- Assertion testing
- Asynchronous context tracking
- Async hooks
- Buffer
- C++ addons
- C/C++ addons with Node-API
- C++ embedder API
- Child processes
- Cluster
- Command-line options
- Console
- Corepack
- Crypto
- Debugger
- Deprecated APIs
- Diagnostics Channel
- DNS
- Domain
- Errors
- Events
- File system
- Globals
- HTTP
- HTTP/2
- HTTPS
- Inspector
- Internationalization
- Modules: CommonJS modules
- Modules: ECMAScript modules
- Modules:
node:module
API - Modules: Packages
- Net
- OS
- Path
- Performance hooks
- Permissions
- Process
- Punycode
- Query strings
- Readline
- REPL
- Report
- Single executable applications
- Stream
- String decoder
- Test runner
- Timers
- TLS/SSL
- Trace events
- TTY
- UDP/datagram
- URL
- Utilities
- V8
- VM
- WASI
- Web Crypto API
- Web Streams API
- Worker threads
- Zlib
Node.js v20.0.0-rc.2 documentation
- Node.js v20.0.0-rc.2
-
►
Table of contents
- Stream
- Organization of this document
- Types of streams
- 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.write(chunk[, encoding][, callback])
- Event:
- Class:
- 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()
readable.pipe(destination[, options])
readable.read([size])
readable.readable
readable.readableAborted
readable.readableDidRead
readable.readableEncoding
readable.readableEnded
readable.errored
readable.readableFlowing
readable.readableHighWaterMark
readable.readableLength
readable.readableObjectMode
readable.resume()
readable.setEncoding(encoding)
readable.unpipe([destination])
readable.unshift(chunk[, encoding])
readable.wrap(stream)
readable[Symbol.asyncIterator]()
readable.compose(stream[, options])
readable.iterator([options])
readable.map(fn[, options])
readable.filter(fn[, options])
readable.forEach(fn[, options])
readable.toArray([options])
readable.some(fn[, options])
readable.find(fn[, options])
readable.every(fn[, options])
readable.flatMap(fn[, options])
readable.drop(limit[, options])
readable.take(limit[, options])
readable.asIndexedPairs([options])
readable.reduce(fn[, initial[, options]])
- Event:
- Duplex and transform streams
stream.finished(stream[, options], callback)
stream.pipeline(source[, ...transforms], destination, callback)
stream.pipeline(streams, callback)
stream.compose(...streams)
stream.Readable.from(iterable[, options])
stream.Readable.fromWeb(readableStream[, options])
stream.Readable.isDisturbed(stream)
stream.isErrored(stream)
stream.isReadable(stream)
stream.Readable.toWeb(streamReadable[, options])
stream.Writable.fromWeb(writableStream[, options])
stream.Writable.toWeb(streamWritable)
stream.Duplex.from(src)
stream.Duplex.fromWeb(pair[, options])
stream.Duplex.toWeb(streamDuplex)
stream.addAbortSignal(signal, stream)
stream.getDefaultHighWaterMark(objectMode)
stream.setDefaultHighWaterMark(objectMode, value)
- Writable streams
- API for stream implementers
- Additional notes
- Stream
-
►
Index
- Assertion testing
- Asynchronous context tracking
- Async hooks
- Buffer
- C++ addons
- C/C++ addons with Node-API
- C++ embedder API
- Child processes
- Cluster
- Command-line options
- Console
- Corepack
- Crypto
- Debugger
- Deprecated APIs
- Diagnostics Channel
- DNS
- Domain
- Errors
- Events
- File system
- Globals
- HTTP
- HTTP/2
- HTTPS
- Inspector
- Internationalization
- Modules: CommonJS modules
- Modules: ECMAScript modules
- Modules:
node:module
API - Modules: Packages
- Net
- OS
- Path
- Performance hooks
- Permissions
- Process
- Punycode
- Query strings
- Readline
- REPL
- Report
- Single executable applications
- Stream
- String decoder
- Test runner
- Timers
- TLS/SSL
- Trace events
- TTY
- UDP/datagram
- URL
- Utilities
- V8
- VM
- WASI
- Web Crypto API
- Web Streams API
- Worker threads
- Zlib
- ► Other versions
- ► Options
Table of contents
- Stream
- Organization of this document
- Types of streams
- 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.write(chunk[, encoding][, callback])
- Event:
- Class:
- 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()
readable.pipe(destination[, options])
readable.read([size])
readable.readable
readable.readableAborted
readable.readableDidRead
readable.readableEncoding
readable.readableEnded
readable.errored
readable.readableFlowing
readable.readableHighWaterMark
readable.readableLength
readable.readableObjectMode
readable.resume()
readable.setEncoding(encoding)
readable.unpipe([destination])
readable.unshift(chunk[, encoding])
readable.wrap(stream)
readable[Symbol.asyncIterator]()
readable.compose(stream[, options])
readable.iterator([options])
readable.map(fn[, options])
readable.filter(fn[, options])
readable.forEach(fn[, options])
readable.toArray([options])
readable.some(fn[, options])
readable.find(fn[, options])
readable.every(fn[, options])
readable.flatMap(fn[, options])
readable.drop(limit[, options])
readable.take(limit[, options])
readable.asIndexedPairs([options])
readable.reduce(fn[, initial[, options]])
- Event:
- Duplex and transform streams
stream.finished(stream[, options], callback)
stream.pipeline(source[, ...transforms], destination, callback)
stream.pipeline(streams, callback)
stream.compose(...streams)
stream.Readable.from(iterable[, options])
stream.Readable.fromWeb(readableStream[, options])
stream.Readable.isDisturbed(stream)
stream.isErrored(stream)
stream.isReadable(stream)
stream.Readable.toWeb(streamReadable[, options])
stream.Writable.fromWeb(writableStream[, options])
stream.Writable.toWeb(streamWritable)
stream.Duplex.from(src)
stream.Duplex.fromWeb(pair[, options])
stream.Duplex.toWeb(streamDuplex)
stream.addAbortSignal(signal, stream)
stream.getDefaultHighWaterMark(objectMode)
stream.setDefaultHighWaterMark(objectMode, value)
- Writable streams
- API for stream implementers
- Additional notes
Stream[src]#
Source Code: lib/stream.js
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 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 bothReadable
andWritable
(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.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(source[, ...transforms], destination[, options])
#
stream.pipeline(streams[, options])
#
streams
<Stream[]> | <Iterable[]> | <AsyncIterable[]> | <Function[]>source
<Stream> | <Iterable> | <AsyncIterable> | <Function>- Returns: <Promise> | <AsyncIterable>
...transforms
<Stream> | <Function>source
<AsyncIterable>- Returns: <Promise> | <AsyncIterable>
destination
<Stream> | <Function>source
<AsyncIterable>- Returns: <Promise> | <AsyncIterable>
options
<Object>signal
<AbortSignal>end
<boolean>
- 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.finished(stream[, options])
#
stream
<Stream>options
<Object>error
<boolean> | <undefined>readable
<boolean> | <undefined>writable
<boolean> | <undefined>signal
: <AbortSignal> | <undefined>
- 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 provides callback version:
Object mode#
All streams created by Node.js APIs operate exclusively on strings and Buffer
(or Uint8Array
) objects. 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. For streams operating
in object mode, the highWaterMark
specifies a total number of objects.
Data is buffered in Readable
streams when the implementation calls
stream.push(chunk)
. If the consumer of the Stream does not
call 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)
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
- crypto streams
- 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)
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'
#
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
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()
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 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()
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()
.
writable.destroy([error])
#
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.closed
#
Is true
after 'close'
has been emitted.
writable.destroyed
#
Is true
after 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> | <Uint8Array> | <any> Optional data to write. For streams not operating in object mode,chunk
must be a string,Buffer
orUint8Array
. For object mode streams,chunk
may be any JavaScript value other thannull
.encoding
<string> The encoding ifchunk
is a stringcallback
<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()
method after calling
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)
#
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();
});
See also: writable.cork()
.
writable.writable
#
Is true
if it is safe to call writable.write()
, which means
the stream has not been destroyed, errored, or ended.
writable.writableAborted
#
Returns whether the stream was destroyed or errored before emitting 'finish'
.
writable.writableEnded
#
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
#
Number of times writable.uncork()
needs to be
called in order to fully uncork the stream.
writable.errored
#
Returns error if the stream has been destroyed with an error.
writable.writableFinished
#
Is set to true
immediately before the 'finish'
event is emitted.
writable.writableHighWaterMark
#
Return the value of highWaterMark
passed when creating this Writable
.
writable.writableLength
#
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
#
Is true
if the stream's buffer has been full and stream will emit 'drain'
.
writable.writableObjectMode
#
Getter for the property objectMode
of a given Writable
stream.
writable.write(chunk[, encoding][, callback])
#
chunk
<string> | <Buffer> | <Uint8Array> | <any> Optional data to write. For streams not operating in object mode,chunk
must be a string,Buffer
orUint8Array
. For object mode streams,chunk
may be any JavaScript value other thannull
.encoding
<string> | <null> The encoding, ifchunk
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; otherwisetrue
.
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 requests, on the server
- fs read streams
- zlib streams
- crypto streams
- 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.
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()
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()
method. - Calling the
stream.pipe()
method to send the data to aWritable
.
The Readable
can switch back to paused mode using one of the following:
- If there are no pipe destinations, by calling the
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()
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()
. 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'
#
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 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 orBuffer
. For streams that are in object mode, the chunk can be any JavaScript value other thannull
.
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()
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'
#
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()
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 or when the end of the stream has been reached. Effectively, the
'readable'
event indicates that the stream has new information. If data is
available, stream.read()
will return that data.
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()
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()
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()
is
called and readableFlowing
is not true
.
readable.destroy([error])
#
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.closed
#
Is true
after 'close'
has been emitted.
readable.destroyed
#
Is true
after 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.
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.
readable.pipe(destination[, options])
#
destination
<stream.Writable> The destination for writing dataoptions
<Object> Pipe optionsend
<boolean> End the writer when the reader ends. Default:true
.
