Stream

Stream

Stability: 2 - Stable

A stream is an abstract interface for working with streaming data in Node.js. The node:stream module provides an API for implementing the stream interface.

There are many stream objects provided by Node.js. For instance, a [request to an HTTP server][http-incoming-message] and [process.stdout][] are both stream instances.

Streams can be readable, writable, or both. All streams are instances of [EventEmitter][].

To access the node:stream module:

const stream = require('node:stream');

The node:stream module is useful for creating new types of stream instances. It is usually not necessary to use the node:stream module to consume streams.

Organization of this document

This document contains two primary sections and a third section for notes. The first section explains how to use existing streams within an application. The second section explains how to create new types of streams.

Types of streams

There are four fundamental stream types within Node.js:

[Writable][]: streams to which data can be written (for example, [fs.createWriteStream()][]).
[Readable][]: streams from which data can be read (for example, [fs.createReadStream()][]).
[Duplex][]: streams that are both Readable and Writable (for example, [net.Socket][]).
[Transform][]: Duplex streams that can modify or transform the data as it is written and read (for example, [zlib.createDeflate()][]).

Additionally, this module includes the utility functions [stream.duplexPair()][], [stream.pipeline()][], [stream.finished()][] [stream.Readable.from()][], and [stream.addAbortSignal()][].

Streams Promises API

The stream/promises API provides an alternative set of asynchronous utility functions for streams that return Promise objects rather than using callbacks. The API is accessible via require('node:stream/promises') or require('node:stream').promises.

`stream.pipeline(streams[, options])`

`stream.pipeline(source[, ...transforms], destination[, options])`

streams {Stream[]|Iterable[]|AsyncIterable[]|Function[]| ReadableStream[]|WritableStream[]|TransformStream[]}
source {Stream|Iterable|AsyncIterable|Function|ReadableStream}
Returns: {Promise|AsyncIterable}
...transforms {Stream|Function|TransformStream}
source {AsyncIterable}
Returns: {Promise|AsyncIterable}
destination {Stream|Function|WritableStream}
source {AsyncIterable}
Returns: {Promise|AsyncIterable}
options {Object} Pipeline options
signal {AbortSignal}
end {boolean} End the destination stream when the source stream ends. Transform streams are always ended, even if this value is false. Default: true.
Returns: {Promise} Fulfills when the pipeline is complete.

const { pipeline } = require('node:stream/promises');
const fs = require('node:fs');
const zlib = require('node:zlib');

async function run() {
  await pipeline(
    fs.createReadStream('archive.tar'),
    zlib.createGzip(),
    fs.createWriteStream('archive.tar.gz'),
  );
  console.log('Pipeline succeeded.');
}

run().catch(console.error);

import { pipeline } from 'node:stream/promises';
import { createReadStream, createWriteStream } from 'node:fs';
import { createGzip } from 'node:zlib';

await pipeline(
  createReadStream('archive.tar'),
  createGzip(),
  createWriteStream('archive.tar.gz'),
);
console.log('Pipeline succeeded.');

To use an AbortSignal, pass it inside an options object, as the last argument. When the signal is aborted, destroy will be called on the underlying pipeline, with an AbortError.

const { pipeline } = require('node:stream/promises');
const fs = require('node:fs');
const zlib = require('node:zlib');

async function run() {
  const ac = new AbortController();
  const signal = ac.signal;

  setImmediate(() => ac.abort());
  await pipeline(
    fs.createReadStream('archive.tar'),
    zlib.createGzip(),
    fs.createWriteStream('archive.tar.gz'),
    { signal },
  );
}

run().catch(console.error); // AbortError

import { pipeline } from 'node:stream/promises';
import { createReadStream, createWriteStream } from 'node:fs';
import { createGzip } from 'node:zlib';

const ac = new AbortController();
const { signal } = ac;
setImmediate(() => ac.abort());
try {
  await pipeline(
    createReadStream('archive.tar'),
    createGzip(),
    createWriteStream('archive.tar.gz'),
    { signal },
  );
} catch (err) {
  console.error(err); // AbortError
}

