Class Cursor<DocType, Options>

Type Parameters

  • DocType = any

  • Options = never

Hierarchy

  • Readable
    • Cursor

Constructors

  • Type Parameters

    • DocType = any

    • Options = never

    Parameters

    • Optional opts: ReadableOptions
      Optional

    Returns Cursor<DocType, Options>

Properties

destroyed: boolean

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

Since

v8.0.0

options: Options
readable: boolean

Is true if it is safe to call readable.read(), which means the stream has not been destroyed or emitted 'error' or 'end'.

Since

v11.4.0

readableAborted: boolean

Returns whether the stream was destroyed or errored before emitting 'end'.

Since

v16.8.0

readableDidRead: boolean

Returns whether 'data' has been emitted.

Since

v16.7.0

readableEncoding: BufferEncoding

Getter for the property encoding of a given Readable stream. The encodingproperty can be set using the readable.setEncoding() method.

Since

v12.7.0

readableEnded: boolean

Becomes true when 'end' event is emitted.

Since

v12.9.0

readableFlowing: boolean

This property reflects the current state of a Readable stream as described in the Three states section.

Since

v9.4.0

readableHighWaterMark: number

Returns the value of highWaterMark passed when creating this Readable.

Since

v9.3.0

readableLength: number

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

Since

v9.4.0

readableObjectMode: boolean

Getter for the property objectMode of a given Readable stream.

Since

v12.3.0

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

  • Returns AsyncIterableIterator<DocType>

  • Type Parameters

    • K

    Parameters

    • error: Error
    • event: string | symbol
    • Rest ...args: AnyRest
      Rest

    Returns void

  • Parameters

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

          • Optional error: Error
            Optional

          Returns void

    Returns void

  • Parameters

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

          • Optional error: Error
            Optional

          Returns void

    Returns void

  • Parameters

    • size: number

    Returns void

  • Adds a cursor flag. Useful for setting the noCursorTimeout and tailable flags.

    Parameters

    Returns Cursor<DocType, Options>

  • Event emitter The defined events on documents including:

    1. close
    2. data
    3. end
    4. error
    5. pause
    6. readable
    7. resume

    Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

    • event: "data"
    • listener: ((chunk) => void)
        • (chunk): void
        • Parameters

          • chunk: any

          Returns void

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

          • err: Error

          Returns void

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

          • Rest ...args: any[]
            Rest

          Returns void

    Returns Cursor<DocType, Options>

  • Marks this cursor as closed. Will stop streaming and subsequent calls to next() will error.

    Returns Promise<void>

  • Destroy the stream. Optionally emit an 'error' event, and emit a 'close'event (unless emitClose is set to false). After this call, the readable stream will release any internal resources and subsequent calls to push()will be ignored.

    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 readable._destroy().

    Parameters

    • Optional error: Error

      Error which will be passed as payload in 'error' event

      Optional

    Returns Cursor<DocType, Options>

    Since

    v8.0.0

  • Execute fn for every document(s) in the cursor. If batchSize is provided fn will be executed for each batch of documents. If fn returns a promise, will wait for the promise to resolve before iterating on to the next one. Returns a promise that resolves when done.

    Parameters

    • fn: ((doc, i) => any)
        • (doc, i): any
        • Parameters

          • doc: DocType[]
          • i: number

          Returns any

    • options: EachAsyncOptions & {
          batchSize: number;
      }

    Returns Promise<void>

  • Parameters

    • fn: ((doc, i) => any)
        • (doc, i): any
        • Parameters

          • doc: DocType
          • i: number

          Returns any

    • Optional options: EachAsyncOptions
      Optional

    Returns Promise<void>

  • Parameters

    • event: "close"

    Returns boolean

  • Parameters

    • event: "data"
    • chunk: any

    Returns boolean

  • Parameters

    • event: "end"

    Returns boolean

  • Parameters

    • event: "error"
    • err: Error

    Returns boolean

  • Parameters

    • event: "pause"

    Returns boolean

  • Parameters

    • event: "readable"

    Returns boolean

  • Parameters

    • event: "resume"

    Returns boolean

  • Parameters

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

    Returns boolean

  • 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 (string | symbol)[]

    Since

    v6.0.0

  • This method allows filtering the stream. For each chunk in the stream the fn function will be called and if it returns a truthy value, the chunk will be passed to the result stream. If the fn function returns a promise - that promise will be awaited.

    Parameters

    • fn: ((data, options?) => boolean | Promise<boolean>)

      a function to filter chunks from the stream. Async or not.

        • (data, options?): boolean | Promise<boolean>
        • Parameters

          • data: any
          • Optional options: Pick<ArrayOptions, "signal">
            Optional

          Returns boolean | Promise<boolean>

    • Optional options: ArrayOptions
      Optional

    Returns Readable

    a stream filtered with the predicate fn.

