Node.js v17.0.0-test20210702a107e9054a documentation

Overview#

The WHATWG Streams Standard (or "web streams") defines an API for handling streaming data. It is similar to the Node.js Streams API but emerged later and has become the "standard" API for streaming data across many JavaScript environments.

There are three primary types of objects

• ReadableStream - Represents a source of streaming data.
• WritableStream - Represents a destination for streaming data.
• TransformStream - Represents an algorithm for transforming streaming data.

Example ReadableStream#

This example creates a simple ReadableStream that pushes the current performance.now() timestamp once every second forever. An async iterable is used to read the data from the stream.

import {
  ReadableStream
} from 'node:stream/web';

import {
  setInterval as every
} from 'node:timers/promises';

import {
  performance
} from 'node:perf_hooks';

const SECOND = 1000;

const stream = new ReadableStream({
  async start(controller) {
    for await (const _ of every(SECOND))
      controller.enqueue(performance.now());
  }
});

for await (const value of stream)
  console.log(value);const {
  ReadableStream
} = require('stream/web');

const {
  setInterval: every
} = require('timers/promises');

const {
  performance
} = require('perf_hooks');

const SECOND = 1000;

const stream = new ReadableStream({
  async start(controller) {
    for await (const _ of every(SECOND))
      controller.enqueue(performance.now());
  }
});

(async () => {
  for await (const value of stream)
    console.log(value);
})();

API#

Class: ReadableStream#

new ReadableStream([underlyingSource [, strategy]])#

• underlyingSource <Object>
  • start <Function> A user-defined function that is invoked immediately when the ReadableStream is created.
    • controller <ReadableStreamDefaultController> | <ReadableByteStreamController>
    • Returns: undefined or a promise fulfilled with undefined.
  • pull <Function> A user-defined function that is called repeatedly when the ReadableStream internal queue is not full. The operation may be sync or async. If async, the function will not be called again until the previously returned promise is fulfilled.
    • controller <ReadableStreamDefaultController> | <ReadableByteStreamController>
    • Returns: A promise fulfilled with undefined.
  • cancel <Function> A user-defined function that is called when the ReadableStream is canceled.
    • reason <any>
    • Returns: A promise fulfilled with undefined.
  • type <string> Must be 'bytes' or undefined.
  • autoAllocateChunkSize <number> Used only when type is equal to 'bytes'.
• strategy <Object>
  • highWaterMark <number> The maximum internal queue size before backpressure is applied.
  • size <Function> A user-defined function used to identify the size of each chunk of data.
    • chunk <any>
    • Returns: <number>

readableStream.locked#

• Type: <boolean> Set to true if there is an active reader for this <ReadableStream>.

The readableStream.locked property is false by default, and is switch to true while there is an active reader consuming the stream's data.

readableStream.cancel([reason])#

• reason <any>
• Returns: A promise fulfilled with undefined once cancelation has been completed.

readableStream.getReader([options])#

• options <Object>
  • mode <string> 'byob' or undefined
• Returns: <ReadableStreamDefaultReader> | <ReadableStreamBYOBReader>

import { ReadableStream } from 'node:stream/web';

const stream = new ReadableStream();

const reader = stream.getReader();

console.log(await reader.read());const { ReadableStream } = require('stream/web');

const stream = new ReadableStream();

const reader = stream.getReader();

reader.read().then(console.log);

Causes the readableStream.locked to be true.

readableStream.pipeThrough(transform[, options])#

• transform <Object>
  • readable <ReadableStream> The ReadableStream to which transform.writable will push the potentially modified data is receives from this ReadableStream.
  • writable <WritableStream> The WritableStream to which this ReadableStream's data will be written.
• options <Object>
  • preventAbort <boolean> When true, errors in this ReadableStream will not cause transform.writable to be aborted.
  • preventCancel <boolean> When true, errors in the destination transform.writable is not cause this ReadableStream to be canceled.
  • preventClose <boolean> When true, closing this ReadableStream will no cause transform.writable to be closed.
  • signal <AbortSignal> Allows the transfer of data to be canceled using an <AbortController>.
• Returns: <ReadableStream> From transform.readable.

Connects this <ReadableStream> to the pair of <ReadableStream> and <WritableStream> provided in the transform argument such that the data from this <ReadableStream> is written in to transform.writable, possibly transformed, then pushed to transform.readable. Once the pipeline is configured, transform.readable is returned.

