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Class: ProgressCallbackTransform

Extends​

  • Transform

Constructors​

Constructor​

new ProgressCallbackTransform(total, cancellationToken, onProgress): ProgressCallbackTransform

Parameters​

total​

number

cancellationToken​

CancellationToken

onProgress​

(info) => any

Returns​

ProgressCallbackTransform

Overrides​

Transform.constructor

Properties​

allowHalfOpen​

allowHalfOpen: boolean

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.

Since​

v0.9.4

Inherited from​

Transform.allowHalfOpen

closed​

readonly closed: boolean

Is true after 'close' has been emitted.

Since​

v18.0.0

Inherited from​

Transform.closed

destroyed​

destroyed: boolean

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

Since​

v8.0.0

Inherited from​

Transform.destroyed

errored​

readonly errored: Error | null

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

Since​

v18.0.0

Inherited from​

Transform.errored

readable​

readable: boolean

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

Since​

v11.4.0

Inherited from​

Transform.readable

readableAborted​

readonly readableAborted: boolean

Experimental

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

Since​

v16.8.0

Inherited from​

Transform.readableAborted

readableDidRead​

readonly readableDidRead: boolean

Experimental

Returns whether 'data' has been emitted.

Since​

v16.7.0, v14.18.0

Inherited from​

Transform.readableDidRead

readableEncoding​

readonly readableEncoding: BufferEncoding | null

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

Since​

v12.7.0

Inherited from​

Transform.readableEncoding

readableEnded​

readonly readableEnded: boolean

Becomes true when 'end' event is emitted.

Since​

v12.9.0

Inherited from​

Transform.readableEnded

readableFlowing​

readonly readableFlowing: boolean | null

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

Since​

v9.4.0

Inherited from​

Transform.readableFlowing

readableHighWaterMark​

readonly readableHighWaterMark: number

Returns the value of highWaterMark passed when creating this Readable.

Since​

v9.3.0

Inherited from​

Transform.readableHighWaterMark

readableLength​

readonly readableLength: number

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

Since​

v9.4.0

Inherited from​

Transform.readableLength

readableObjectMode​

readonly readableObjectMode: boolean

Getter for the property objectMode of a given Readable stream.

Since​

v12.3.0

Inherited from​

Transform.readableObjectMode

writable​

readonly writable: boolean

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

Since​

v11.4.0

Inherited from​

Transform.writable

writableCorked​

readonly writableCorked: number

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

Since​

v13.2.0, v12.16.0

Inherited from​

Transform.writableCorked

writableEnded​

readonly writableEnded: boolean

Is true after writable.end() has been called. This property does not indicate whether the data has been flushed, for this use writable.writableFinished instead.

Since​

v12.9.0

Inherited from​

Transform.writableEnded

writableFinished​

readonly writableFinished: boolean

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

Since​

v12.6.0

Inherited from​

Transform.writableFinished

writableHighWaterMark​

readonly writableHighWaterMark: number

Return the value of highWaterMark passed when creating this Writable.

Since​

v9.3.0

Inherited from​

Transform.writableHighWaterMark

writableLength​

readonly writableLength: number

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

Since​

v9.4.0

Inherited from​

Transform.writableLength

writableNeedDrain​

readonly writableNeedDrain: boolean

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

Since​

v15.2.0, v14.17.0

Inherited from​

Transform.writableNeedDrain

writableObjectMode​

readonly writableObjectMode: boolean

Getter for the property objectMode of a given Writable stream.

Since​

v12.3.0

Inherited from​

Transform.writableObjectMode

captureRejections​

static captureRejections: boolean

Value: boolean

Change the default captureRejections option on all new EventEmitter objects.

Since​

v13.4.0, v12.16.0

Inherited from​

Transform.captureRejections

captureRejectionSymbol​

readonly static captureRejectionSymbol: typeof captureRejectionSymbol

Value: Symbol.for('nodejs.rejection')

See how to write a custom rejection handler.

Since​

v13.4.0, v12.16.0

Inherited from​

Transform.captureRejectionSymbol

defaultMaxListeners​

static defaultMaxListeners: number

By default, a maximum of 10 listeners can be registered for any single event. This limit can be changed for individual EventEmitter instances using the emitter.setMaxListeners(n) method. To change the default for allEventEmitter instances, the events.defaultMaxListeners property can be used. If this value is not a positive number, a RangeError is thrown.