    Since

    v17.4.0, v16.14.0

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

    Returns number

    Since

    v1.0.0

  • The readable.isPaused() method returns the current operating state of theReadable. This is used primarily by the mechanism that underlies thereadable.pipe() method. In most typical cases, there will be no reason to use this method directly.

    const readable = new stream.Readable();

    readable.isPaused(); // === false
    readable.pause();
    readable.isPaused(); // === true
    readable.resume();
    readable.isPaused(); // === false

    Returns boolean

    Since

    v0.11.14

  • The iterator created by this method gives users the option to cancel the destruction of the stream if the for await...of loop is exited by return, break, or throw, or if the iterator should destroy the stream if the stream emitted an error during iteration.

    Parameters

    • Optional options: {
          destroyOnReturn?: boolean;
      }
      Optional
      • Optional destroyOnReturn?: boolean

        When set to false, calling return on the async iterator, or exiting a for await...of iteration using a break, return, or throw will not destroy the stream. Default: true.

    Returns AsyncIterator<any, any, any>

    Since

    v16.3.0

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

    Type Parameters

    • K

    Parameters

    • eventName: string | symbol

      The name of the event being listened for

    • Optional listener: Function
      Optional

    Returns number

    Since

    v3.2.0

  • 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] ]

    Type Parameters

    • K

    Parameters

    • eventName: string | symbol

    Returns Function[]

    Since

    v0.1.26

  • Registers a transform function which subsequently maps documents retrieved via the streams interface or .next()

    Type Parameters

    • ResultType

    Parameters

    • fn: ((res) => ResultType)
        • (res): ResultType
        • Parameters

          • res: DocType

          Returns ResultType

    Returns Cursor<ResultType, Options>

  • Get the next document from this cursor. Will return null when there are no documents left.

    Returns Promise<DocType>

  • Alias for emitter.removeListener().

    Type Parameters

    • K

    Parameters

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

          • Rest ...args: any[]
            Rest

          Returns void

    Returns Cursor<DocType, Options>

    Since

    v10.0.0

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

    • event: "data"
    • listener: ((chunk) => void)
        • (chunk): void
        • Parameters

          • chunk: any

          Returns void

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

          • err: Error

          Returns void

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

          • Rest ...args: any[]
            Rest

          Returns void

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

    • event: "data"
    • listener: ((chunk) => void)
        • (chunk): void
        • Parameters

          • chunk: any

          Returns void

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

          • err: Error

          Returns void

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

          • Rest ...args: any[]
            Rest

          Returns void

    Returns Cursor<DocType, Options>

  • The readable.pause() method will cause a stream in flowing mode to stop emitting 'data' events, switching out of flowing mode. Any data that becomes available will remain in the internal buffer.

    const readable = getReadableStreamSomehow();
    readable.on('data', (chunk) => {
    console.log(`Received ${chunk.length} bytes of data.`);
    readable.pause();
    console.log('There will be no additional data for 1 second.');
    setTimeout(() => {
    console.log('Now data will start flowing again.');
    readable.resume();
    }, 1000);
    });

    The readable.pause() method has no effect if there is a 'readable'event listener.

    Returns Cursor<DocType, Options>

    Since

    v0.9.4

  • Type Parameters

    • T extends WritableStream<T>

    Parameters

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

    Returns T

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

    • event: "data"
    • listener: ((chunk) => void)
        • (chunk): void
        • Parameters

          • chunk: any

          Returns void

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

          • err: Error

          Returns void

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

          • Rest ...args: any[]
            Rest

          Returns void

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

    • event: "data"
    • listener: ((chunk) => void)
        • (chunk): void
        • Parameters

          • chunk: any

          Returns void

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

          • err: Error

          Returns void

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

          • Rest ...args: any[]
            Rest

          Returns void

    Returns Cursor<DocType, Options>

  • Parameters

    • chunk: any
    • Optional encoding: BufferEncoding
      Optional

    Returns boolean

  • 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');

    Type Parameters

    • K

    Parameters

    • eventName: string | symbol

    Returns Function[]

    Since

    v9.4.0

  • The readable.read() method pulls some data out of the internal buffer and returns it. If no data available to be read, null is returned. By default, the data will be returned as a Buffer object unless an encoding has been specified using the readable.setEncoding() method or the stream is operating in object mode.

    The optional size argument specifies a specific number of bytes to read. Ifsize bytes are not available to be read, null will be returned _unless_the stream has ended, in which case all of the data remaining in the internal buffer will be returned.

    If the size argument is not specified, all of the data contained in the internal buffer will be returned.

    The size argument must be less than or equal to 1 GiB.