- Returns: <stream.Writable> The destination, allowing for a chain of pipes if
it is a
Duplex
or aTransform
stream
The readable.pipe()
method attaches a Writable
stream to the readable
,
causing it to switch automatically into flowing mode and push all of its data
to the attached Writable
. The flow of data will be automatically managed
so that the destination Writable
stream is not overwhelmed by a faster
Readable
stream.
The following example pipes all of the data from the readable
into a file
named file.txt
:
const fs = require('node:fs');
const readable = getReadableStreamSomehow();
const writable = fs.createWriteStream('file.txt');
// All the data from readable goes into 'file.txt'.
readable.pipe(writable);
It is possible to attach multiple Writable
streams to a single Readable
stream.
The readable.pipe()
method returns a reference to the destination stream
making it possible to set up chains of piped streams:
const fs = require('node:fs');
const zlib = require('node:zlib');
const r = fs.createReadStream('file.txt');
const z = zlib.createGzip();
const w = fs.createWriteStream('file.txt.gz');
r.pipe(z).pipe(w);
By default, stream.end()
is called on the destination Writable
stream when the source Readable
stream emits 'end'
, so that the
destination is no longer writable. To disable this default behavior, the end
option can be passed as false
, causing the destination stream to remain open:
reader.pipe(writer, { end: false });
reader.on('end', () => {
writer.end('Goodbye\n');
});
One important caveat is that if the Readable
stream emits an error during
processing, the Writable
destination is not closed automatically. If an
error occurs, it will be necessary to manually close each stream in order
to prevent memory leaks.
The process.stderr
and process.stdout
Writable
streams are never
closed until the Node.js process exits, regardless of the specified options.
readable.read([size])
#
size
<number> Optional argument to specify how much data to read.- Returns: <string> | <Buffer> | <null> | <any>
The readable.read()
method reads data out of the internal buffer and
returns it. If no data is available to be read, null
is returned. By default,
the data is 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. If
size
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 the
size
argument.
If the readable.read()
method returns a chunk of data, a 'data'
event will
also be emitted.
Calling stream.read([size])
after the 'end'
event has
been emitted will return null
. No runtime error will be raised.
readable.readable
#
Is true
if it is safe to call readable.read()
, which means
the stream has not been destroyed or emitted 'error'
or 'end'
.
readable.readableAborted
#
Returns whether the stream was destroyed or errored before emitting 'end'
.
readable.readableDidRead
#
Returns whether 'data'
has been emitted.
readable.readableEncoding
#
Getter for the property encoding
of a given Readable
stream. The encoding
property can be set using the readable.setEncoding()
method.
readable.readableEnded
#
Becomes true
when 'end'
event is emitted.
readable.errored
#
Returns error if the stream has been destroyed with an error.
readable.readableFlowing
#
This property reflects the current state of a Readable
stream as described
in the Three states section.
readable.readableHighWaterMark
#
Returns the value of highWaterMark
passed when creating this Readable
.
readable.readableLength
#
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
.
readable.readableObjectMode
#
Getter for the property objectMode
of a given Readable
stream.
readable.resume()
#
- Returns: <this>
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.
readable.setEncoding(encoding)
#
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 as
Buffer
objects. Setting an encoding causes the stream data
to be returned as strings of the specified encoding rather than as Buffer
objects. For instance, calling readable.setEncoding('utf8')
will cause the
output data to be interpreted as UTF-8 data, and passed as strings. Calling
readable.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);
});
readable.unpipe([destination])
#
destination
<stream.Writable> Optional specific stream to unpipe- Returns: <this>
The readable.unpipe()
method detaches a Writable
stream previously attached
using the stream.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.
const fs = require('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);
readable.unshift(chunk[, encoding])
#
chunk
<Buffer> | <Uint8Array> | <string> | <null> | <any> Chunk of data to unshift onto the read queue. For streams not operating in object mode,chunk
must be a string,Buffer
,Uint8Array
, ornull
. For object mode streams,chunk
may be any JavaScript value.encoding
<string> Encoding of string chunks. Must be a validBuffer
encoding, such as'utf8'
or'ascii'
.
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).
const { StringDecoder } = require('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.includes('\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);
return;
}
// Still reading the header.
header += str;
}
}
}
Unlike stream.push(chunk)
, 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 stream._read()
implementation on a
custom stream). Following the call to readable.unshift()
with an immediate
stream.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.
readable.wrap(stream)
#
Prior to Node.js 0.10, streams did not implement the entire node: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
stream.pause()
method that is advisory only, the
readable.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.
const { OldReader } = require('./old-api-module.js');
const { Readable } = require('node:stream');
const oreader = new OldReader();
const myReader = new Readable().wrap(oreader);
myReader.on('readable', () => {
myReader.read(); // etc.
});
readable[Symbol.asyncIterator]()
#
- Returns: <AsyncIterator> to fully consume the stream.
const fs = require('node:fs');
async function print(readable) {
readable.setEncoding('utf8');
let data = '';
for await (const chunk of readable) {
data += chunk;
}
console.log(data);
}
print(fs.createReadStream('file')).catch(console.error);
If the loop terminates with a break
, return
, or a throw
, the stream will
be destroyed. In other terms, iterating over a stream will consume the stream
fully. The stream will be read in chunks of size equal to the highWaterMark
option. In the code example above, data will be in a single chunk if the file
has less then 64 KiB of data because no highWaterMark
option is provided to
fs.createReadStream()
.
readable.compose(stream[, options])
#
stream
<Stream> | <Iterable> | <AsyncIterable> | <Function>options
<Object>signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Duplex> a stream composed with the stream
stream
.
import { Readable } from 'node:stream';
async function* splitToWords(source) {
for await (const chunk of source) {
const words = String(chunk).split(' ');
for (const word of words) {
yield word;
}
}
}
const wordsStream = Readable.from(['this is', 'compose as operator']).compose(splitToWords);
const words = await wordsStream.toArray();
console.log(words); // prints ['this', 'is', 'compose', 'as', 'operator']
See stream.compose
for more information.
readable.iterator([options])
#
options
<Object>destroyOnReturn
<boolean> When set tofalse
, callingreturn
on the async iterator, or exiting afor await...of
iteration using abreak
,return
, orthrow
will not destroy the stream. Default:true
.
- Returns: <AsyncIterator> to consume the stream.
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.
const { Readable } = require('node:stream');
async function printIterator(readable) {
for await (const chunk of readable.iterator({ destroyOnReturn: false })) {
console.log(chunk); // 1
break;
}
console.log(readable.destroyed); // false
for await (const chunk of readable.iterator({ destroyOnReturn: false })) {
console.log(chunk); // Will print 2 and then 3
}
console.log(readable.destroyed); // True, stream was totally consumed
}
async function printSymbolAsyncIterator(readable) {
for await (const chunk of readable) {
console.log(chunk); // 1
break;
}
console.log(readable.destroyed); // true
}
async function showBoth() {
await printIterator(Readable.from([1, 2, 3]));
await printSymbolAsyncIterator(Readable.from([1, 2, 3]));
}
showBoth();
readable.map(fn[, options])
#
fn
<Function> | <AsyncFunction> a function to map over every chunk in the stream.data
<any> a chunk of data from the stream.options
<Object>signal
<AbortSignal> aborted if the stream is destroyed allowing to abort thefn
call early.
options
<Object>concurrency
<number> the maximum concurrent invocation offn
to call on the stream at once. Default:1
.signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Readable> a stream mapped with the function
fn
.
This method allows mapping over the stream. The fn
function will be called
for every chunk in the stream. If the fn
function returns a promise - that
promise will be await
ed before being passed to the result stream.
import { Readable } from 'node:stream';
import { Resolver } from 'node:dns/promises';
// With a synchronous mapper.
for await (const chunk of Readable.from([1, 2, 3, 4]).map((x) => x * 2)) {
console.log(chunk); // 2, 4, 6, 8
}
// With an asynchronous mapper, making at most 2 queries at a time.
const resolver = new Resolver();
const dnsResults = Readable.from([
'nodejs.org',
'openjsf.org',
'www.linuxfoundation.org',
]).map((domain) => resolver.resolve4(domain), { concurrency: 2 });
for await (const result of dnsResults) {
console.log(result); // Logs the DNS result of resolver.resolve4.
}
readable.filter(fn[, options])
#
fn
<Function> | <AsyncFunction> a function to filter chunks from the stream.data
<any> a chunk of data from the stream.options
<Object>signal
<AbortSignal> aborted if the stream is destroyed allowing to abort thefn
call early.
options
<Object>concurrency
<number> the maximum concurrent invocation offn
to call on the stream at once. Default:1
.signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Readable> a stream filtered with the predicate
fn
.
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 await
ed.
import { Readable } from 'node:stream';
import { Resolver } from 'node:dns/promises';
// With a synchronous predicate.
for await (const chunk of Readable.from([1, 2, 3, 4]).filter((x) => x > 2)) {
console.log(chunk); // 3, 4
}
// With an asynchronous predicate, making at most 2 queries at a time.
const resolver = new Resolver();
const dnsResults = Readable.from([
'nodejs.org',
'openjsf.org',
'www.linuxfoundation.org',
]).filter(async (domain) => {
const { address } = await resolver.resolve4(domain, { ttl: true });
return address.ttl > 60;
}, { concurrency: 2 });
for await (const result of dnsResults) {
// Logs domains with more than 60 seconds on the resolved dns record.
console.log(result);
}
readable.forEach(fn[, options])
#
fn
<Function> | <AsyncFunction> a function to call on each chunk of the stream.data
<any> a chunk of data from the stream.options
<Object>signal
<AbortSignal> aborted if the stream is destroyed allowing to abort thefn
call early.
options
<Object>concurrency
<number> the maximum concurrent invocation offn
to call on the stream at once. Default:1
.signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Promise> a promise for when the stream has finished.
This method allows iterating a stream. For each chunk in the stream the
fn
function will be called. If the fn
function returns a promise - that
promise will be await
ed.