Take caution when setting the events.defaultMaxListeners because the change affects all EventEmitter instances, including those created before the change is made. However, calling emitter.setMaxListeners(n) still has precedence over events.defaultMaxListeners.

This is not a hard limit. The EventEmitter instance will allow more listeners to be added but will output a trace warning to stderr indicating that a "possible EventEmitter memory leak" has been detected. For any single EventEmitter, the emitter.getMaxListeners() and emitter.setMaxListeners() methods can be used to temporarily avoid this warning:

import { EventEmitter } from 'node:events';
const emitter = new EventEmitter();
emitter.setMaxListeners(emitter.getMaxListeners() + 1);
emitter.once('event', () => {
  // do stuff
  emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));
});

The --trace-warnings command-line flag can be used to display the stack trace for such warnings.

The emitted warning can be inspected with process.on('warning') and will have the additional emitter, type, and count properties, referring to the event emitter instance, the event's name and the number of attached listeners, respectively. Its name property is set to 'MaxListenersExceededWarning'.

Since​

v0.11.2

Inherited from​

Transform.defaultMaxListeners

errorMonitor​

readonly static errorMonitor: typeof errorMonitor

This symbol shall be used to install a listener for only monitoring 'error' events. Listeners installed using this symbol are called before the regular 'error' listeners are called.

Installing a listener using this symbol does not change the behavior once an 'error' event is emitted. Therefore, the process will still crash if no regular 'error' listener is installed.

Since​

v13.6.0, v12.17.0

Inherited from​

Transform.errorMonitor

Methods​

_construct()?​

optional _construct(callback): void

Parameters​

callback​

(error?) => void

Returns​

void

Inherited from​

Transform._construct

_destroy()​

_destroy(error, callback): void

Parameters​

error​

Error | null

callback​

(error?) => void

Returns​

void

Inherited from​

Transform._destroy

_final()​

_final(callback): void

Parameters​

callback​

(error?) => void

Returns​

void

Inherited from​

Transform._final

_flush()​

_flush(callback): void

Parameters​

callback​

any

Returns​

void

Overrides​

Transform._flush

_read()​

_read(size): void

Parameters​

size​

number

Returns​

void

Inherited from​

Transform._read

_transform()​

_transform(chunk, encoding, callback): void

Parameters​

chunk​

any

encoding​

string

callback​

any

Returns​

void

Overrides​

Transform._transform

_write()​

_write(chunk, encoding, callback): void

Parameters​

chunk​

any

encoding​

BufferEncoding

callback​

(error?) => void

Returns​

void

Inherited from​

Transform._write

_writev()?​

optional _writev(chunks, callback): void

Parameters​

chunks​

object[]

callback​

(error?) => void

Returns​

void

Inherited from​

Transform._writev

[asyncDispose]()​

[asyncDispose](): Promise<void>

Calls readable.destroy() with an AbortError and returns a promise that fulfills when the stream is finished.

Returns​

Promise<void>

Since​

v20.4.0

Inherited from​

Transform.[asyncDispose]

[asyncIterator]()​

[asyncIterator](): AsyncIterator<any>

Returns​

AsyncIterator<any>

Inherited from​

Transform.[asyncIterator]

[captureRejectionSymbol]()?​

optional [captureRejectionSymbol]<K>(error, event, ...args): void

Type Parameters​

K​

K

Parameters​

error​

Error

event​

string | symbol

args​

...AnyRest

Returns​

void

Inherited from​

Transform.[captureRejectionSymbol]

addListener()​

Call Signature​

addListener(event, listener): this

Event emitter The defined events on documents including:

  1. close
  2. data
  3. drain
  4. end
  5. error
  6. finish
  7. pause
  8. pipe
  9. readable
  10. resume
  11. unpipe
Parameters​
event​

"close"

listener​

() => void

Returns​

this

Inherited from​

Transform.addListener

Call Signature​

addListener(event, listener): this

Event emitter The defined events on documents including:

  1. close
  2. data
  3. drain
  4. end
  5. error
  6. finish
  7. pause
  8. pipe
  9. readable
  10. resume
  11. unpipe
Parameters​
event​

"data"

listener​

(chunk) => void

Returns​

this

Inherited from​

Transform.addListener

Call Signature​

addListener(event, listener): this

Event emitter The defined events on documents including:

  1. close
  2. data
  3. drain
  4. end
  5. error
  6. finish
  7. pause
  8. pipe
  9. readable
  10. resume
  11. unpipe
Parameters​
event​