    The readable.read() method should only be called on Readable streams operating in paused mode. In flowing mode, readable.read() is called automatically until the internal buffer is fully drained.

    const readable = getReadableStreamSomehow();

    // 'readable' may be triggered multiple times as data is buffered in
    readable.on('readable', () => {
    let chunk;
    console.log('Stream is readable (new data received in buffer)');
    // Use a loop to make sure we read all currently available data
    while (null !== (chunk = readable.read())) {
    console.log(`Read ${chunk.length} bytes of data...`);
    }
    });

    // 'end' will be triggered once when there is no more data available
    readable.on('end', () => {
    console.log('Reached end of stream.');
    });

    Each call to readable.read() returns a chunk of data, or null. The chunks are not concatenated. A while loop is necessary to consume all data currently in the buffer. When reading a large file .read() may return null, having consumed all buffered content so far, but there is still more data to come not yet buffered. In this case a new 'readable' event will be emitted when there is more data in the buffer. Finally the 'end' event will be emitted when there is no more data to come.

    Therefore to read a file's whole contents from a readable, it is necessary to collect chunks across multiple 'readable' events:

    const chunks = [];

    readable.on('readable', () => {
    let chunk;
    while (null !== (chunk = readable.read())) {
    chunks.push(chunk);
    }
    });

    readable.on('end', () => {
    const content = chunks.join('');
    });

    A Readable stream in object mode will always return a single item from a call to readable.read(size), regardless of the value of thesize argument.

    If the readable.read() method returns a chunk of data, a 'data' event will also be emitted.

    Calling read after the 'end' event has been emitted will return null. No runtime error will be raised.

    Parameters

    • Optional size: number

      Optional argument to specify how much data to read.

      Optional

    Returns any

    Since

    v0.9.4

  • 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
      Optional

    Returns Cursor<DocType, Options>

    Since

    v0.1.26

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

    • event: "data"
    • listener: ((chunk) => void)
        • (chunk): void
        • Parameters

          • chunk: any

          Returns void

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

          • err: Error

          Returns void

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

    Returns Cursor<DocType, Options>

  • Parameters

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

          • Rest ...args: any[]
            Rest

          Returns void

    Returns Cursor<DocType, Options>

  • The readable.resume() method causes an explicitly paused Readable stream to resume emitting 'data' events, switching the stream into flowing mode.

    The readable.resume() method can be used to fully consume the data from a stream without actually processing any of that data:

    getReadableStreamSomehow()
    .resume()
    .on('end', () => {
    console.log('Reached the end, but did not read anything.');
    });

    The readable.resume() method has no effect if there is a 'readable'event listener.

    Returns Cursor<DocType, Options>

    Since

    v0.9.4

  • Rewind this cursor to its uninitialized state. Any options that are present on the cursor will remain in effect. Iterating this cursor will cause new queries to be sent to the server, even if the resultant data has already been retrieved by this cursor.

    Returns Cursor<DocType, Options>

  • The readable.setEncoding() method sets the character encoding for data read from the Readable stream.

    By default, no encoding is assigned and stream data will be returned asBuffer objects. Setting an encoding causes the stream data to be returned as strings of the specified encoding rather than as Bufferobjects. For instance, calling readable.setEncoding('utf8') will cause the output data to be interpreted as UTF-8 data, and passed as strings. Callingreadable.setEncoding('hex') will cause the data to be encoded in hexadecimal string format.

    The Readable stream will properly handle multi-byte characters delivered through the stream that would otherwise become improperly decoded if simply pulled from the stream as Buffer objects.

    const readable = getReadableStreamSomehow();
    readable.setEncoding('utf8');
    readable.on('data', (chunk) => {
    assert.equal(typeof chunk, 'string');
    console.log('Got %d characters of string data:', chunk.length);
    });

    Parameters

    • encoding: BufferEncoding

      The encoding to use.

    Returns Cursor<DocType, Options>

    Since

    v0.9.4

  • 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 Cursor<DocType, Options>

    Since

    v0.3.5

  • The readable.unpipe() method detaches a Writable stream previously attached using the pipe method.

    If the destination is not specified, then all pipes are detached.

    If the destination is specified, but no pipe is set up for it, then the method does nothing.

    import fs from 'node:fs';
    const readable = getReadableStreamSomehow();
    const writable = fs.createWriteStream('file.txt');
    // All the data from readable goes into 'file.txt',
    // but only for the first second.
    readable.pipe(writable);
    setTimeout(() => {
    console.log('Stop writing to file.txt.');
    readable.unpipe(writable);
    console.log('Manually close the file stream.');
    writable.end();
    }, 1000);

    Parameters

    • Optional destination: WritableStream

      Optional specific stream to unpipe

      Optional

    Returns Cursor<DocType, Options>

    Since

    v0.9.4

  • Passing chunk as null signals the end of the stream (EOF) and behaves the same as readable.push(null), after which no more data can be written. The EOF signal is put at the end of the buffer and any buffered data will still be flushed.