The pipeline API also supports async generators:

const { pipeline } = require('node:stream/promises');
const fs = require('node:fs');

async function run() {
  await pipeline(
    fs.createReadStream('lowercase.txt'),
    async function* (source, { signal }) {
      source.setEncoding('utf8');  // Work with strings rather than `Buffer`s.
      for await (const chunk of source) {
        yield await processChunk(chunk, { signal });
      }
    },
    fs.createWriteStream('uppercase.txt'),
  );
  console.log('Pipeline succeeded.');
}

run().catch(console.error);

import { pipeline } from 'node:stream/promises';
import { createReadStream, createWriteStream } from 'node:fs';

await pipeline(
  createReadStream('lowercase.txt'),
  async function* (source, { signal }) {
    source.setEncoding('utf8');  // Work with strings rather than `Buffer`s.
    for await (const chunk of source) {
      yield await processChunk(chunk, { signal });
    }
  },
  createWriteStream('uppercase.txt'),
);
console.log('Pipeline succeeded.');

Remember to handle the signal argument passed into the async generator. Especially in the case where the async generator is the source for the pipeline (i.e. first argument) or the pipeline will never complete.

const { pipeline } = require('node:stream/promises');
const fs = require('node:fs');

async function run() {
  await pipeline(
    async function* ({ signal }) {
      await someLongRunningfn({ signal });
      yield 'asd';
    },
    fs.createWriteStream('uppercase.txt'),
  );
  console.log('Pipeline succeeded.');
}

run().catch(console.error);

import { pipeline } from 'node:stream/promises';
import fs from 'node:fs';
await pipeline(
  async function* ({ signal }) {
    await someLongRunningfn({ signal });
    yield 'asd';
  },
  fs.createWriteStream('uppercase.txt'),
);
console.log('Pipeline succeeded.');

The pipeline API provides [callback version][stream-pipeline]:

`stream.finished(stream[, options])`

stream {Stream|ReadableStream|WritableStream} A readable and/or writable stream/webstream.
options {Object}
error {boolean|undefined}
readable {boolean|undefined}
writable {boolean|undefined}
signal {AbortSignal|undefined}
cleanup {boolean|undefined} If true, removes the listeners registered by this function before the promise is fulfilled. Default: false.
Returns: {Promise} Fulfills when the stream is no longer readable or writable.

const { finished } = require('node:stream/promises');
const fs = require('node:fs');

const rs = fs.createReadStream('archive.tar');

async function run() {
  await finished(rs);
  console.log('Stream is done reading.');
}

run().catch(console.error);
rs.resume(); // Drain the stream.

import { finished } from 'node:stream/promises';
import { createReadStream } from 'node:fs';

const rs = createReadStream('archive.tar');

async function run() {
  await finished(rs);
  console.log('Stream is done reading.');
}

run().catch(console.error);
rs.resume(); // Drain the stream.

The finished API also provides a [callback version][stream-finished].

stream.finished() leaves dangling event listeners (in particular 'error', 'end', 'finish' and 'close') after the returned promise is resolved or rejected. The reason for this is so that unexpected 'error' events (due to incorrect stream implementations) do not cause unexpected crashes. If this is unwanted behavior then options.cleanup should be set to true:

await finished(rs, { cleanup: true });

Object mode

All streams created by Node.js APIs operate exclusively on strings, {Buffer}, {TypedArray} and {DataView} objects:

Strings and Buffers are the most common types used with streams.
TypedArray and DataView lets you handle binary data with types like Int32Array or Uint8Array. When you write a TypedArray or DataView to a stream, Node.js processes the raw bytes.

It is possible, however, for stream implementations to work with other types of JavaScript values (with the exception of null, which serves a special purpose within streams). Such streams are considered to operate in “object mode”.

Stream instances are switched into object mode using the objectMode option when the stream is created. Attempting to switch an existing stream into object mode is not safe.

Buffering

Both [Writable][] and [Readable][] streams will store data in an internal buffer.

The amount of data potentially buffered depends on the highWaterMark option passed into the stream's constructor. For normal streams, the highWaterMark option specifies a [total number of bytes][hwm-gotcha]. For streams operating in object mode, the highWaterMark specifies a total number of objects. For streams operating on (but not decoding) strings, the highWaterMark specifies a total number of UTF-16 code units.

Data is buffered in Readable streams when the implementation calls [stream.push(chunk)][stream-push]. If the consumer of the Stream does not call [stream.read()][stream-read], the data will sit in the internal queue until it is consumed.

