: "open"
    • listener: ((fd: number) => void)
        • (fd: number): void
        • Parameters

          • fd: number

          Returns void

    Returns WriteStream

  • Parameters

    Returns WriteStream

  • Parameters

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

    Returns WriteStream

  • Parameters

    Returns WriteStream

  • Parameters

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

          • Rest ...args: any[]

          Returns void

    Returns WriteStream

  • 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');

    Since

    v9.4.0

    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.

    Since

    v0.1.26

    Parameters

    • Optional event: string | symbol

    Returns WriteStream

  • 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.

    Since

    v0.1.26

    Parameters

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

    Returns WriteStream

  • Parameters

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

    Returns WriteStream

  • Parameters

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

          Returns void

    Returns WriteStream

  • Parameters

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

    Returns WriteStream

  • Parameters

    Returns WriteStream

  • Parameters

    Returns WriteStream

  • Parameters

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

          • Rest ...args: any[]

          Returns void

    Returns WriteStream

  • 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.

    Since

    v0.3.5

    Parameters

    • n: number

    Returns WriteStream

  • 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().

    Since

    v0.11.2

    Returns void

  • 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.

    Since

    v0.9.4

    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: undefined | null | Error) => void)

      Callback for when this chunk of data is flushed.

        • (error: undefined | null | Error): void
        • Parameters

          • error: undefined | null | Error

          Returns void

    Returns boolean

  • Parameters

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

          • error: undefined | null | Error

          Returns void

    Returns boolean

  • Experimental

    A utility method for creating a Writable from a web WritableStream.

    Since

    v17.0.0

    Parameters

    Returns Writable

  • 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]
    }

    Since

    v15.2.0, v14.17.0

    Parameters

    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

    Since

    v0.9.12

    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());

    Since

    v13.6.0, v12.16.0

    Returns

    that iterates eventName events emitted by the emitter

    Parameters

    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!

    Since

    v11.13.0, v10.16.0

    Parameters

    Returns Promise<any[]>

  • Parameters

    Returns Promise<any[]>

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

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

    setMaxListeners(5, target, emitter);

    Since

    v15.4.0

    Parameters

    • Optional n: number

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

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

    Returns void

  • Experimental

    A utility method for creating a web WritableStream from a Writable.

    Since

    v17.0.0

    Parameters

    Returns WritableStream<any>