This method is different from for await...of
loops in that it can optionally
process chunks concurrently. In addition, a forEach
iteration can only be
stopped by having passed a signal
option and aborting the related
AbortController
while for await...of
can be stopped with break
or
return
. In either case the stream will be destroyed.
This method is different from listening to the 'data'
event in that it
uses the readable
event in the underlying machinary and can limit the
number of concurrent fn
calls.
import { Readable } from 'node:stream';
import { Resolver } from 'node:dns/promises';
// With a synchronous predicate.
for await (const chunk of Readable.from([1, 2, 3, 4]).filter((x) => x > 2)) {
console.log(chunk); // 3, 4
}
// With an asynchronous predicate, making at most 2 queries at a time.
const resolver = new Resolver();
const dnsResults = Readable.from([
'nodejs.org',
'openjsf.org',
'www.linuxfoundation.org',
]).map(async (domain) => {
const { address } = await resolver.resolve4(domain, { ttl: true });
return address;
}, { concurrency: 2 });
await dnsResults.forEach((result) => {
// Logs result, similar to `for await (const result of dnsResults)`
console.log(result);
});
console.log('done'); // Stream has finished
readable.toArray([options])
#
options
<Object>signal
<AbortSignal> allows cancelling the toArray operation if the signal is aborted.
- Returns: <Promise> a promise containing an array with the contents of the stream.
This method allows easily obtaining the contents of a stream.
As this method reads the entire stream into memory, it negates the benefits of streams. It's intended for interoperability and convenience, not as the primary way to consume streams.
import { Readable } from 'node:stream';
import { Resolver } from 'node:dns/promises';
await Readable.from([1, 2, 3, 4]).toArray(); // [1, 2, 3, 4]
// Make dns queries concurrently using .map and collect
// the results into an array using toArray
const dnsResults = await Readable.from([
'nodejs.org',
'openjsf.org',
'www.linuxfoundation.org',
]).map(async (domain) => {
const { address } = await resolver.resolve4(domain, { ttl: true });
return address;
}, { concurrency: 2 }).toArray();
readable.some(fn[, options])
#
fn
<Function> | <AsyncFunction> a function to call on each chunk of the stream.data
<any> a chunk of data from the stream.options
<Object>signal
<AbortSignal> aborted if the stream is destroyed allowing to abort thefn
call early.
options
<Object>concurrency
<number> the maximum concurrent invocation offn
to call on the stream at once. Default:1
.signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Promise> a promise evaluating to
true
iffn
returned a truthy value for at least one of the chunks.
This method is similar to Array.prototype.some
and calls fn
on each chunk
in the stream until the awaited return value is true
(or any truthy value).
Once an fn
call on a chunk awaited return value is truthy, the stream is
destroyed and the promise is fulfilled with true
. If none of the fn
calls on the chunks return a truthy value, the promise is fulfilled with
false
.
import { Readable } from 'node:stream';
import { stat } from 'node:fs/promises';
// With a synchronous predicate.
await Readable.from([1, 2, 3, 4]).some((x) => x > 2); // true
await Readable.from([1, 2, 3, 4]).some((x) => x < 0); // false
// With an asynchronous predicate, making at most 2 file checks at a time.
const anyBigFile = await Readable.from([
'file1',
'file2',
'file3',
]).some(async (fileName) => {
const stats = await stat(fileName);
return stats.size > 1024 * 1024;
}, { concurrency: 2 });
console.log(anyBigFile); // `true` if any file in the list is bigger than 1MB
console.log('done'); // Stream has finished
readable.find(fn[, options])
#
fn
<Function> | <AsyncFunction> a function to call on each chunk of the stream.data
<any> a chunk of data from the stream.options
<Object>signal
<AbortSignal> aborted if the stream is destroyed allowing to abort thefn
call early.
options
<Object>concurrency
<number> the maximum concurrent invocation offn
to call on the stream at once. Default:1
.signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Promise> a promise evaluating to the first chunk for which
fn
evaluated with a truthy value, orundefined
if no element was found.
This method is similar to Array.prototype.find
and calls fn
on each chunk
in the stream to find a chunk with a truthy value for fn
. Once an fn
call's
awaited return value is truthy, the stream is destroyed and the promise is
fulfilled with value for which fn
returned a truthy value. If all of the
fn
calls on the chunks return a falsy value, the promise is fulfilled with
undefined
.
import { Readable } from 'node:stream';
import { stat } from 'node:fs/promises';
// With a synchronous predicate.
await Readable.from([1, 2, 3, 4]).find((x) => x > 2); // 3
await Readable.from([1, 2, 3, 4]).find((x) => x > 0); // 1
await Readable.from([1, 2, 3, 4]).find((x) => x > 10); // undefined
// With an asynchronous predicate, making at most 2 file checks at a time.
const foundBigFile = await Readable.from([
'file1',
'file2',
'file3',
]).find(async (fileName) => {
const stats = await stat(fileName);
return stats.size > 1024 * 1024;
}, { concurrency: 2 });
console.log(foundBigFile); // File name of large file, if any file in the list is bigger than 1MB
console.log('done'); // Stream has finished
readable.every(fn[, options])
#
fn
<Function> | <AsyncFunction> a function to call on each chunk of the stream.data
<any> a chunk of data from the stream.options
<Object>signal
<AbortSignal> aborted if the stream is destroyed allowing to abort thefn
call early.
options
<Object>concurrency
<number> the maximum concurrent invocation offn
to call on the stream at once. Default:1
.signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Promise> a promise evaluating to
true
iffn
returned a truthy value for all of the chunks.
This method is similar to Array.prototype.every
and calls fn
on each chunk
in the stream to check if all awaited return values are truthy value for fn
.
Once an fn
call on a chunk awaited return value is falsy, the stream is
destroyed and the promise is fulfilled with false
. If all of the fn
calls
on the chunks return a truthy value, the promise is fulfilled with true
.
import { Readable } from 'node:stream';
import { stat } from 'node:fs/promises';
// With a synchronous predicate.
await Readable.from([1, 2, 3, 4]).every((x) => x > 2); // false
await Readable.from([1, 2, 3, 4]).every((x) => x > 0); // true
// With an asynchronous predicate, making at most 2 file checks at a time.
const allBigFiles = await Readable.from([
'file1',
'file2',
'file3',
]).every(async (fileName) => {
const stats = await stat(fileName);
return stats.size > 1024 * 1024;
}, { concurrency: 2 });
// `true` if all files in the list are bigger than 1MiB
console.log(allBigFiles);
console.log('done'); // Stream has finished
readable.flatMap(fn[, options])
#
fn
<Function> | <AsyncGeneratorFunction> | <AsyncFunction> a function to map over every chunk in the stream.data
<any> a chunk of data from the stream.options
<Object>signal
<AbortSignal> aborted if the stream is destroyed allowing to abort thefn
call early.
options
<Object>concurrency
<number> the maximum concurrent invocation offn
to call on the stream at once. Default:1
.signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Readable> a stream flat-mapped with the function
fn
.
This method returns a new stream by applying the given callback to each chunk of the stream and then flattening the result.
It is possible to return a stream or another iterable or async iterable from
fn
and the result streams will be merged (flattened) into the returned
stream.
import { Readable } from 'node:stream';
import { createReadStream } from 'node:fs';
// With a synchronous mapper.
for await (const chunk of Readable.from([1, 2, 3, 4]).flatMap((x) => [x, x])) {
console.log(chunk); // 1, 1, 2, 2, 3, 3, 4, 4
}
// With an asynchronous mapper, combine the contents of 4 files
const concatResult = Readable.from([
'./1.mjs',
'./2.mjs',
'./3.mjs',
'./4.mjs',
]).flatMap((fileName) => createReadStream(fileName));
for await (const result of concatResult) {
// This will contain the contents (all chunks) of all 4 files
console.log(result);
}
readable.drop(limit[, options])
#
limit
<number> the number of chunks to drop from the readable.options
<Object>signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Readable> a stream with
limit
chunks dropped.
This method returns a new stream with the first limit
chunks dropped.
import { Readable } from 'node:stream';
await Readable.from([1, 2, 3, 4]).drop(2).toArray(); // [3, 4]
readable.take(limit[, options])
#
limit
<number> the number of chunks to take from the readable.options
<Object>signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Readable> a stream with
limit
chunks taken.
This method returns a new stream with the first limit
chunks.
import { Readable } from 'node:stream';
await Readable.from([1, 2, 3, 4]).take(2).toArray(); // [1, 2]
readable.asIndexedPairs([options])
#
options
<Object>signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Readable> a stream of indexed pairs.
This method returns a new stream with chunks of the underlying stream paired
with a counter in the form [index, chunk]
. The first index value is 0 and it
increases by 1 for each chunk produced.
import { Readable } from 'node:stream';
const pairs = await Readable.from(['a', 'b', 'c']).asIndexedPairs().toArray();
console.log(pairs); // [[0, 'a'], [1, 'b'], [2, 'c']]
readable.reduce(fn[, initial[, options]])
#
fn
<Function> | <AsyncFunction> a reducer function to call over every chunk in the stream.previous
<any> the value obtained from the last call tofn
or theinitial
value if specified or the first chunk of the stream otherwise.data
<any> a chunk of data from the stream.options
<Object>signal
<AbortSignal> aborted if the stream is destroyed allowing to abort thefn
call early.
initial
<any> the initial value to use in the reduction.options
<Object>signal
<AbortSignal> allows destroying the stream if the signal is aborted.
- Returns: <Promise> a promise for the final value of the reduction.
This method calls fn
on each chunk of the stream in order, passing it the
result from the calculation on the previous element. It returns a promise for
the final value of the reduction.