Once the total size of the internal read buffer reaches the threshold specified by highWaterMark, the stream will temporarily stop reading data from the underlying resource until the data currently buffered can be consumed (that is, the stream will stop calling the internal [readable._read()][] method that is used to fill the read buffer).

Data is buffered in Writable streams when the [writable.write(chunk)][stream-write] method is called repeatedly. While the total size of the internal write buffer is below the threshold set by highWaterMark, calls to writable.write() will return true. Once the size of the internal buffer reaches or exceeds the highWaterMark, false will be returned.

A key goal of the stream API, particularly the [stream.pipe()][] method, is to limit the buffering of data to acceptable levels such that sources and destinations of differing speeds will not overwhelm the available memory.

The highWaterMark option is a threshold, not a limit: it dictates the amount of data that a stream buffers before it stops asking for more data. It does not enforce a strict memory limitation in general. Specific stream implementations may choose to enforce stricter limits but doing so is optional.

Because [Duplex][] and [Transform][] streams are both Readable and Writable, each maintains two separate internal buffers used for reading and writing, allowing each side to operate independently of the other while maintaining an appropriate and efficient flow of data. For example, [net.Socket][] instances are [Duplex][] streams whose Readable side allows consumption of data received from the socket and whose Writable side allows writing data to the socket. Because data may be written to the socket at a faster or slower rate than data is received, each side should operate (and buffer) independently of the other.

The mechanics of the internal buffering are an internal implementation detail and may be changed at any time. However, for certain advanced implementations, the internal buffers can be retrieved using writable.writableBuffer or readable.readableBuffer. Use of these undocumented properties is discouraged.

API for stream consumers

Almost all Node.js applications, no matter how simple, use streams in some manner. The following is an example of using streams in a Node.js application that implements an HTTP server:

const http = require('node:http');

const server = http.createServer((req, res) => {
  // `req` is an http.IncomingMessage, which is a readable stream.
  // `res` is an http.ServerResponse, which is a writable stream.

  let body = '';
  // Get the data as utf8 strings.
  // If an encoding is not set, Buffer objects will be received.
  req.setEncoding('utf8');

  // Readable streams emit 'data' events once a listener is added.
  req.on('data', (chunk) => {
    body += chunk;
  });

  // The 'end' event indicates that the entire body has been received.
  req.on('end', () => {
    try {
      const data = JSON.parse(body);
      // Write back something interesting to the user:
      res.write(typeof data);
      res.end();
    } catch (er) {
      // uh oh! bad json!
      res.statusCode = 400;
      return res.end(`error: ${er.message}`);
    }
  });
});

server.listen(1337);

// $ curl localhost:1337 -d "{}"
// object
// $ curl localhost:1337 -d "\"foo\""
// string
// $ curl localhost:1337 -d "not json"
// error: Unexpected token 'o', "not json" is not valid JSON

[Writable][] streams (such as res in the example) expose methods such as write() and end() that are used to write data onto the stream.

[Readable][] streams use the [EventEmitter][] API for notifying application code when data is available to be read off the stream. That available data can be read from the stream in multiple ways.

Both [Writable][] and [Readable][] streams use the [EventEmitter][] API in various ways to communicate the current state of the stream.

[Duplex][] and [Transform][] streams are both [Writable][] and [Readable][].

Applications that are either writing data to or consuming data from a stream are not required to implement the stream interfaces directly and will generally have no reason to call require('node:stream').

Developers wishing to implement new types of streams should refer to the section [API for stream implementers][].

Writable streams

Writable streams are an abstraction for a destination to which data is written.

Examples of [Writable][] streams include:

[HTTP requests, on the client][]
[HTTP responses, on the server][]
[fs write streams][]
[zlib streams][zlib]
[crypto streams][crypto]
[TCP sockets][]
[child process stdin][]
[process.stdout][], [process.stderr][]

Some of these examples are actually [Duplex][] streams that implement the [Writable][] interface.

All [Writable][] streams implement the interface defined by the stream.Writable class.

While specific instances of [Writable][] streams may differ in various ways, all Writable streams follow the same fundamental usage pattern as illustrated in the example below:

const myStream = getWritableStreamSomehow();
myStream.write('some data');
myStream.write('some more data');
myStream.end('done writing data');

Class: `stream.Writable`

Event: `'close'`

The 'close' event is emitted when the stream and any of its underlying resources (a file descriptor, for example) have been closed. The event indicates that no more events will be emitted, and no further computation will occur.