"drain"

listener​

() => void

Returns​

this

Inherited from​

Transform.addListener

Call Signature​

addListener(event, listener): this

Event emitter The defined events on documents including:

  1. close
  2. data
  3. drain
  4. end
  5. error
  6. finish
  7. pause
  8. pipe
  9. readable
  10. resume
  11. unpipe
Parameters​
event​

"end"

listener​

() => void

Returns​

this

Inherited from​

Transform.addListener

Call Signature​

addListener(event, listener): this

Event emitter The defined events on documents including:

  1. close
  2. data
  3. drain
  4. end
  5. error
  6. finish
  7. pause
  8. pipe
  9. readable
  10. resume
  11. unpipe
Parameters​
event​

"error"

listener​

(err) => void

Returns​

this

Inherited from​

Transform.addListener

Call Signature​

addListener(event, listener): this

Event emitter The defined events on documents including:

  1. close
  2. data
  3. drain
  4. end
  5. error
  6. finish
  7. pause
  8. pipe
  9. readable
  10. resume
  11. unpipe
Parameters​
event​

"finish"

listener​

() => void

Returns​

this

Inherited from​

Transform.addListener

Call Signature​

addListener(event, listener): this

Event emitter The defined events on documents including:

  1. close
  2. data
  3. drain
  4. end
  5. error
  6. finish
  7. pause
  8. pipe
  9. readable
  10. resume
  11. unpipe
Parameters​
event​

"pause"

listener​

() => void

Returns​

this

Inherited from​

Transform.addListener

Call Signature​

addListener(event, listener): this

Event emitter The defined events on documents including:

  1. close
  2. data
  3. drain
  4. end
  5. error
  6. finish
  7. pause
  8. pipe
  9. readable
  10. resume
  11. unpipe
Parameters​
event​

"pipe"

listener​

(src) => void

Returns​

this

Inherited from​

Transform.addListener

Call Signature​

addListener(event, listener): this

Event emitter The defined events on documents including:

  1. close
  2. data
  3. drain
  4. end
  5. error
  6. finish
  7. pause
  8. pipe
  9. readable
  10. resume
  11. unpipe
Parameters​
event​

"readable"

listener​

() => void

Returns​

this

Inherited from​

Transform.addListener

Call Signature​

addListener(event, listener): this

Event emitter The defined events on documents including:

  1. close
  2. data
  3. drain
  4. end
  5. error
  6. finish
  7. pause
  8. pipe
  9. readable
  10. resume
  11. unpipe
Parameters​
event​

"resume"

listener​

() => void

Returns​

this

Inherited from​

Transform.addListener

Call Signature​

addListener(event, listener): this

Event emitter The defined events on documents including:

  1. close
  2. data
  3. drain
  4. end
  5. error
  6. finish
  7. pause
  8. pipe
  9. readable
  10. resume
  11. unpipe
Parameters​
event​

"unpipe"

listener​

(src) => void

Returns​

this

Inherited from​

Transform.addListener

Call Signature​

addListener(event, listener): this

Event emitter The defined events on documents including:

  1. close
  2. data
  3. drain
  4. end
  5. error
  6. finish
  7. pause
  8. pipe
  9. readable
  10. resume
  11. unpipe
Parameters​
event​

string | symbol

listener​

(...args) => void

Returns​

this

Inherited from​

Transform.addListener

asIndexedPairs()​

asIndexedPairs(options?): Readable

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.

Parameters​

options?​

Pick<ArrayOptions, "signal">

Returns​

Readable

a stream of indexed pairs.

Since​

v17.5.0

Inherited from​

Transform.asIndexedPairs

compose()​

compose<T>(stream, options?): T

Type Parameters​

T​

T extends ReadableStream

Parameters​

stream​

ComposeFnParam | T | Iterable<T, any, any> | AsyncIterable<T, any, any>

options?​
signal​

AbortSignal

Returns​

T

Inherited from​

Transform.compose

cork()​

cork(): void

The writable.cork() method forces all written data to be buffered in memory. The buffered data will be flushed when either the uncork or end methods are called.

The primary intent of writable.cork() is to accommodate a situation in which several small chunks are written to the stream in rapid succession. Instead of immediately forwarding them to the underlying destination, writable.cork() buffers all the chunks until writable.uncork() is called, which will pass them all to writable._writev(), if present. This prevents a head-of-line blocking situation where data is being buffered while waiting for the first small chunk to be processed. However, use of writable.cork() without implementing writable._writev() may have an adverse effect on throughput.