If no initial
value is supplied the first chunk of the stream is used as the
initial value. If the stream is empty, the promise is rejected with a
TypeError
with the ERR_INVALID_ARGS
code property.
import { Readable } from 'node:stream';
import { readdir, stat } from 'node:fs/promises';
import { join } from 'node:path';
const directoryPath = './src';
const filesInDir = await readdir(directoryPath);
const folderSize = await Readable.from(filesInDir)
.reduce(async (totalSize, file) => {
const { size } = await stat(join(directoryPath, file));
return totalSize + size;
}, 0);
console.log(folderSize);
The reducer function iterates the stream element-by-element which means that
there is no concurrency
parameter or parallelism. To perform a reduce
concurrently, you can extract the async function to readable.map
method.
import { Readable } from 'node:stream';
import { readdir, stat } from 'node:fs/promises';
import { join } from 'node:path';
const directoryPath = './src';
const filesInDir = await readdir(directoryPath);
const folderSize = await Readable.from(filesInDir)
.map((file) => stat(join(directoryPath, file)), { concurrency: 2 })
.reduce((totalSize, { size }) => totalSize + size, 0);
console.log(folderSize);
Duplex and transform streams#
Class: stream.Duplex
#
Duplex streams are streams that implement both the Readable
and
Writable
interfaces.
Examples of Duplex
streams include:
duplex.allowHalfOpen
#
If false
then the stream will automatically end the writable side when the
readable side ends. Set initially by the allowHalfOpen
constructor option,
which defaults to true
.
This can be changed manually to change the half-open behavior of an existing
Duplex
stream instance, but must be changed before the 'end'
event is
emitted.
Class: stream.Transform
#
Transform streams are Duplex
streams where the output is in some way
related to the input. Like all Duplex
streams, Transform
streams
implement both the Readable
and Writable
interfaces.
Examples of Transform
streams include:
transform.destroy([error])
#
Destroy the stream, and optionally emit an 'error'
event. After this call, the
transform stream would release any internal resources.
Implementors should not override this method, but instead implement
readable._destroy()
.
The default implementation of _destroy()
for Transform
also emit 'close'
unless emitClose
is set in false.
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'
.
stream.finished(stream[, options], callback)
#
stream
<Stream> | <ReadableStream> | <WritableStream>
A readable and/or writable stream/webstream.
-
options
<Object>error
<boolean> If set tofalse
, then a call toemit('error', err)
is not treated as finished. Default:true
.readable
<boolean> When set tofalse
, the callback will be called when the stream ends even though the stream might still be readable. Default:true
.writable
<boolean> When set tofalse
, the callback will be called when the stream ends even though the stream might still be writable. Default:true
.signal
<AbortSignal> allows aborting the wait for the stream finish. The underlying stream will not be aborted if the signal is aborted. The callback will get called with anAbortError
. All registered listeners added by this function will also be removed.cleanup
<boolean> remove all registered stream listeners. Default:false
.
-
callback
<Function> A callback function that takes an optional error argument. -
Returns: <Function> A cleanup function which removes all registered listeners.
A function to get notified when a stream is no longer readable, writable or has experienced an error or a premature close event.
const { finished } = require('node:stream');
const fs = require('node:fs');
const rs = fs.createReadStream('archive.tar');
finished(rs, (err) => {
if (err) {
console.error('Stream failed.', err);
} else {
console.log('Stream is done reading.');
}
});
rs.resume(); // Drain the stream.
Especially useful in error handling scenarios where a stream is destroyed
prematurely (like an aborted HTTP request), and will not emit 'end'
or 'finish'
.
The finished
API provides promise version.
stream.finished()
leaves dangling event listeners (in particular
'error'
, 'end'
, 'finish'
and 'close'
) after callback
has been
invoked. 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 the returned cleanup function needs to be
invoked in the callback:
const cleanup = finished(rs, (err) => {
cleanup();
// ...
});
stream.pipeline(source[, ...transforms], destination, callback)
#
stream.pipeline(streams, callback)
#
streams
<Stream[]> | <Iterable[]> | <AsyncIterable[]> | <Function[]> | <ReadableStream[]> | <WritableStream[]> | <TransformStream[]>source
<Stream> | <Iterable> | <AsyncIterable> | <Function> | <ReadableStream>- Returns: <Iterable> | <AsyncIterable>
...transforms
<Stream> | <Function> | <TransformStream>source
<AsyncIterable>- Returns: <AsyncIterable>
destination
<Stream> | <Function> | <WritableStream>source
<AsyncIterable>- Returns: <AsyncIterable> | <Promise>
callback
<Function> Called when the pipeline is fully done.err
<Error>val
Resolved value ofPromise
returned bydestination
.
- Returns: <Stream>
A module method to pipe between streams and generators forwarding errors and properly cleaning up and provide a callback when the pipeline is complete.
const { pipeline } = require('node:stream');
const fs = require('node:fs');
const zlib = require('node:zlib');
// Use the pipeline API to easily pipe a series of streams
// together and get notified when the pipeline is fully done.
// A pipeline to gzip a potentially huge tar file efficiently:
pipeline(
fs.createReadStream('archive.tar'),
zlib.createGzip(),
fs.createWriteStream('archive.tar.gz'),
(err) => {
if (err) {
console.error('Pipeline failed.', err);
} else {
console.log('Pipeline succeeded.');
}
},
);
The pipeline
API provides a promise version.
stream.pipeline()
will call stream.destroy(err)
on all streams except:
Readable
streams which have emitted'end'
or'close'
.Writable
streams which have emitted'finish'
or'close'
.
stream.pipeline()
leaves dangling event listeners on the streams
after the callback
has been invoked. In the case of reuse of streams after
failure, this can cause event listener leaks and swallowed errors. If the last
stream is readable, dangling event listeners will be removed so that the last
stream can be consumed later.
stream.pipeline()
closes all the streams when an error is raised.
The IncomingRequest
usage with pipeline
could lead to an unexpected behavior
once it would destroy the socket without sending the expected response.
See the example below:
const fs = require('node:fs');
const http = require('node:http');
const { pipeline } = require('node:stream');
const server = http.createServer((req, res) => {
const fileStream = fs.createReadStream('./fileNotExist.txt');
pipeline(fileStream, res, (err) => {
if (err) {
console.log(err); // No such file
// this message can't be sent once `pipeline` already destroyed the socket
return res.end('error!!!');
}
});
});
stream.compose(...streams)
#
stream.compose
is experimental.streams
<Stream[]> | <Iterable[]> | <AsyncIterable[]> | <Function[]> | <ReadableStream[]> | <WritableStream[]> | <TransformStream[]>- Returns: <stream.Duplex>
Combines two or more streams into a Duplex
stream that writes to the
first stream and reads from the last. Each provided stream is piped into
the next, using stream.pipeline
. If any of the streams error then all
are destroyed, including the outer Duplex
stream.
Because stream.compose
returns a new stream that in turn can (and
should) be piped into other streams, it enables composition. In contrast,
when passing streams to stream.pipeline
, typically the first stream is
a readable stream and the last a writable stream, forming a closed
circuit.
If passed a Function
it must be a factory method taking a source
Iterable
.
import { compose, Transform } from 'node:stream';
const removeSpaces = new Transform({
transform(chunk, encoding, callback) {
callback(null, String(chunk).replace(' ', ''));
},
});
async function* toUpper(source) {
for await (const chunk of source) {
yield String(chunk).toUpperCase();
}
}
let res = '';
for await (const buf of compose(removeSpaces, toUpper).end('hello world')) {
res += buf;
}
console.log(res); // prints 'HELLOWORLD'
stream.compose
can be used to convert async iterables, generators and
functions into streams.
AsyncIterable
converts into a readableDuplex
. Cannot yieldnull
.AsyncGeneratorFunction
converts into a readable/writable transformDuplex
. Must take a sourceAsyncIterable
as first parameter. Cannot yieldnull
.AsyncFunction
converts into a writableDuplex
. Must return eithernull
orundefined
.
import { compose } from 'node:stream';
import { finished } from 'node:stream/promises';
// Convert AsyncIterable into readable Duplex.
const s1 = compose(async function*() {
yield 'Hello';
yield 'World';
}());
// Convert AsyncGenerator into transform Duplex.
const s2 = compose(async function*(source) {
for await (const chunk of source) {
yield String(chunk).toUpperCase();
}
});
let res = '';
// Convert AsyncFunction into writable Duplex.
const s3 = compose(async function(source) {
for await (const chunk of source) {
res += chunk;
}
});
await finished(compose(s1, s2, s3));
console.log(res); // prints 'HELLOWORLD'
See readable.compose(stream)
for stream.compose
as operator.
stream.Readable.from(iterable[, options])
#
iterable
<Iterable> Object implementing theSymbol.asyncIterator
orSymbol.iterator
iterable protocol. Emits an 'error' event if a null value is passed.options
<Object> Options provided tonew stream.Readable([options])
. By default,Readable.from()
will setoptions.objectMode
totrue
, unless this is explicitly opted out by settingoptions.objectMode
tofalse
.- Returns: <stream.Readable>
A utility method for creating readable streams out of iterators.
const { Readable } = require('node:stream');
async function * generate() {
yield 'hello';
yield 'streams';
}
const readable = Readable.from(generate());
readable.on('data', (chunk) => {
console.log(chunk);
});
Calling Readable.from(string)
or Readable.from(buffer)
will not have
the strings or buffers be iterated to match the other streams semantics
for performance reasons.
If an Iterable
object containing promises is passed as an argument,
it might result in unhandled rejection.
const { Readable } = require('node:stream');
Readable.from([
new Promise((resolve) => setTimeout(resolve('1'), 1500)),
new Promise((_, reject) => setTimeout(reject(new Error('2')), 1000)), // Unhandled rejection
]);
stream.Readable.fromWeb(readableStream[, options])
#
readableStream
<ReadableStream>options
<Object>encoding
<string>highWaterMark
<number>objectMode
<boolean>signal
<AbortSignal>
- Returns: <stream.Readable>
stream.Readable.isDisturbed(stream)
#
stream
<stream.Readable> | <ReadableStream>- Returns:
boolean
Returns whether the stream has been read from or cancelled.
stream.isErrored(stream)
#
stream
<Readable> | <Writable> | <Duplex> | <WritableStream> | <ReadableStream>- Returns: <boolean>
Returns whether the stream has encountered an error.
stream.isReadable(stream)
#
stream
<Readable> | <Duplex> | <ReadableStream>- Returns: <boolean>
Returns whether the stream is readable.
stream.Readable.toWeb(streamReadable[, options])
#
streamReadable
<stream.Readable>options
<Object>strategy
<Object>highWaterMark
<number> The maximum internal queue size (of the createdReadableStream
) before backpressure is applied in reading from the givenstream.Readable
. If no value is provided, it will be taken from the givenstream.Readable
.size
<Function> A function that size of the given chunk of data. If no value is provided, the size will be1
for all the chunks.