Causes the readableStream.locked to be true while the pipe operation is active.

import {
  ReadableStream,
  TransformStream,
} from 'node:stream/web';

const stream = new ReadableStream({
  start(controller) {
    controller.enqueue('a');
  },
});

const transform = new TransformStream({
  transform(chunk, controller) {
    controller.enqueue(chunk.toUpperCase());
  }
});

const transformedStream = stream.pipeThrough(transform);

for await (const chunk of transformedStream)
  console.log(chunk);const {
  ReadableStream,
  TransformStream,
} = require('stream/web');

const stream = new ReadableStream({
  start(controller) {
    controller.enqueue('a');
  },
});

const transform = new TransformStream({
  transform(chunk, controller) {
    controller.enqueue(chunk.toUpperCase());
  }
});

const transformedStream = stream.pipeThrough(transform);

(async () => {
  for await (const chunk of transformedStream)
    console.log(chunk);
})();

readableStream.pipeTo(destination, options)#

• destination <WritableStream> A <WritableStream> to which this ReadableStream's data will be written.
• options <Object>
  • preventAbort <boolean> When true, errors in this ReadableStream will not cause transform.writable to be aborted.
  • preventCancel <boolean> When true, errors in the destination transform.writable is not cause this ReadableStream to be canceled.
  • preventClose <boolean> When true, closing this ReadableStream will no cause transform.writable to be closed.
  • signal <AbortSignal> Allows the transfer of data to be canceled using an <AbortController>.
• Returns: A promise fulfilled with undefined

Causes the readableStream.locked to be true while the pipe operation is active.

readableStream.tee()#

• Returns: <ReadableStream[]>

Returns a pair of new <ReadableStream> instances to which this ReadableStream's data will be forwarded. Each will receive the same data.

Causes the readableStream.locked to be true.

readableStream.values([options])#

• options <Object>
  • preventCancel <boolean> When true, prevents the <ReadableStream> from being closed when the async iterator abruptly terminates. Defaults: false

Creates and returns an async iterator usable for consuming this ReadableStream's data.

Causes the readableStream.locked to be true while the async iterator is active.

import { Buffer } from 'node:buffer';

const stream = new ReadableStream(getSomeSource());

for await (const chunk of stream.values({ preventCancel: true }))
  console.log(Buffer.from(chunk).toString());

Async Iteration#

The <ReadableStream> object supports the async iterator protocol using for await syntax.

import { Buffer } from 'buffer';

const stream = new ReadableStream(getSomeSource());

for await (const chunk of stream)
  console.log(Buffer.from(chunk).toString());

The async iterator will consume the <ReadableStream> until it terminates.

By default, if the async iterator exits early (via either a break, return, or a throw), the <ReadableStream> will be closed. To prevent automatic closing of the <ReadableStream>, use the readableStream.values() method to acquire the async iterator and set the preventCancel option to true.

The <ReadableStream> must not be locked (that is, it must not have an existing active reader). During the async iteration, the <ReadableStream> will be locked.

Transfering with postMessage()#

A <ReadableStream> instance can be transferred using a <MessagePort>.

const stream = new ReadableStream(getReadableSourceSomehow());

const { port1, port2 } = new MessageChannel();

port1.onmessage = ({ data }) => {
  data.getReader().read().then((chunk) => {
    console.log(chunk);
  });
};

port2.postMessage(stream, [stream]);

Class: ReadableStreamDefaultReader#

By default, calling readableStream.getReader() with no arguments will return an instance of ReadableStreamDefaultReader. The default reader treats the chunks of data passed through the stream as opaque values, which allows the <ReadableStream> to work with generally any JavaScript value.

new ReadableStreamDefaultReader(stream)#

• stream <ReadableStream>

Creates a new <ReadableStreamDefaultReader> that is locked to the given <ReadableStream>.

readableStreamDefaultReader.cancel([reason])#

• reason <any>
• Returns: A promise fulfilled with undefined.

Cancels the <ReadableStream> and returns a promise that is fulfilled when the underlying stream has been canceled.

readableStreamDefaultReader.closed#

• Type: <Promise> Fulfilled with undefined when the associated <ReadableStream> is closed or this reader's lock is released.

readableStreamDefaultReader.read()#

• Returns: A promise fulfilled with an object:
  • value <ArrayBuffer>
  • done <boolean>

Requests the next chunk of data from the underlying <ReadableStream> and returns a promise that is fulfilled with the data once it is available.

readableStreamDefaultReader.releaseLock()#

Releases this reader's lock on the underlying <ReadableStream>.