See also: writable.uncork(), writable._writev().

Returns​

void

Since​

v0.11.2

Inherited from​

Transform.cork

destroy()​

destroy(error?): this

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

Once destroy() has been called any further calls will be a no-op and no further errors except from _destroy() may be emitted as 'error'.

Implementors should not override this method, but instead implement readable._destroy().

Parameters​

error?​

Error

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

Returns​

this

Since​

v8.0.0

Inherited from​

Transform.destroy

drop()​

drop(limit, options?): Readable

This method returns a new stream with the first limit chunks dropped from the start.

Parameters​

limit​

number

the number of chunks to drop from the readable.

options?​

Pick<ArrayOptions, "signal">

Returns​

Readable

a stream with limit chunks dropped from the start.

Since​

v17.5.0

Inherited from​

Transform.drop

emit()​

Call Signature​

emit(event): boolean

Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.

Returns true if the event had listeners, false otherwise.

import { EventEmitter } from 'node:events';
const myEmitter = new EventEmitter();

// First listener
myEmitter.on('event', function firstListener() {
  console.log('Helloooo! first listener');
});
// Second listener
myEmitter.on('event', function secondListener(arg1, arg2) {
  console.log(`event with parameters ${arg1}, ${arg2} in second listener`);
});
// Third listener
myEmitter.on('event', function thirdListener(...args) {
  const parameters = args.join(', ');
  console.log(`event with parameters ${parameters} in third listener`);
});

console.log(myEmitter.listeners('event'));

myEmitter.emit('event', 1, 2, 3, 4, 5);

// Prints:
// [
//   [Function: firstListener],
//   [Function: secondListener],
//   [Function: thirdListener]
// ]
// Helloooo! first listener
// event with parameters 1, 2 in second listener
// event with parameters 1, 2, 3, 4, 5 in third listener
Parameters​
event​

"close"

Returns​

boolean

Since​

v0.1.26

Inherited from​

Transform.emit

Call Signature​

emit(event, chunk): boolean

Parameters​
event​

"data"

chunk​

any

Returns​

boolean

Inherited from​

Transform.emit

Call Signature​

emit(event): boolean

Parameters​
event​

"drain"

Returns​

boolean

Inherited from​

Transform.emit

Call Signature​

emit(event): boolean

Parameters​
event​

"end"

Returns​

boolean

Inherited from​

Transform.emit

Call Signature​

emit(event, err): boolean

Parameters​
event​

"error"

err​

Error

Returns​

boolean

Inherited from​

Transform.emit

Call Signature​

emit(event): boolean

Parameters​
event​

"finish"

Returns​

boolean

Inherited from​

Transform.emit

Call Signature​

emit(event): boolean

Parameters​
event​

"pause"

Returns​

boolean

Inherited from​

Transform.emit

Call Signature​

emit(event, src): boolean

Parameters​
event​

"pipe"

src​

Readable

Returns​

boolean

Inherited from​

Transform.emit

Call Signature​

emit(event): boolean

Parameters​
event​

"readable"

Returns​

boolean

Inherited from​

Transform.emit

Call Signature​

emit(event): boolean

Parameters​
event​

"resume"

Returns​

boolean

Inherited from​

Transform.emit

Call Signature​

emit(event, src): boolean

Parameters​
event​

"unpipe"

src​

Readable

Returns​

boolean

Inherited from​

Transform.emit

Call Signature​

emit(event, ...args): boolean

Parameters​
event​

string | symbol

args​

...any[]

Returns​

boolean

Inherited from​

Transform.emit

end()​

Call Signature​

end(cb?): 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 write method after calling end will raise an error.

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

() => void

Returns​

this

Since​

v0.9.4

Inherited from​

Transform.end

Call Signature​

end(chunk, cb?): 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 write method after calling end will raise an error.

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

any

Optional data to write. For streams not operating in object mode, chunk must be a {string}, {Buffer}, {TypedArray} or {DataView}. For object mode streams, chunk may be any JavaScript value other than null.

cb?​

() => void

Returns​

this

Since​

v0.9.4

Inherited from​

Transform.end

Call Signature​

end(chunk, encoding, cb?): 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 write method after calling end will raise an error.