A [Writable][] stream will always emit the 'close' event if it is created with the emitClose option.

Event: `'drain'`

If a call to [stream.write(chunk)][stream-write] returns false, the 'drain' event will be emitted when it is appropriate to resume writing data to the stream.

// Write the data to the supplied writable stream one million times.
// Be attentive to back-pressure.
function writeOneMillionTimes(writer, data, encoding, callback) {
  let i = 1000000;
  write();
  function write() {
    let ok = true;
    do {
      i--;
      if (i === 0) {
        // Last time!
        writer.write(data, encoding, callback);
      } else {
        // See if we should continue, or wait.
        // Don't pass the callback, because we're not done yet.
        ok = writer.write(data, encoding);
      }
    } while (i > 0 && ok);
    if (i > 0) {
      // Had to stop early!
      // Write some more once it drains.
      writer.once('drain', write);
    }
  }
}

Event: `'error'`

Type: {Error}

The 'error' event is emitted if an error occurred while writing or piping data. The listener callback is passed a single Error argument when called.

The stream is closed when the 'error' event is emitted unless the [autoDestroy][writable-new] option was set to false when creating the stream.

After 'error', no further events other than 'close' should be emitted (including 'error' events).

Event: `'finish'`

The 'finish' event is emitted after the [stream.end()][stream-end] method has been called, and all data has been flushed to the underlying system.

const writer = getWritableStreamSomehow();
for (let i = 0; i < 100; i++) {
  writer.write(`hello, #${i}!\n`);
}
writer.on('finish', () => {
  console.log('All writes are now complete.');
});
writer.end('This is the end\n');

Event: `'pipe'`

src {stream.Readable} source stream that is piping to this writable

The 'pipe' event is emitted when the [stream.pipe()][] method is called on a readable stream, adding this writable to its set of destinations.

const writer = getWritableStreamSomehow();
const reader = getReadableStreamSomehow();
writer.on('pipe', (src) => {
  console.log('Something is piping into the writer.');
  assert.equal(src, reader);
});
reader.pipe(writer);

Event: `'unpipe'`

src {stream.Readable} The source stream that [unpiped][stream.unpipe()] this writable

The 'unpipe' event is emitted when the [stream.unpipe()][] method is called on a [Readable][] stream, removing this [Writable][] from its set of destinations.

This is also emitted in case this [Writable][] stream emits an error when a [Readable][] stream pipes into it.

const writer = getWritableStreamSomehow();
const reader = getReadableStreamSomehow();
writer.on('unpipe', (src) => {
  console.log('Something has stopped piping into the writer.');
  assert.equal(src, reader);
});
reader.pipe(writer);
reader.unpipe(writer);

`writable.cork()`

The writable.cork() method forces all written data to be buffered in memory. The buffered data will be flushed when either the [stream.uncork()][] or [stream.end()][stream-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 implementing writable._writev() may have an adverse effect on throughput.

See also: [writable.uncork()][], [writable._writev()][stream-_writev].

`writable.destroy([error])`

error {Error} Optional, an error to emit with 'error' event.
Returns: {this}

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 to write() 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.

const { Writable } = require('node:stream');

const myStream = new Writable();

const fooErr = new Error('foo error');
myStream.destroy(fooErr);
myStream.on('error', (fooErr) => console.error(fooErr.message)); // foo error

const { Writable } = require('node:stream');

const myStream = new Writable();

myStream.destroy();
myStream.on('error', function wontHappen() {});

const { Writable } = require('node:stream');

const myStream = new Writable();
myStream.destroy();

myStream.write('foo', (error) => console.error(error.code));
// ERR_STREAM_DESTROYED

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()][writable-_destroy].

`writable.closed`

Type: {boolean}

Is true after 'close' has been emitted.