Class: ReadableStreamBYOBReader#

The ReadableStreamBYOBReader is an alternative consumer for byte-oriented <ReadableStream>'s (those that are created with underlyingSource.type set equal to 'bytes`` when the ReadableStream` was created).

The BYOB is short for "bring your own buffer". This is a pattern that allows for more efficient reading of byte-oriented data that avoids extraneous copying.

import {
  open
} from 'node:fs/promises';

import {
  ReadableStream
} from 'node:stream/web';

import { Buffer } from 'node:buffer';

class Source {
  type = 'bytes';
  autoAllocateChunkSize = 1024;

  async start(controller) {
    this.file = await open(new URL(import.meta.url));
    this.controller = controller;
  }

  async pull(controller) {
    const view = controller.byobRequest?.view;
    const {
      bytesRead,
    } = await this.file.read({
      buffer: view,
      offset: view.byteOffset,
      length: view.byteLength
    });

    if (bytesRead === 0) {
      await this.file.close();
      this.controller.close();
    }
    controller.byobRequest.respond(bytesRead);
  }
}

const stream = new ReadableStream(new Source());

async function read(stream) {
  const reader = stream.getReader({ mode: 'byob' });

  const chunks = [];
  let result;
  do {
    result = await reader.read(Buffer.alloc(100));
    if (result.value !== undefined)
      chunks.push(Buffer.from(result.value));
  } while (!result.done);

  return Buffer.concat(chunks);
}

const data = await read(stream);
console.log(Buffer.from(data).toString());

new ReadableStreamBYOBReader(stream)#

• stream <ReadableStream>

Creates a new ReadableStreamBYOBReader that is locked to the given <ReadableStream>.

readableStreamBYOBReader.cancel([reason])#

• reason <any>
• Returns: A promise fulfilled with undefined.

Cancels the <ReadableStream> and returns a promise that is fulfilled when the underlying stream has been canceled.

readableStreamBYOBReader.closed#

• Type: <Promise> Fulfilled with undefined when the associated <ReadableStream> is closed or this reader's lock is released.

readableStreamBYOBReader.read(view)#

• view <Buffer> | <TypedArray> | <DataView>
• Returns: A promise fulfilled with an object:
  • value <ArrayBuffer>
  • done <boolean>

Requests the next chunk of data from the underlying <ReadableStream> and returns a promise that is fulfilled with the data once it is available.

Do not pass a pooled <Buffer> object instance in to this method. Pooled Buffer objects are created using Buffer.allocUnsafe(), or Buffer.from(), or are often returned by various fs module callbacks. These types of Buffers use a shared underlying <ArrayBuffer> object that contains all of the data from all of the pooled Buffer instances. When a Buffer, <TypedArray>, or <DataView> is passed in to readableStreamBYOBReader.read(), the view's underlying ArrayBuffer is detached, invalidating all existing views that may exist on that ArrayBuffer. This can have disasterous consequences for your application.

readableStreamBYOBReader.releaseLock()#

Releases this reader's lock on the underlying <ReadableStream>.

Class: ReadableStreamDefaultController#

Every <ReadableStream> has a controller that is responsible for the internal state and management of the stream's queue. The ReadableStreamDefaultController is the default controller implementation for ReadableStreams that are not byte-oriented.

readableStreamDefaultController.close()#

Closes the <ReadableStream> to which this controller is associated.

readableStreamDefaultController.desiredSize#

• Type: <number>

Returns the amount of data remaining to fill the <ReadableStream>'s queue.

readableStreamDefaultController.enqueue(chunk)#

• chunk <any>

Appends a new chunk of data to the <ReadableStream>'s queue.

readableStreamDefaultController.error(error)#

• error <any>

Signals an error that causes the <ReadableStream> to error and close.

Class: ReadableByteStreamController#

Every <ReadableStream> has a controller that is responsible for the internal state and management of the stream's queue. The ReadableByteStreamController is for byte-oriented ReadableStreams.

readableByteStreamController.byobRequest#

• Type: <ReadableStreamBYOBRequest>

readableByteStreamController.close()#

Closes the <ReadableStream> to which this controller is associated.

readableByteStreamController.desiredSize#

• Type: <number>

Returns the amount of data remaining to fill the <ReadableStream>'s queue.

readableByteStreamController.enqueue(chunk)#

• chunk: <Buffer> | <TypedArray> | <DataView>

Appends a new chunk of data to the <ReadableStream>'s queue.

readableByteStreamController.error(error)#

• error <any>

Signals an error that causes the <ReadableStream> to error and close.