- Returns: <ReadableStream>
stream.Writable.fromWeb(writableStream[, options])
#
writableStream
<WritableStream>options
<Object>decodeStrings
<boolean>highWaterMark
<number>objectMode
<boolean>signal
<AbortSignal>
- Returns: <stream.Writable>
stream.Writable.toWeb(streamWritable)
#
streamWritable
<stream.Writable>- Returns: <WritableStream>
stream.Duplex.from(src)
#
src
<Stream> | <Blob> | <ArrayBuffer> | <string> | <Iterable> | <AsyncIterable> | <AsyncGeneratorFunction> | <AsyncFunction> | <Promise> | <Object> | <ReadableStream> | <WritableStream>
A utility method for creating duplex streams.
Stream
converts writable stream into writableDuplex
and readable stream toDuplex
.Blob
converts into readableDuplex
.string
converts into readableDuplex
.ArrayBuffer
converts into readableDuplex
.AsyncIterable
converts into a readableDuplex
. Cannot yieldnull
.AsyncGeneratorFunction
converts into a readable/writable transformDuplex
. Must take a sourceAsyncIterable
as first parameter. Cannot yieldnull
.AsyncFunction
converts into a writableDuplex
. Must return eithernull
orundefined
Object ({ writable, readable })
convertsreadable
andwritable
intoStream
and then combines them intoDuplex
where theDuplex
will write to thewritable
and read from thereadable
.Promise
converts into readableDuplex
. Valuenull
is ignored.ReadableStream
converts into readableDuplex
.WritableStream
converts into writableDuplex
.- Returns: <stream.Duplex>
If an Iterable
object containing promises is passed as an argument,
it might result in unhandled rejection.
const { Duplex } = require('node:stream');
Duplex.from([
new Promise((resolve) => setTimeout(resolve('1'), 1500)),
new Promise((_, reject) => setTimeout(reject(new Error('2')), 1000)), // Unhandled rejection
]);
stream.Duplex.fromWeb(pair[, options])
#
pair
<Object>readable
<ReadableStream>writable
<WritableStream>
options
<Object>- Returns: <stream.Duplex>
import { Duplex } from 'node:stream';
import {
ReadableStream,
WritableStream,
} from 'node:stream/web';
const readable = new ReadableStream({
start(controller) {
controller.enqueue('world');
},
});
const writable = new WritableStream({
write(chunk) {
console.log('writable', chunk);
},
});
const pair = {
readable,
writable,
};
const duplex = Duplex.fromWeb(pair, { encoding: 'utf8', objectMode: true });
duplex.write('hello');
for await (const chunk of duplex) {
console.log('readable', chunk);
}
const { Duplex } = require('node:stream');
const {
ReadableStream,
WritableStream,
} = require('node:stream/web');
const readable = new ReadableStream({
start(controller) {
controller.enqueue('world');
},
});
const writable = new WritableStream({
write(chunk) {
console.log('writable', chunk);
},
});
const pair = {
readable,
writable,
};
const duplex = Duplex.fromWeb(pair, { encoding: 'utf8', objectMode: true });
duplex.write('hello');
duplex.once('readable', () => console.log('readable', duplex.read()));
stream.Duplex.toWeb(streamDuplex)
#
streamDuplex
<stream.Duplex>- Returns: <Object>
readable
<ReadableStream>writable
<WritableStream>
import { Duplex } from 'node:stream';
const duplex = Duplex({
objectMode: true,
read() {
this.push('world');
this.push(null);
},
write(chunk, encoding, callback) {
console.log('writable', chunk);
callback();
},
});
const { readable, writable } = Duplex.toWeb(duplex);
writable.getWriter().write('hello');
const { value } = await readable.getReader().read();
console.log('readable', value);
const { Duplex } = require('node:stream');
const duplex = Duplex({
objectMode: true,
read() {
this.push('world');
this.push(null);
},
write(chunk, encoding, callback) {
console.log('writable', chunk);
callback();
},
});
const { readable, writable } = Duplex.toWeb(duplex);
writable.getWriter().write('hello');
readable.getReader().read().then((result) => {
console.log('readable', result.value);
});
stream.addAbortSignal(signal, stream)
#
signal
<AbortSignal> A signal representing possible cancellationstream
<Stream> | <ReadableStream> | <WritableStream>
A stream to attach a signal to.
Attaches an AbortSignal to a readable or writeable stream. This lets code
control stream destruction using an AbortController
.
Calling abort
on the AbortController
corresponding to the passed
AbortSignal
will behave the same way as calling .destroy(new AbortError())
on the stream, and controller.error(new AbortError())
for webstreams.
const fs = require('node:fs');
const controller = new AbortController();
const read = addAbortSignal(
controller.signal,
fs.createReadStream(('object.json')),
);
// Later, abort the operation closing the stream
controller.abort();
Or using an AbortSignal
with a readable stream as an async iterable:
const controller = new AbortController();
setTimeout(() => controller.abort(), 10_000); // set a timeout
const stream = addAbortSignal(
controller.signal,
fs.createReadStream(('object.json')),
);
(async () => {
try {
for await (const chunk of stream) {
await process(chunk);
}
} catch (e) {
if (e.name === 'AbortError') {
// The operation was cancelled
} else {
throw e;
}
}
})();
Or using an AbortSignal
with a ReadableStream:
const controller = new AbortController();
const rs = new ReadableStream({
start(controller) {
controller.enqueue('hello');
controller.enqueue('world');
controller.close();
},
});
addAbortSignal(controller.signal, rs);
finished(rs, (err) => {
if (err) {
if (err.name === 'AbortError') {
// The operation was cancelled
}
}
});
const reader = rs.getReader();
reader.read().then(({ value, done }) => {
console.log(value); // hello
console.log(done); // false
controller.abort();
});
stream.getDefaultHighWaterMark(objectMode)
#
Returns the default highWaterMark used by streams.
Defaults to 16384
(16 KiB), or 16
for objectMode
.
stream.setDefaultHighWaterMark(objectMode, value)
#
Sets the default highWaterMark used by streams.
API for stream implementers#
The node:stream
module API has been designed to make it possible to easily
implement streams using JavaScript's prototypal inheritance model.
First, a stream developer would declare a new JavaScript class that extends one
of the four basic stream classes (stream.Writable
, stream.Readable
,
stream.Duplex
, or stream.Transform
), making sure they call the appropriate
parent class constructor:
const { Writable } = require('node:stream');
class MyWritable extends Writable {
constructor({ highWaterMark, ...options }) {
super({ highWaterMark });
// ...
}
}
When extending streams, keep in mind what options the user
can and should provide before forwarding these to the base constructor. For
example, if the implementation makes assumptions in regard to the
autoDestroy
and emitClose
options, do not allow the
user to override these. Be explicit about what
options are forwarded instead of implicitly forwarding all options.
The new stream class must then implement one or more specific methods, depending on the type of stream being created, as detailed in the chart below:
Use-case | Class | Method(s) to implement |
---|---|---|
Reading only | Readable | _read() |
Writing only | Writable | _write() , _writev() , _final() |
Reading and writing | Duplex | _read() , _write() , _writev() , _final() |
Operate on written data, then read the result | Transform | _transform() , _flush() , _final() |
The implementation code for a stream should never call the "public" methods of a stream that are intended for use by consumers (as described in the API for stream consumers section). Doing so may lead to adverse side effects in application code consuming the stream.
Avoid overriding public methods such as write()
, end()
, cork()
,
uncork()
, read()
and destroy()
, or emitting internal events such
as 'error'
, 'data'
, 'end'
, 'finish'
and 'close'
through .emit()
.
Doing so can break current and future stream invariants leading to behavior
and/or compatibility issues with other streams, stream utilities, and user
expectations.
Simplified construction#
For many simple cases, it is possible to create a stream without relying on
inheritance. This can be accomplished by directly creating instances of the
stream.Writable
, stream.Readable
, stream.Duplex
, or stream.Transform
objects and passing appropriate methods as constructor options.
const { Writable } = require('node:stream');
const myWritable = new Writable({
construct(callback) {
// Initialize state and load resources...
},
write(chunk, encoding, callback) {
// ...
},
destroy() {
// Free resources...
},
});
Implementing a writable stream#
The stream.Writable
class is extended to implement a Writable
stream.
Custom Writable
streams must call the new stream.Writable([options])
constructor and implement the writable._write()
and/or writable._writev()
method.
new stream.Writable([options])
#
options
<Object>highWaterMark
<number> Buffer level whenstream.write()
starts returningfalse
. Default:16384
(16 KiB), or16
forobjectMode
streams.decodeStrings
<boolean> Whether to encodestring
s passed tostream.write()
toBuffer
s (with the encoding specified in thestream.write()
call) before passing them tostream._write()
. Other types of data are not converted (i.e.Buffer
s are not decoded intostring
s). Setting to false will preventstring
s from being converted. Default:true
.defaultEncoding
<string> The default encoding that is used when no encoding is specified as an argument tostream.write()
. Default:'utf8'
.objectMode
<boolean> Whether or not thestream.write(anyObj)
is a valid operation. When set, it becomes possible to write JavaScript values other than string,Buffer
orUint8Array
if supported by the stream implementation. Default:false
.emitClose
<boolean> Whether or not the stream should emit'close'
after it has been destroyed. Default:true
.write
<Function> Implementation for thestream._write()
method.writev
<Function> Implementation for thestream._writev()
method.destroy
<Function> Implementation for thestream._destroy()
method.final
<Function> Implementation for thestream._final()
method.construct
<Function> Implementation for thestream._construct()
method.autoDestroy
<boolean> Whether this stream should automatically call.destroy()
on itself after ending. Default:true
.signal
<AbortSignal> A signal representing possible cancellation.
const { Writable } = require('node:stream');
class MyWritable extends Writable {
constructor(options) {
// Calls the stream.Writable() constructor.
super(options);
// ...