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

any

Optional data to write. For streams not operating in object mode, chunk must be a {string}, {Buffer}, {TypedArray} or {DataView}. For object mode streams, chunk may be any JavaScript value other than null.

encoding​

BufferEncoding

The encoding if chunk is a string

cb?​

() => void

Returns​

this

Since​

v0.9.4

Inherited from​

Transform.end

eventNames()​

eventNames(): (string | symbol)[]

Returns an array listing the events for which the emitter has registered listeners. The values in the array are strings or Symbols.

import { EventEmitter } from 'node:events';

const myEE = new EventEmitter();
myEE.on('foo', () => {});
myEE.on('bar', () => {});

const sym = Symbol('symbol');
myEE.on(sym, () => {});

console.log(myEE.eventNames());
// Prints: [ 'foo', 'bar', Symbol(symbol) ]

Returns​

(string | symbol)[]

Since​

v6.0.0

Inherited from​

Transform.eventNames

every()​

every(fn, options?): Promise<boolean>

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.

Parameters​

fn​

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

a function to call on each chunk of the stream. Async or not.

options?​

ArrayOptions

Returns​

Promise<boolean>

a promise evaluating to true if fn returned a truthy value for every one of the chunks.

Since​

v17.5.0

Inherited from​

Transform.every

filter()​

filter(fn, options?): Readable

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

Parameters​

fn​

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

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

options?​

ArrayOptions

Returns​

Readable

a stream filtered with the predicate fn.

Since​

v17.4.0, v16.14.0

Inherited from​

Transform.filter

find()​

Call Signature​

find<T>(fn, options?): Promise<T | undefined>

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.

Type Parameters​
T​

T

Parameters​
fn​

(data, options?) => data is T

a function to call on each chunk of the stream. Async or not.

options?​

ArrayOptions

Returns​

Promise<T | undefined>

a promise evaluating to the first chunk for which fn evaluated with a truthy value, or undefined if no element was found.

Since​

v17.5.0

Inherited from​

Transform.find

Call Signature​

find(fn, options?): Promise<any>

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.

Parameters​
fn​

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

a function to call on each chunk of the stream. Async or not.

options?​

ArrayOptions

Returns​

Promise<any>

a promise evaluating to the first chunk for which fn evaluated with a truthy value, or undefined if no element was found.

Since​

v17.5.0

Inherited from​

Transform.find

flatMap()​

flatMap(fn, options?): Readable

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.

Parameters​

fn​

(data, options?) => any

a function to map over every chunk in the stream. May be async. May be a stream or generator.

options?​

ArrayOptions

Returns​

Readable

a stream flat-mapped with the function fn.

Since​

v17.5.0

Inherited from​

Transform.flatMap

forEach()​

forEach(fn, options?): Promise<void>

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 awaited.

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.

Parameters​

fn​

(data, options?) => void | Promise<void>

a function to call on each chunk of the stream. Async or not.

options?​

ArrayOptions

Returns​

Promise<void>

a promise for when the stream has finished.

Since​

v17.5.0

Inherited from​

Transform.forEach

getMaxListeners()​

getMaxListeners(): number

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

Returns​

number

Since​

v1.0.0

Inherited from​

Transform.getMaxListeners

isPaused()​

isPaused(): 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

Returns​

boolean

Since​

v0.11.14

Inherited from​

Transform.isPaused

iterator()​

iterator(options?): AsyncIterator<any>

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

Parameters​

options?​
destroyOnReturn?​

boolean

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

Returns​

AsyncIterator<any>

Since​

v16.3.0

Inherited from​

Transform.iterator

listenerCount()​

listenerCount<K>(eventName, listener?): number

Returns the number of listeners listening for the event named eventName. If listener is provided, it will return how many times the listener is found in the list of the listeners of the event.

Type Parameters​

K​

K

Parameters​

eventName​

string | symbol

The name of the event being listened for

listener?​

Function

The event handler function

Returns​

number

Since​

v3.2.0

Inherited from​

Transform.listenerCount

listeners()​

listeners<K>(eventName): Function[]

Returns a copy of the array of listeners for the event named eventName.

server.on('connection', (stream) => {
  console.log('someone connected!');
});
console.log(util.inspect(server.listeners('connection')));
// Prints: [ [Function] ]

Type Parameters​

K​

K

Parameters​

eventName​

string | symbol

Returns​

Function[]

Since​

v0.1.26

Inherited from​

Transform.listeners

map()​

map(fn, options?): Readable

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 awaited before being passed to the result stream.

Parameters​

fn​

(data, options?) => any

a function to map over every chunk in the stream. Async or not.

options?​

ArrayOptions

Returns​

Readable

a stream mapped with the function fn.