Class: ReadableStreamBYOBRequest#

When using ReadableByteStreamController in byte-oriented streams, and when using the ReadableStreamBYOBReader, the readableByteStreamController.byobRequest property provides access to a ReadableStreamBYOBRequest instance that represents the current read request. The object is used to gain access to the ArrayBuffer/TypedArray that has been provided for the read request to fill, and provides methods for signaling that the data has been provided.

readableStreamBYOBRequest.respond(bytesWritten)#

• bytesWritten <number>

Signals that a bytesWritten number of bytes have been written to readableStreamBYOBRequest.view.

readableStreamBYOBRequest.respondWithNewView(view)#

• view <Buffer> | <TypedArray> | <DataView>

Signals that the request has been fulfilled with bytes written to a new Buffer, TypedArray, or DataView.

readableStreamBYOBRequest.view#

• Type: <Buffer> | <TypedArray> | <DataView>

Class: WritableStream#

The WritableStream is a destination to which stream data is sent.

import {
  WritableStream
} from 'node:stream/web';

const stream = new WritableStream({
  write(chunk) {
    console.log(chunk);
  }
});

await stream.getWriter().write('Hello World');

new WritableStream([underlyingSink[, strategy]])#

• underlyingSink <Object>
  • start <Function> A user-defined function that is invoked immediately when the WritableStream is created.
    • controller <WritableStreamDefaultController>
    • Returns: undefined or a promise fulfilled with undefined.
  • write <Function> A user-defined function that is invoked when a chunk of data has been written to the WritableStream.
    • chunk <any>
    • controller <WritableStreamDefaultController>
    • Returns: A promise fulfilled with undefined.
  • close <Function> A user-defined function that is called when the WritableStream is closed.
    • Returns: A promise fulfilled with undefined.
  • abort <Function> A user-defined function that is called to abruptly close the WritableStream.
    • reason <any>
    • Returns: A promise fulfilled with undefined.
  • type <any> The type option is reserved for future use and must be undefined.
• strategy <Object>
  • highWaterMark <number> The maximum internal queue size before backpressure is applied.
  • size <Function> A user-defined function used to identify the size of each chunk of data.
    • chunk <any>
    • Returns: <number>

writableStream.abort([reason])#

• reason <any>
• Returns: A promise fulfilled with undefined.

Abruptly terminates the WritableStream. All queued writes will be canceled with their associated promises rejected.

writableStream.close()#

• Returns: A promise fulfilled with undefined.

Closes the WritableStream when no additional writes are expected.

writableStream.getWriter()#

• Returns: <WritableStreamDefaultWriter>

Creates and creates a new writer instance that can be used to write data into the WritableStream.

writableStream.locked#

• Type: <boolean>

The writableStream.locked property is false by default, and is switched to true while there is an active writer attached to this WritableStream.

Transfering with postMessage()#

A <WritableStream> instance can be transferred using a <MessagePort>.

const stream = new WritableStream(getWritableSinkSomehow());

const { port1, port2 } = new MessageChannel();

port1.onmessage = ({ data }) => {
  data.getWriter().write('hello');
};

port2.postMessage(stream, [stream]);

Class: WritableStreamDefaultWriter#

new WritableStreamDefaultWriter(stream)#

• stream <WritableStream>

Creates a new WritableStreamDefaultWriter that is locked to the given WritableStream.

writableStreamDefaultWriter.abort([reason])#

• reason <any>
• Returns: A promise fulfilled with undefined.

Abruptly terminates the WritableStream. All queued writes will be canceled with their associated promises rejected.

writableStreamDefaultWriter.close()#

• Returns: A promise fulfilled with undefined.

Closes the WritableStream when no additional writes are expected.

writableStreamDefaultWriter.closed#

• Type: A promise that is fulfilled with undefined when the associated <WritableStream> is closed or this writer's lock is released.

writableStreamDefaultWriter.desiredSize#

• Type: <number>

The amount of data required to fill the <WritableStream>'s queue.

writableStreamDefaultWriter.ready#

• type: A promise that is fulfilled with undefined when the writer is ready to be used.

writableStreamDefaultWriter.releaseLock()#

Releases this writer's lock on the underlying <ReadableStream>.

writableStreamDefaultWriter.write([chunk])#

• chunk: <any>
• Returns: A promise fulfilled with undefined.