}
}
Or, when using pre-ES6 style constructors:
const { Writable } = require('node:stream');
const util = require('node:util');
function MyWritable(options) {
if (!(this instanceof MyWritable))
return new MyWritable(options);
Writable.call(this, options);
}
util.inherits(MyWritable, Writable);
Or, using the simplified constructor approach:
const { Writable } = require('node:stream');
const myWritable = new Writable({
write(chunk, encoding, callback) {
// ...
},
writev(chunks, callback) {
// ...
},
});
Calling abort
on the AbortController
corresponding to the passed
AbortSignal
will behave the same way as calling .destroy(new AbortError())
on the writeable stream.
const { Writable } = require('node:stream');
const controller = new AbortController();
const myWritable = new Writable({
write(chunk, encoding, callback) {
// ...
},
writev(chunks, callback) {
// ...
},
signal: controller.signal,
});
// Later, abort the operation closing the stream
controller.abort();
writable._construct(callback)
#
callback
<Function> Call this function (optionally with an error argument) when the stream has finished initializing.
The _construct()
method MUST NOT be called directly. It may be implemented
by child classes, and if so, will be called by the internal Writable
class methods only.
This optional function will be called in a tick after the stream constructor
has returned, delaying any _write()
, _final()
and _destroy()
calls until
callback
is called. This is useful to initialize state or asynchronously
initialize resources before the stream can be used.
const { Writable } = require('node:stream');
const fs = require('node:fs');
class WriteStream extends Writable {
constructor(filename) {
super();
this.filename = filename;
this.fd = null;
}
_construct(callback) {
fs.open(this.filename, (err, fd) => {
if (err) {
callback(err);
} else {
this.fd = fd;
callback();
}
});
}
_write(chunk, encoding, callback) {
fs.write(this.fd, chunk, callback);
}
_destroy(err, callback) {
if (this.fd) {
fs.close(this.fd, (er) => callback(er || err));
} else {
callback(err);
}
}
}
writable._write(chunk, encoding, callback)
#
chunk
<Buffer> | <string> | <any> TheBuffer
to be written, converted from thestring
passed tostream.write()
. If the stream'sdecodeStrings
option isfalse
or the stream is operating in object mode, the chunk will not be converted & will be whatever was passed tostream.write()
.encoding
<string> If the chunk is a string, thenencoding
is the character encoding of that string. If chunk is aBuffer
, or if the stream is operating in object mode,encoding
may be ignored.callback
<Function> Call this function (optionally with an error argument) when processing is complete for the supplied chunk.
All Writable
stream implementations must provide a
writable._write()
and/or
writable._writev()
method to send data to the underlying
resource.
Transform
streams provide their own implementation of the
writable._write()
.
This function MUST NOT be called by application code directly. It should be
implemented by child classes, and called by the internal Writable
class
methods only.
The callback
function must be called synchronously inside of
writable._write()
or asynchronously (i.e. different tick) to signal either
that the write completed successfully or failed with an error.
The first argument passed to the callback
must be the Error
object if the
call failed or null
if the write succeeded.
All calls to writable.write()
that occur between the time writable._write()
is called and the callback
is called will cause the written data to be
buffered. When the callback
is invoked, the stream might emit a 'drain'
event. If a stream implementation is capable of processing multiple chunks of
data at once, the writable._writev()
method should be implemented.
If the decodeStrings
property is explicitly set to false
in the constructor
options, then chunk
will remain the same object that is passed to .write()
,
and may be a string rather than a Buffer
. This is to support implementations
that have an optimized handling for certain string data encodings. In that case,
the encoding
argument will indicate the character encoding of the string.
Otherwise, the encoding
argument can be safely ignored.
The writable._write()
method is prefixed with an underscore because it is
internal to the class that defines it, and should never be called directly by
user programs.
writable._writev(chunks, callback)
#
chunks
<Object[]> The data to be written. The value is an array of <Object> that each represent a discrete chunk of data to write. The properties of these objects are:chunk
<Buffer> | <string> A buffer instance or string containing the data to be written. Thechunk
will be a string if theWritable
was created with thedecodeStrings
option set tofalse
and a string was passed towrite()
.encoding
<string> The character encoding of thechunk
. Ifchunk
is aBuffer
, theencoding
will be'buffer'
.
callback
<Function> A callback function (optionally with an error argument) to be invoked when processing is complete for the supplied chunks.
This function MUST NOT be called by application code directly. It should be
implemented by child classes, and called by the internal Writable
class
methods only.
The writable._writev()
method may be implemented in addition or alternatively
to writable._write()
in stream implementations that are capable of processing
multiple chunks of data at once. If implemented and if there is buffered data
from previous writes, _writev()
will be called instead of _write()
.
The writable._writev()
method is prefixed with an underscore because it is
internal to the class that defines it, and should never be called directly by
user programs.
writable._destroy(err, callback)
#
err
<Error> A possible error.callback
<Function> A callback function that takes an optional error argument.
The _destroy()
method is called by writable.destroy()
.
It can be overridden by child classes but it must not be called directly.
Furthermore, the callback
should not be mixed with async/await
once it is executed when a promise is resolved.
writable._final(callback)
#
callback
<Function> Call this function (optionally with an error argument) when finished writing any remaining data.
The _final()
method must not be called directly. It may be implemented
by child classes, and if so, will be called by the internal Writable
class methods only.
This optional function will be called before the stream closes, delaying the
'finish'
event until callback
is called. This is useful to close resources
or write buffered data before a stream ends.
Errors while writing#
Errors occurring during the processing of the writable._write()
,
writable._writev()
and writable._final()
methods must be propagated
by invoking the callback and passing the error as the first argument.
Throwing an Error
from within these methods or manually emitting an 'error'
event results in undefined behavior.
If a Readable
stream pipes into a Writable
stream when Writable
emits an
error, the Readable
stream will be unpiped.
const { Writable } = require('node:stream');
const myWritable = new Writable({
write(chunk, encoding, callback) {
if (chunk.toString().indexOf('a') >= 0) {
callback(new Error('chunk is invalid'));
} else {
callback();
}
},
});
An example writable stream#
The following illustrates a rather simplistic (and somewhat pointless) custom
Writable
stream implementation. While this specific Writable
stream instance
is not of any real particular usefulness, the example illustrates each of the
required elements of a custom Writable
stream instance:
const { Writable } = require('node:stream');
class MyWritable extends Writable {
_write(chunk, encoding, callback) {
if (chunk.toString().indexOf('a') >= 0) {
callback(new Error('chunk is invalid'));
} else {
callback();
}
}
}
Decoding buffers in a writable stream#
Decoding buffers is a common task, for instance, when using transformers whose
input is a string. This is not a trivial process when using multi-byte
characters encoding, such as UTF-8. The following example shows how to decode
multi-byte strings using StringDecoder
and Writable
.
const { Writable } = require('node:stream');
const { StringDecoder } = require('node:string_decoder');
class StringWritable extends Writable {
constructor(options) {
super(options);
this._decoder = new StringDecoder(options && options.defaultEncoding);
this.data = '';
}
_write(chunk, encoding, callback) {
if (encoding === 'buffer') {
chunk = this._decoder.write(chunk);
}
this.data += chunk;
callback();
}
_final(callback) {
this.data += this._decoder.end();
callback();
}
}
const euro = [[0xE2, 0x82], [0xAC]].map(Buffer.from);
const w = new StringWritable();
w.write('currency: ');
w.write(euro[0]);
w.end(euro[1]);
console.log(w.data); // currency: €
Implementing a readable stream#
The stream.Readable
class is extended to implement a Readable
stream.
Custom Readable
streams must call the new stream.Readable([options])
constructor and implement the readable._read()
method.
new stream.Readable([options])
#
options
<Object>highWaterMark
<number> The maximum number of bytes to store in the internal buffer before ceasing to read from the underlying resource. Default:16384
(16 KiB), or16
forobjectMode
streams.encoding
<string> If specified, then buffers will be decoded to strings using the specified encoding. Default:null
.objectMode
<boolean> Whether this stream should behave as a stream of objects. Meaning thatstream.read(n)
returns a single value instead of aBuffer
of sizen
. Default:false
.emitClose
<boolean> Whether or not the stream should emit'close'
after it has been destroyed. Default:true
.read
<Function> Implementation for thestream._read()
method.destroy
<Function> Implementation for thestream._destroy()
method.construct
<Function> Implementation for thestream._construct()
method.autoDestroy
<boolean> Whether this stream should automatically call.destroy()
on itself after ending. Default:true
.signal
<AbortSignal> A signal representing possible cancellation.
const { Readable } = require('node:stream');
class MyReadable extends Readable {
constructor(options) {
// Calls the stream.Readable(options) constructor.
super(options);
// ...
}
}
Or, when using pre-ES6 style constructors:
const { Readable } = require('node:stream');
const util = require('node:util');
function MyReadable(options) {
if (!(this instanceof MyReadable))
return new MyReadable(options);
Readable.call(this, options);
}
util.inherits(MyReadable, Readable);
Or, using the simplified constructor approach:
const { Readable } = require('node:stream');
const myReadable = new Readable({
read(size) {
// ...
},
});
Calling abort
on the AbortController
corresponding to the passed
AbortSignal
will behave the same way as calling .destroy(new AbortError())
on the readable created.
const { Readable } = require('node:stream');
const controller = new AbortController();
const read = new Readable({
read(size) {
// ...
},
signal: controller.signal,
});
// Later, abort the operation closing the stream
controller.abort();
readable._construct(callback)
#
callback
<Function> Call this function (optionally with an error argument) when the stream has finished initializing.
The _construct()
method MUST NOT be called directly. It may be implemented
by child classes, and if so, will be called by the internal Readable
class methods only.