`writable.destroyed`

Type: {boolean}

Is true after [writable.destroy()][writable-destroy] has been called.

const { Writable } = require('node:stream');

const myStream = new Writable();

console.log(myStream.destroyed); // false
myStream.destroy();
console.log(myStream.destroyed); // true

`writable.end([chunk[, encoding]][, callback])`

chunk {string|Buffer|TypedArray|DataView|any} Optional data to write. For streams not operating in object mode, chunk must be a {string}, {Buffer}, {TypedArray} or {DataView}. For object mode streams, chunk may be any JavaScript value other than null.
encoding {string} The encoding if chunk is a string
callback {Function} Callback for when the stream is finished.
Returns: {this}

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 [stream.write()][stream-write] method after calling [stream.end()][stream-end] will raise an error.

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

`writable.setDefaultEncoding(encoding)`

encoding {string} The new default encoding
Returns: {this}

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

`writable.uncork()`

The writable.uncork() method flushes all data buffered since [stream.cork()][] was called.

When using [writable.cork()][] and writable.uncork() to manage the buffering of writes to a stream, defer calls to writable.uncork() using process.nextTick(). Doing so allows batching of all writable.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();
});

`writable.writable`

Type: {boolean}

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

`writable.writableAborted`

Type: {boolean}

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

`writable.writableEnded`

Type: {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.

`writable.writableCorked`

Type: {integer}

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

`writable.errored`

Type: {Error}

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

`writable.writableFinished`

Type: {boolean}

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

`writable.writableHighWaterMark`

Type: {number}

Return the value of highWaterMark passed when creating this Writable.

`writable.writableLength`

Type: {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.

`writable.writableNeedDrain`

Type: {boolean}

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

`writable.writableObjectMode`

Type: {boolean}

Getter for the property objectMode of a given Writable stream.

`writable[Symbol.asyncDispose]()`

Calls [writable.destroy()][writable-destroy] with an AbortError and returns a promise that fulfills when the stream is finished.

`writable.write(chunk[, encoding][, callback])`

chunk {string|Buffer|TypedArray|DataView|any} Optional data to write. For streams not operating in object mode, chunk must be a {string}, {Buffer}, {TypedArray} or {DataView}. For object mode streams, chunk may be any JavaScript value other than null.
encoding {string|null} The encoding, if chunk is a string. Default: 'utf8'
callback {Function} Callback for when this chunk of data is flushed.
Returns: {boolean} 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.

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 the highWaterMark 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 [stream.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.

Readable streams

Readable streams are an abstraction for a source from which data is consumed.

Examples of Readable streams include:

[HTTP responses, on the client][http-incoming-message]
[HTTP requests, on the server][http-incoming-message]
[fs read streams][]
[zlib streams][zlib]
[crypto streams][crypto]
[TCP sockets][]
[child process stdout and stderr][]
[process.stdin][]

All [Readable][] streams implement the interface defined by the stream.Readable class.

Two reading modes

Readable streams effectively operate in one of two modes: flowing and paused. These modes are separate from [object mode][object-mode]. A [Readable][] stream can be in object mode or not, regardless of whether it is in flowing mode or paused mode.

In flowing mode, data is read from the underlying system automatically and provided to an application as quickly as possible using events via the [EventEmitter][] interface.
In paused mode, the [stream.read()][stream-read] method must be called explicitly to read chunks of data from the stream.

All [Readable][] streams begin in paused mode but can be switched to flowing mode in one of the following ways:

Adding a ['data'][] event handler.
Calling the [stream.resume()][stream-resume] method.
Calling the [stream.pipe()][] method to send the data to a [Writable][].

The Readable can switch back to paused mode using one of the following:

If there are no pipe destinations, by calling the [stream.pause()][stream-pause] method.
If there are pipe destinations, by removing all pipe destinations. Multiple pipe destinations may be removed by calling the [stream.unpipe()][] method.

The important concept to remember is that a Readable will not generate data until a mechanism for either consuming or ignoring that data is provided. If the consuming mechanism is disabled or taken away, the Readable will attempt to stop generating the data.

For backward compatibility reasons, removing ['data'][] event handlers will not automatically pause the stream. Also, if there are piped destinations, then calling [stream.pause()][stream-pause] will not guarantee that the stream will remain paused once those destinations drain and ask for more data.

If a [Readable][] is switched into flowing mode and there are no consumers available to handle the data, that data will be lost. This can occur, for instance, when the readable.resume() method is called without a listener attached to the 'data' event, or when a 'data' event handler is removed from the stream.