Appends a new chunk of data to the <WritableStream>'s queue.

Class: WritableStreamDefaultController#

The WritableStreamDefaultController manage's the <WritableStream>'s internal state.

writableStreamDefaultController.abortReason#

• Type: <any> The reason value passed to writableStream.abort().

writableStreamDefaultController.error(error)#

• error <any>

Called by user-code to signal that an error has occurred while processing the WritableStream data. When called, the <WritableStream> will be aborted, with currently pending writes canceled.

writableStreamDefaultController.signal#

• Type: <AbortSignal> An AbortSignal that can be used to cancel pending write or close operations when a <WritableStream> is aborted.

Class: TransformStream#

A TransformStream consists of a <ReadableStream> and a <WritableStream> that are connected such that the data written to the WritableStream is received, and potentially transformed, before being pushed into the ReadableStream's queue.

import {
  TransformStream
} from 'node:stream/web';

const transform = new TransformStream({
  transform(chunk, controller) {
    controller.enqueue(chunk.toUpperCase());
  }
});

await Promise.all([
  transform.writable.getWriter().write('A'),
  transform.readable.getReader().read(),
]);

new TransformStream([transformer[, writableStrategy[, readableStrategy]]])#

• transformer <Object>
  • start <Function> A user-defined function that is invoked immediately when the TransformStream is created.
    • controller <TransformStreamDefaultController>
    • Returns: undefined or a promise fulfilled with undefined
  • transform <Function> A user-defined function that receives, and potentially modifies, a chunk of data written to transformStream.writable, before forwarding that on to transformStream.readable.
    • chunk <any>
    • controller <TransformStreamDefaultController>
    • Returns: A promise fulfilled with undefined.
  • flush <Function> A user-defined function that is called immediately before the writable side of the TransformStream is closed, signaling the end of the transformation process.
    • controller <TransformStreamDefaultController>
    • Returns: A promise fulfilled with undefined.
  • readableType <any> the readableType option is reserved for future use and must be `undefined.
  • writableType <any> the writableType option is reserved for future use and must be `undefined.
• writableStrategy <Object>
  • highWaterMark <number> The maximum internal queue size before backpressure is applied.
  • size <Function> A user-defined function used to identify the size of each chunk of data.
    • chunk <any>
    • Returns: <number>
• readableStrategy <Object>
  • highWaterMark <number> The maximum internal queue size before backpressure is applied.
  • size <Function> A user-defined function used to identify the size of each chunk of data.
    • chunk <any>
    • Returns: <number>

transformStream.readable#

• Type: <ReadableStream>

transformStream.writable#

• Type: <WritableStream>

Transfering with postMessage()#

A <TransformStream> instance can be transferred using a <MessagePort>.

const stream = new TransformStream();

const { port1, port2 } = new MessageChannel();

port1.onmessage = ({ data }) => {
  const { writable, readable } = data;
  // ...
};

port2.postMessage(stream, [stream]);

Class: TransformStreamDefaultController#

The TransformStreamDefaultController manages the internal state of the TransformStream.

transformStreamDefaultController.desiredSize#

• Type: <number>

The amount of data required to fill the readable side's queue.

transformStreamDefaultController.enqueue([chunk])#

• chunk <any>

Appends a chunk of data to the readable side's queue.

transformStreamDefaultController.error([reason])#

• reason <any>

Signals to both the readable and writable side that an error has occured while processing the transform data, causing both sides to be abruptly closed.

transformStreamDefaultController.terminate()#

Closes the readable side of the transport and causes the writable side to be abruptly closed with an error.

Class: ByteLengthQueuingStrategy#

new ByteLengthQueuingStrategy(options)#

• options <Object>
  • highWaterMark <number>

byteLengthQueuingStrategy.highWaterMark#

• Type: <number>

byteLengthQueuingStrategy.size#

• Type: <Function>
  • chunk <any>
  • Returns: <number>

Class: CountQueuingStrategy#

new CountQueuingStrategy(options)#

• options <Object>
  • highWaterMark <number>

countQueuingStrategy.highWaterMark#

• Type: <number>

countQueuingStrategy.size#

• Type: <Function>
  • chunk <any>
  • Returns: <number>