    The readable.unshift() method pushes a chunk of data back into the internal buffer. This is useful in certain situations where a stream is being consumed by code that needs to "un-consume" some amount of data that it has optimistically pulled out of the source, so that the data can be passed on to some other party.

    The stream.unshift(chunk) method cannot be called after the 'end' event has been emitted or a runtime error will be thrown.

    Developers using stream.unshift() often should consider switching to use of a Transform stream instead. See the API for stream implementers section for more information.

    // Pull off a header delimited by \n\n.
    // Use unshift() if we get too much.
    // Call the callback with (error, header, stream).
    import { StringDecoder } from 'node:string_decoder';
    function parseHeader(stream, callback) {
    stream.on('error', callback);
    stream.on('readable', onReadable);
    const decoder = new StringDecoder('utf8');
    let header = '';
    function onReadable() {
    let chunk;
    while (null !== (chunk = stream.read())) {
    const str = decoder.write(chunk);
    if (str.match(/\n\n/)) {
    // Found the header boundary.
    const split = str.split(/\n\n/);
    header += split.shift();
    const remaining = split.join('\n\n');
    const buf = Buffer.from(remaining, 'utf8');
    stream.removeListener('error', callback);
    // Remove the 'readable' listener before unshifting.
    stream.removeListener('readable', onReadable);
    if (buf.length)
    stream.unshift(buf);
    // Now the body of the message can be read from the stream.
    callback(null, header, stream);
    } else {
    // Still reading the header.
    header += str;
    }
    }
    }
    }

    Unlike push, stream.unshift(chunk) will not end the reading process by resetting the internal reading state of the stream. This can cause unexpected results if readable.unshift() is called during a read (i.e. from within a _read implementation on a custom stream). Following the call to readable.unshift() with an immediate push will reset the reading state appropriately, however it is best to simply avoid calling readable.unshift() while in the process of performing a read.

    Parameters

    • chunk: any

      Chunk of data to unshift onto the read queue. For streams not operating in object mode, chunk must be a string, Buffer, Uint8Array or null. For object mode streams, chunk may be any JavaScript value.

    • Optional encoding: BufferEncoding

      Encoding of string chunks. Must be a valid Buffer encoding, such as 'utf8' or 'ascii'.

      Optional

    Returns void

    Since

    v0.9.11

  • Prior to Node.js 0.10, streams did not implement the entire stream module API as it is currently defined. (See Compatibility for more information.)

    When using an older Node.js library that emits 'data' events and has a pause method that is advisory only, thereadable.wrap() method can be used to create a Readable stream that uses the old stream as its data source.

    It will rarely be necessary to use readable.wrap() but the method has been provided as a convenience for interacting with older Node.js applications and libraries.

    import { OldReader } from './old-api-module.js';
    import { Readable } from 'node:stream';
    const oreader = new OldReader();
    const myReader = new Readable().wrap(oreader);

    myReader.on('readable', () => {
    myReader.read(); // etc.
    });

    Parameters

    • stream: ReadableStream

      An "old style" readable stream

    Returns Cursor<DocType, Options>

    Since

    v0.9.4

  • A utility method for creating Readable Streams out of iterators.

    Parameters

    • iterable: Iterable<any> | AsyncIterable<any>
    • Optional options: ReadableOptions
      Optional

    Returns Readable

  • 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);
    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<DefaultEventMap> | DOMEventTarget
    • name: string | symbol

    Returns Function[]

    Since

    v15.2.0

  • Returns whether the stream has been read from or cancelled.

    Parameters

    • stream: Readable | ReadableStream

    Returns boolean

    Since

    v16.8.0

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

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

    Parameters

    • emitter: EventEmitter<DefaultEventMap>

      The emitter to query

    • eventName: string | symbol

      The event name

    Returns number

    Since

    v0.9.12

    Deprecated

    Since v3.2.0 - Use listenerCount instead.

  • import { on, EventEmitter } from 'node: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:

    import { on, EventEmitter } from 'node: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());

    Parameters

    • emitter: EventEmitter<DefaultEventMap>
    • eventName: string

      The name of the event being listened for

    • Optional options: StaticEventEmitterOptions
      Optional

    Returns AsyncIterator<any, any, any>

    that iterates eventName events emitted by the emitter

    Since

    v13.6.0, v12.16.0

  • 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';

    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:

    import { EventEmitter, once } from 'node: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:

    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!

    Parameters

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

    Returns Promise<any[]>

    Since

    v11.13.0, v10.16.0

  • Parameters

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

    Returns Promise<any[]>

  • import {
    setMaxListeners,
    EventEmitter
    } from 'node: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.

      Optional
    • Rest ...eventTargets: (EventEmitter<DefaultEventMap> | DOMEventTarget)[]

      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.

      Rest

    Returns void

    Since

    v15.4.0

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