Adding a ['readable'][] event handler automatically makes the stream stop flowing, and the data has to be consumed via [readable.read()][stream-read]. If the ['readable'][] event handler is removed, then the stream will start flowing again if there is a ['data'][] event handler.

Three states

The “two modes” of operation for a Readable stream are a simplified abstraction for the more complicated internal state management that is happening within the Readable stream implementation.

Specifically, at any given point in time, every Readable is in one of three possible states:

readable.readableFlowing === null
readable.readableFlowing === false
readable.readableFlowing === true

When readable.readableFlowing is null, no mechanism for consuming the stream's data is provided. Therefore, the stream will not generate data. While in this state, attaching a listener for the 'data' event, calling the readable.pipe() method, or calling the readable.resume() method will switch readable.readableFlowing to true, causing the Readable to begin actively emitting events as data is generated.

Calling readable.pause(), readable.unpipe(), or receiving backpressure will cause the readable.readableFlowing to be set as false, temporarily halting the flowing of events but not halting the generation of data. While in this state, attaching a listener for the 'data' event will not switch readable.readableFlowing to true.

const { PassThrough, Writable } = require('node:stream');
const pass = new PassThrough();
const writable = new Writable();

pass.pipe(writable);
pass.unpipe(writable);
// readableFlowing is now false.

pass.on('data', (chunk) => { console.log(chunk.toString()); });
// readableFlowing is still false.
pass.write('ok');  // Will not emit 'data'.
pass.resume();     // Must be called to make stream emit 'data'.
// readableFlowing is now true.

While readable.readableFlowing is false, data may be accumulating within the stream's internal buffer.

Choose one API style

The Readable stream API evolved across multiple Node.js versions and provides multiple methods of consuming stream data. In general, developers should choose one of the methods of consuming data and should never use multiple methods to consume data from a single stream. Specifically, using a combination of on('data'), on('readable'), pipe(), or async iterators could lead to unintuitive behavior.

Class: `stream.Readable`

Event: `'close'`

A [Readable][] stream will always emit the 'close' event if it is created with the emitClose option.

Event: `'data'`

chunk {Buffer|string|any} The chunk of data. For streams that are not operating in object mode, the chunk will be either a string or Buffer. For streams that are in object mode, the chunk can be any JavaScript value other than null.

The 'data' event is emitted whenever the stream is relinquishing ownership of a chunk of data to a consumer. This may occur whenever the stream is switched in flowing mode by calling readable.pipe(), readable.resume(), or by attaching a listener callback to the 'data' event. The 'data' event will also be emitted whenever the readable.read() method is called and a chunk of data is available to be returned.

Attaching a 'data' event listener to a stream that has not been explicitly paused will switch the stream into flowing mode. Data will then be passed as soon as it is available.

The listener callback will be passed the chunk of data as a string if a default encoding has been specified for the stream using the readable.setEncoding() method; otherwise the data will be passed as a Buffer.

const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
  console.log(`Received ${chunk.length} bytes of data.`);
});

Event: `'end'`

The 'end' event is emitted when there is no more data to be consumed from the stream.

The 'end' event will not be emitted unless the data is completely consumed. This can be accomplished by switching the stream into flowing mode, or by calling [stream.read()][stream-read] repeatedly until all data has been consumed.

const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
  console.log(`Received ${chunk.length} bytes of data.`);
});
readable.on('end', () => {
  console.log('There will be no more data.');
});

Event: `'error'`

Type: {Error}

The 'error' event may be emitted by a Readable implementation at any time. Typically, this may occur if the underlying stream is unable to generate data due to an underlying internal failure, or when a stream implementation attempts to push an invalid chunk of data.

The listener callback will be passed a single Error object.

Event: `'pause'`

The 'pause' event is emitted when [stream.pause()][stream-pause] is called and readableFlowing is not false.

Event: `'readable'`

The 'readable' event is emitted when there is data available to be read from the stream, up to the configured high water mark (state.highWaterMark). Effectively, it indicates that the stream has new information within the buffer. If data is available within this buffer, [stream.read()][stream-read] can be called to retrieve that data. Additionally, the 'readable' event may also be emitted when the end of the stream has been reached.

const readable = getReadableStreamSomehow();
readable.on('readable', function() {
  // There is some data to read now.
  let data;

  while ((data = this.read()) !== null) {
    console.log(data);
  }
});

If the end of the stream has been reached, calling [stream.read()][stream-read] will return null and trigger the 'end' event. This is also true if there never was any data to be read. For instance, in the following example, foo.txt is an empty file:

const fs = require('node:fs');
const rr = fs.createReadStream('foo.txt');
rr.on('readable', () => {
  console.log(`readable: ${rr.read()}`);
});
rr.on('end', () => {
  console.log('end');
});

The output of running this script is:

$ node test.js
readable: null
end

In some cases, attaching a listener for the 'readable' event will cause some amount of data to be read into an internal buffer.