This optional function will be scheduled in the next tick by the stream
constructor, delaying any _read()
and _destroy()
calls until callback
is
called. This is useful to initialize state or asynchronously initialize
resources before the stream can be used.
const { Readable } = require('node:stream');
const fs = require('node:fs');
class ReadStream extends Readable {
constructor(filename) {
super();
this.filename = filename;
this.fd = null;
}
_construct(callback) {
fs.open(this.filename, (err, fd) => {
if (err) {
callback(err);
} else {
this.fd = fd;
callback();
}
});
}
_read(n) {
const buf = Buffer.alloc(n);
fs.read(this.fd, buf, 0, n, null, (err, bytesRead) => {
if (err) {
this.destroy(err);
} else {
this.push(bytesRead > 0 ? buf.slice(0, bytesRead) : null);
}
});
}
_destroy(err, callback) {
if (this.fd) {
fs.close(this.fd, (er) => callback(er || err));
} else {
callback(err);
}
}
}
readable._read(size)
#
size
<number> Number of bytes to read asynchronously
This function MUST NOT be called by application code directly. It should be
implemented by child classes, and called by the internal Readable
class
methods only.
All Readable
stream implementations must provide an implementation of the
readable._read()
method to fetch data from the underlying resource.
When readable._read()
is called, if data is available from the resource,
the implementation should begin pushing that data into the read queue using the
this.push(dataChunk)
method. _read()
will be called again
after each call to this.push(dataChunk)
once the stream is
ready to accept more data. _read()
may continue reading from the resource and
pushing data until readable.push()
returns false
. Only when _read()
is
called again after it has stopped should it resume pushing additional data into
the queue.
Once the readable._read()
method has been called, it will not be called
again until more data is pushed through the readable.push()
method. Empty data such as empty buffers and strings will not cause
readable._read()
to be called.
The size
argument is advisory. For implementations where a "read" is a
single operation that returns data can use the size
argument to determine how
much data to fetch. Other implementations may ignore this argument and simply
provide data whenever it becomes available. There is no need to "wait" until
size
bytes are available before calling stream.push(chunk)
.
The readable._read()
method is prefixed with an underscore because it is
internal to the class that defines it, and should never be called directly by
user programs.
readable._destroy(err, callback)
#
err
<Error> A possible error.callback
<Function> A callback function that takes an optional error argument.
The _destroy()
method is called by readable.destroy()
.
It can be overridden by child classes but it must not be called directly.
readable.push(chunk[, encoding])
#
chunk
<Buffer> | <Uint8Array> | <string> | <null> | <any> Chunk of data to push into the read queue. For streams not operating in object mode,chunk
must be a string,Buffer
orUint8Array
. For object mode streams,chunk
may be any JavaScript value.encoding
<string> Encoding of string chunks. Must be a validBuffer
encoding, such as'utf8'
or'ascii'
.- Returns: <boolean>
true
if additional chunks of data may continue to be pushed;false
otherwise.
When chunk
is a Buffer
, Uint8Array
, or string
, the chunk
of data will
be added to the internal queue for users of the stream to consume.
Passing chunk
as null
signals the end of the stream (EOF), after which no
more data can be written.
When the Readable
is operating in paused mode, the data added with
readable.push()
can be read out by calling the
readable.read()
method when the 'readable'
event is
emitted.
When the Readable
is operating in flowing mode, the data added with
readable.push()
will be delivered by emitting a 'data'
event.
The readable.push()
method is designed to be as flexible as possible. For
example, when wrapping a lower-level source that provides some form of
pause/resume mechanism, and a data callback, the low-level source can be wrapped
by the custom Readable
instance:
// `_source` is an object with readStop() and readStart() methods,
// and an `ondata` member that gets called when it has data, and
// an `onend` member that gets called when the data is over.
class SourceWrapper extends Readable {
constructor(options) {
super(options);
this._source = getLowLevelSourceObject();
// Every time there's data, push it into the internal buffer.
this._source.ondata = (chunk) => {
// If push() returns false, then stop reading from source.
if (!this.push(chunk))
this._source.readStop();
};
// When the source ends, push the EOF-signaling `null` chunk.
this._source.onend = () => {
this.push(null);
};
}
// _read() will be called when the stream wants to pull more data in.
// The advisory size argument is ignored in this case.
_read(size) {
this._source.readStart();
}
}
The readable.push()
method is used to push the content
into the internal buffer. It can be driven by the readable._read()
method.
For streams not operating in object mode, if the chunk
parameter of
readable.push()
is undefined
, it will be treated as empty string or
buffer. See readable.push('')
for more information.
Errors while reading#
Errors occurring during processing of the readable._read()
must be
propagated through the readable.destroy(err)
method.
Throwing an Error
from within readable._read()
or manually emitting an
'error'
event results in undefined behavior.
const { Readable } = require('node:stream');
const myReadable = new Readable({
read(size) {
const err = checkSomeErrorCondition();
if (err) {
this.destroy(err);
} else {
// Do some work.
}
},
});
An example counting stream#
The following is a basic example of a Readable
stream that emits the numerals
from 1 to 1,000,000 in ascending order, and then ends.
const { Readable } = require('node:stream');
class Counter extends Readable {
constructor(opt) {
super(opt);
this._max = 1000000;
this._index = 1;
}
_read() {
const i = this._index++;
if (i > this._max)
this.push(null);
else {
const str = String(i);
const buf = Buffer.from(str, 'ascii');
this.push(buf);
}
}
}
Implementing a duplex stream#
A Duplex
stream is one that implements both Readable
and
Writable
, such as a TCP socket connection.
Because JavaScript does not have support for multiple inheritance, the
stream.Duplex
class is extended to implement a Duplex
stream (as opposed
to extending the stream.Readable
and stream.Writable
classes).
The stream.Duplex
class prototypically inherits from stream.Readable
and
parasitically from stream.Writable
, but instanceof
will work properly for
both base classes due to overriding Symbol.hasInstance
on
stream.Writable
.
Custom Duplex
streams must call the new stream.Duplex([options])
constructor and implement both the readable._read()
and
writable._write()
methods.
new stream.Duplex(options)
#
options
<Object> Passed to bothWritable
andReadable
constructors. Also has the following fields:allowHalfOpen
<boolean> If set tofalse
, then the stream will automatically end the writable side when the readable side ends. Default:true
.readable
<boolean> Sets whether theDuplex
should be readable. Default:true
.writable
<boolean> Sets whether theDuplex
should be writable. Default:true
.readableObjectMode
<boolean> SetsobjectMode
for readable side of the stream. Has no effect ifobjectMode
istrue
. Default:false
.writableObjectMode
<boolean> SetsobjectMode
for writable side of the stream. Has no effect ifobjectMode
istrue
. Default:false
.readableHighWaterMark
<number> SetshighWaterMark
for the readable side of the stream. Has no effect ifhighWaterMark
is provided.writableHighWaterMark
<number> SetshighWaterMark
for the writable side of the stream. Has no effect ifhighWaterMark
is provided.
const { Duplex } = require('node:stream');
class MyDuplex extends Duplex {
constructor(options) {
super(options);
// ...
}
}
Or, when using pre-ES6 style constructors:
const { Duplex } = require('node:stream');
const util = require('node:util');
function MyDuplex(options) {
if (!(this instanceof MyDuplex))
return new MyDuplex(options);
Duplex.call(this, options);
}
util.inherits(MyDuplex, Duplex);
Or, using the simplified constructor approach:
const { Duplex } = require('node:stream');
const myDuplex = new Duplex({
read(size) {
// ...
},
write(chunk, encoding, callback) {
// ...
},
});
When using pipeline:
const { Transform, pipeline } = require('node:stream');
const fs = require('node:fs');
pipeline(
fs.createReadStream('object.json')
.setEncoding('utf8'),
new Transform({
decodeStrings: false, // Accept string input rather than Buffers
construct(callback) {
this.data = '';
callback();
},
transform(chunk, encoding, callback) {
this.data += chunk;
callback();
},
flush(callback) {
try {
// Make sure is valid json.
JSON.parse(this.data);
this.push(this.data);
callback();
} catch (err) {
callback(err);
}
},
}),
fs.createWriteStream('valid-object.json'),
(err) => {
if (err) {
console.error('failed', err);
} else {
console.log('completed');
}
},
);
An example duplex stream#
The following illustrates a simple example of a Duplex
stream that wraps a
hypothetical lower-level source object to which data can be written, and
from which data can be read, albeit using an API that is not compatible with
Node.js streams.
The following illustrates a simple example of a Duplex
stream that buffers
incoming written data via the Writable
interface that is read back out
via the Readable
interface.
const { Duplex } = require('node:stream');
const kSource = Symbol('source');
class MyDuplex extends Duplex {
constructor(source, options) {
super(options);
this[kSource] = source;
}
_write(chunk, encoding, callback) {
// The underlying source only deals with strings.
if (Buffer.isBuffer(chunk))
chunk = chunk.toString();
this[kSource].writeSomeData(chunk);
callback();
}
_read(size) {
this[kSource].fetchSomeData(size, (data, encoding) => {
this.push(Buffer.from(data, encoding));
});
}
}
The most important aspect of a Duplex
stream is that the Readable
and
Writable
sides operate independently of one another despite co-existing within
a single object instance.
Object mode duplex streams#
For Duplex
streams, objectMode
can be set exclusively for either the
Readable
or Writable
side using the readableObjectMode
and
writableObjectMode
options respectively.
In the following example, for instance, a new Transform
stream (which is a
type of Duplex
stream) is created that has an object mode Writable
side
that accepts JavaScript numbers that are converted to hexadecimal strings on
the Readable
side.
const { Transform } = require('node:stream');
// All Transform streams are also Duplex Streams.
const myTransform = new Transform({
writableObjectMode: true,
transform(chunk, encoding, callback) {
// Coerce the chunk to a number if necessary.
chunk |= 0;
// Transform the chunk into something else.
const data = chunk.toString(16);
// Push the data onto the readable queue.
callback(null, '0'.repeat(data.length % 2) + data);
},
});
myTransform.setEncoding('ascii');
myTransform.on('data', (chunk) => console.log(chunk));
myTransform.write(1);
// Prints: 01
myTransform.write(10);
// Prints: 0a
myTransform.write(100);
// Prints: 64
Implementing a transform stream#
A Transform
stream is a Duplex
stream where the output is computed
in some way from the input. Examples include zlib streams or crypto
streams that compress, encrypt, or decrypt data.