In general, the readable.pipe() and 'data' event mechanisms are easier to understand than the 'readable' event. However, handling 'readable' might result in increased throughput.

If both 'readable' and ['data'][] are used at the same time, 'readable' takes precedence in controlling the flow, i.e. 'data' will be emitted only when [stream.read()][stream-read] is called. The readableFlowing property would become false. If there are 'data' listeners when 'readable' is removed, the stream will start flowing, i.e. 'data' events will be emitted without calling .resume().

Event: `'resume'`

The 'resume' event is emitted when [stream.resume()][stream-resume] is called and readableFlowing is not true.

`readable.destroy([error])`

error {Error} Error which will be passed as payload in 'error' event
Returns: {this}

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()][readable-_destroy].

`readable.closed`

Type: {boolean}

Is true after 'close' has been emitted.

`readable.destroyed`

Type: {boolean}

Is true after [readable.destroy()][readable-destroy] has been called.

`readable.isPaused()`

Returns: {boolean}

The readable.isPaused() method returns the current operating state of the Readable. This is used primarily by the mechanism that underlies the readable.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

`readable.pause()`

Returns: {this}

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.

``js 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.'

（本文超出站点单文档体积上限，已截断）

Stream

Stream

Organization of this document

Types of streams

Streams Promises API

stream.pipeline(streams[, options])

stream.pipeline(source[, ...transforms], destination[, options])

stream.finished(stream[, options])

Object mode

Buffering

API for stream consumers

Writable streams

Class: stream.Writable

Event: 'close'

Event: 'drain'

Event: 'error'

Event: 'finish'

Event: 'pipe'

Event: 'unpipe'

writable.cork()

writable.destroy([error])

writable.closed

writable.destroyed

writable.end([chunk[, encoding]][, callback])

writable.setDefaultEncoding(encoding)

writable.uncork()

writable.writable

writable.writableAborted

writable.writableEnded

writable.writableCorked

writable.errored

writable.writableFinished

writable.writableHighWaterMark

writable.writableLength

writable.writableNeedDrain

writable.writableObjectMode

writable[Symbol.asyncDispose]()

writable.write(chunk[, encoding][, callback])

Readable streams

Two reading modes

Three states

Choose one API style

Class: stream.Readable

Event: 'close'

Event: 'data'

Event: 'end'

Event: 'error'

Event: 'pause'

Event: 'readable'

Event: 'resume'

readable.destroy([error])

readable.closed

readable.destroyed

readable.isPaused()

readable.pause()

`stream.pipeline(streams[, options])`

`stream.pipeline(source[, ...transforms], destination[, options])`

`stream.finished(stream[, options])`

Class: `stream.Writable`

Event: `'close'`

Event: `'drain'`

Event: `'error'`

Event: `'finish'`

Event: `'pipe'`

Event: `'unpipe'`

`writable.cork()`

`writable.destroy([error])`

`writable.closed`

`writable.destroyed`

`writable.end([chunk[, encoding]][, callback])`

`writable.setDefaultEncoding(encoding)`

`writable.uncork()`

`writable.writable`

`writable.writableAborted`

`writable.writableEnded`

`writable.writableCorked`

`writable.errored`

`writable.writableFinished`

`writable.writableHighWaterMark`

`writable.writableLength`

`writable.writableNeedDrain`

`writable.writableObjectMode`

`writable[Symbol.asyncDispose]()`

`writable.write(chunk[, encoding][, callback])`

Class: `stream.Readable`

Event: `'close'`

Event: `'data'`

Event: `'end'`

Event: `'error'`

Event: `'pause'`

Event: `'readable'`

Event: `'resume'`

`readable.destroy([error])`

`readable.closed`

`readable.destroyed`

`readable.isPaused()`

`readable.pause()`