There is no requirement that the output be the same size as the input, the same
number of chunks, or arrive at the same time. For example, a Hash
stream will
only ever have a single chunk of output which is provided when the input is
ended. A zlib
stream will produce output that is either much smaller or much
larger than its input.
The stream.Transform
class is extended to implement a Transform
stream.
The stream.Transform
class prototypically inherits from stream.Duplex
and
implements its own versions of the writable._write()
and
readable._read()
methods. Custom Transform
implementations must
implement the transform._transform()
method and may
also implement the transform._flush()
method.
Care must be taken when using Transform
streams in that data written to the
stream can cause the Writable
side of the stream to become paused if the
output on the Readable
side is not consumed.
new stream.Transform([options])
#
options
<Object> Passed to bothWritable
andReadable
constructors. Also has the following fields:transform
<Function> Implementation for thestream._transform()
method.flush
<Function> Implementation for thestream._flush()
method.
const { Transform } = require('node:stream');
class MyTransform extends Transform {
constructor(options) {
super(options);
// ...
}
}
Or, when using pre-ES6 style constructors:
const { Transform } = require('node:stream');
const util = require('node:util');
function MyTransform(options) {
if (!(this instanceof MyTransform))
return new MyTransform(options);
Transform.call(this, options);
}
util.inherits(MyTransform, Transform);
Or, using the simplified constructor approach:
const { Transform } = require('node:stream');
const myTransform = new Transform({
transform(chunk, encoding, callback) {
// ...
},
});
Event: 'end'
#
The 'end'
event is from the stream.Readable
class. The 'end'
event is
emitted after all data has been output, which occurs after the callback in
transform._flush()
has been called. In the case of an error,
'end'
should not be emitted.
Event: 'finish'
#
The 'finish'
event is from the stream.Writable
class. The 'finish'
event is emitted after stream.end()
is called and all chunks
have been processed by stream._transform()
. In the case
of an error, 'finish'
should not be emitted.
transform._flush(callback)
#
callback
<Function> A callback function (optionally with an error argument and data) to be called when remaining data has been flushed.
This function MUST NOT be called by application code directly. It should be
implemented by child classes, and called by the internal Readable
class
methods only.
In some cases, a transform operation may need to emit an additional bit of
data at the end of the stream. For example, a zlib
compression stream will
store an amount of internal state used to optimally compress the output. When
the stream ends, however, that additional data needs to be flushed so that the
compressed data will be complete.
Custom Transform
implementations may implement the transform._flush()
method. This will be called when there is no more written data to be consumed,
but before the 'end'
event is emitted signaling the end of the
Readable
stream.
Within the transform._flush()
implementation, the transform.push()
method
may be called zero or more times, as appropriate. The callback
function must
be called when the flush operation is complete.
The transform._flush()
method is prefixed with an underscore because it is
internal to the class that defines it, and should never be called directly by
user programs.
transform._transform(chunk, encoding, callback)
#
chunk
<Buffer> | <string> | <any> TheBuffer
to be transformed, converted from thestring
passed tostream.write()
. If the stream'sdecodeStrings
option isfalse
or the stream is operating in object mode, the chunk will not be converted & will be whatever was passed tostream.write()
.encoding
<string> If the chunk is a string, then this is the encoding type. If chunk is a buffer, then this is the special value'buffer'
. Ignore it in that case.callback
<Function> A callback function (optionally with an error argument and data) to be called after the suppliedchunk
has been processed.
This function MUST NOT be called by application code directly. It should be
implemented by child classes, and called by the internal Readable
class
methods only.
All Transform
stream implementations must provide a _transform()
method to accept input and produce output. The transform._transform()
implementation handles the bytes being written, computes an output, then passes
that output off to the readable portion using the transform.push()
method.
The transform.push()
method may be called zero or more times to generate
output from a single input chunk, depending on how much is to be output
as a result of the chunk.
It is possible that no output is generated from any given chunk of input data.
The callback
function must be called only when the current chunk is completely
consumed. The first argument passed to the callback
must be an Error
object
if an error occurred while processing the input or null
otherwise. If a second
argument is passed to the callback
, it will be forwarded on to the
transform.push()
method. In other words, the following are equivalent:
transform.prototype._transform = function(data, encoding, callback) {
this.push(data);
callback();
};
transform.prototype._transform = function(data, encoding, callback) {
callback(null, data);
};
The transform._transform()
method is prefixed with an underscore because it
is internal to the class that defines it, and should never be called directly by
user programs.
transform._transform()
is never called in parallel; streams implement a
queue mechanism, and to receive the next chunk, callback
must be
called, either synchronously or asynchronously.
Class: stream.PassThrough
#
The stream.PassThrough
class is a trivial implementation of a Transform
stream that simply passes the input bytes across to the output. Its purpose is
primarily for examples and testing, but there are some use cases where
stream.PassThrough
is useful as a building block for novel sorts of streams.
Additional notes#
Streams compatibility with async generators and async iterators#
With the support of async generators and iterators in JavaScript, async generators are effectively a first-class language-level stream construct at this point.
Some common interop cases of using Node.js streams with async generators and async iterators are provided below.
Consuming readable streams with async iterators#
(async function() {
for await (const chunk of readable) {
console.log(chunk);
}
})();
Async iterators register a permanent error handler on the stream to prevent any unhandled post-destroy errors.
Creating readable streams with async generators#
A Node.js readable stream can be created from an asynchronous generator using
the Readable.from()
utility method:
const { Readable } = require('node:stream');
const ac = new AbortController();
const signal = ac.signal;
async function * generate() {
yield 'a';
await someLongRunningFn({ signal });
yield 'b';
yield 'c';
}
const readable = Readable.from(generate());
readable.on('close', () => {
ac.abort();
});
readable.on('data', (chunk) => {
console.log(chunk);
});
Piping to writable streams from async iterators#
When writing to a writable stream from an async iterator, ensure correct
handling of backpressure and errors. stream.pipeline()
abstracts away
the handling of backpressure and backpressure-related errors:
const fs = require('node:fs');
const { pipeline } = require('node:stream');
const { pipeline: pipelinePromise } = require('node:stream/promises');
const writable = fs.createWriteStream('./file');
const ac = new AbortController();
const signal = ac.signal;
const iterator = createIterator({ signal });
// Callback Pattern
pipeline(iterator, writable, (err, value) => {
if (err) {
console.error(err);
} else {
console.log(value, 'value returned');
}
}).on('close', () => {
ac.abort();
});
// Promise Pattern
pipelinePromise(iterator, writable)
.then((value) => {
console.log(value, 'value returned');
})
.catch((err) => {
console.error(err);
ac.abort();
});
Compatibility with older Node.js versions#
Prior to Node.js 0.10, the Readable
stream interface was simpler, but also
less powerful and less useful.
- Rather than waiting for calls to the
stream.read()
method,'data'
events would begin emitting immediately. Applications that would need to perform some amount of work to decide how to handle data were required to store read data into buffers so the data would not be lost. - The
stream.pause()
method was advisory, rather than guaranteed. This meant that it was still necessary to be prepared to receive'data'
events even when the stream was in a paused state.
In Node.js 0.10, the Readable
class was added. For backward
compatibility with older Node.js programs, Readable
streams switch into
"flowing mode" when a 'data'
event handler is added, or when the
stream.resume()
method is called. The effect is that, even
when not using the new stream.read()
method and
'readable'
event, it is no longer necessary to worry about losing
'data'
chunks.
While most applications will continue to function normally, this introduces an edge case in the following conditions:
- No
'data'
event listener is added. - The
stream.resume()
method is never called. - The stream is not piped to any writable destination.
For example, consider the following code:
// WARNING! BROKEN!
net.createServer((socket) => {
// We add an 'end' listener, but never consume the data.
socket.on('end', () => {
// It will never get here.
socket.end('The message was received but was not processed.\n');
});
}).listen(1337);
Prior to Node.js 0.10, the incoming message data would be simply discarded. However, in Node.js 0.10 and beyond, the socket remains paused forever.
The workaround in this situation is to call the
stream.resume()
method to begin the flow of data:
// Workaround.
net.createServer((socket) => {
socket.on('end', () => {
socket.end('The message was received but was not processed.\n');
});
// Start the flow of data, discarding it.
socket.resume();
}).listen(1337);
In addition to new Readable
streams switching into flowing mode,
pre-0.10 style streams can be wrapped in a Readable
class using the
readable.wrap()
method.
readable.read(0)
#
There are some cases where it is necessary to trigger a refresh of the
underlying readable stream mechanisms, without actually consuming any
data. In such cases, it is possible to call readable.read(0)
, which will
always return null
.
If the internal read buffer is below the highWaterMark
, and the
stream is not currently reading, then calling stream.read(0)
will trigger
a low-level stream._read()
call.
While most applications will almost never need to do this, there are
situations within Node.js where this is done, particularly in the
Readable
stream class internals.
readable.push('')
#
Use of readable.push('')
is not recommended.
Pushing a zero-byte string, Buffer
, or Uint8Array
to a stream that is not in
object mode has an interesting side effect. Because it is a call to
readable.push()
, the call will end the reading process.
However, because the argument is an empty string, no data is added to the
readable buffer so there is nothing for a user to consume.
highWaterMark
discrepancy after calling readable.setEncoding()
#
The use of readable.setEncoding()
will change the behavior of how the
highWaterMark
operates in non-object mode.
Typically, the size of the current buffer is measured against the
highWaterMark
in bytes. However, after setEncoding()
is called, the
comparison function will begin to measure the buffer's size in characters.
This is not a problem in common cases with latin1
or ascii
. But it is
advised to be mindful about this behavior when working with strings that could
contain multi-byte characters.