Since​

v17.4.0, v16.14.0

Inherited from​

Transform.map

off()​

off<K>(eventName, listener): this

Alias for emitter.removeListener().

Type Parameters​

K​

K

Parameters​

eventName​

string | symbol

listener​

(...args) => void

Returns​

this

Since​

v10.0.0

Inherited from​

Transform.off

on()​

Call Signature​

on(event, listener): this

Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.

server.on('connection', (stream) => {
  console.log('someone connected!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

import { EventEmitter } from 'node:events';
const myEE = new EventEmitter();
myEE.on('foo', () => console.log('a'));
myEE.prependListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a
Parameters​
event​

"close"

listener​

() => void

The callback function

Returns​

this

Since​

v0.1.101

Inherited from​

Transform.on

Call Signature​

on(event, listener): this

Parameters​
event​

"data"

listener​

(chunk) => void

Returns​

this

Inherited from​

Transform.on

Call Signature​

on(event, listener): this

Parameters​
event​

"drain"

listener​

() => void

Returns​

this

Inherited from​

Transform.on

Call Signature​

on(event, listener): this

Parameters​
event​

"end"

listener​

() => void

Returns​

this

Inherited from​

Transform.on

Call Signature​

on(event, listener): this

Parameters​
event​

"error"

listener​

(err) => void

Returns​

this

Inherited from​

Transform.on

Call Signature​

on(event, listener): this

Parameters​
event​

"finish"

listener​

() => void

Returns​

this

Inherited from​

Transform.on

Call Signature​

on(event, listener): this

Parameters​
event​

"pause"

listener​

() => void

Returns​

this

Inherited from​

Transform.on

Call Signature​

on(event, listener): this

Parameters​
event​

"pipe"

listener​

(src) => void

Returns​

this

Inherited from​

Transform.on

Call Signature​

on(event, listener): this

Parameters​
event​

"readable"

listener​

() => void

Returns​

this

Inherited from​

Transform.on

Call Signature​

on(event, listener): this

Parameters​
event​

"resume"

listener​

() => void

Returns​

this

Inherited from​

Transform.on

Call Signature​

on(event, listener): this

Parameters​
event​

"unpipe"

listener​

(src) => void

Returns​

this

Inherited from​

Transform.on

Call Signature​

on(event, listener): this

Parameters​
event​

string | symbol

listener​

(...args) => void

Returns​

this

Inherited from​

Transform.on

once()​

Call Signature​

once(event, listener): this

Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.

server.once('connection', (stream) => {
  console.log('Ah, we have our first user!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

By default, event listeners are invoked in the order they are added. The emitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

import { EventEmitter } from 'node:events';
const myEE = new EventEmitter();
myEE.once('foo', () => console.log('a'));
myEE.prependOnceListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
//   b
//   a
Parameters​
event​

"close"

listener​

() => void

The callback function

Returns​

this

Since​

v0.3.0

Inherited from​

Transform.once

Call Signature​

once(event, listener): this

Parameters​
event​

"data"

listener​

(chunk) => void

Returns​

this

Inherited from​

Transform.once

Call Signature​

once(event, listener): this

Parameters​
event​

"drain"

listener​

() => void

Returns​

this

Inherited from​

Transform.once

Call Signature​

once(event, listener): this

Parameters​
event​

"end"

listener​

() => void

Returns​

this

Inherited from​

Transform.once

Call Signature​

once(event, listener): this

Parameters​
event​

"error"

listener​

(err) => void

Returns​

this

Inherited from​

Transform.once

Call Signature​

once(event, listener): this

Parameters​
event​

"finish"

listener​

() => void

Returns​

this

Inherited from​

Transform.once

Call Signature​

once(event, listener): this

Parameters​
event​

"pause"

listener​

() => void

Returns​

this

Inherited from​

Transform.once

Call Signature​

once(event, listener): this

Parameters​
event​

"pipe"

listener​

(src) => void

Returns​

this

Inherited from​

Transform.once

Call Signature​

once(event, listener): this

Parameters​
event​

"readable"

listener​

() => void

Returns​

this

Inherited from​

Transform.once

Call Signature​

once(event, listener): this

Parameters​
event​

"resume"

listener​

() => void

Returns​

this

Inherited from​

Transform.once

Call Signature​

once(event, listener): this

Parameters​
event​

"unpipe"

listener​

(src) => void

Returns​

this

Inherited from​

Transform.once

Call Signature​

once(event, listener): this

Parameters​
event​

string | symbol

listener​

(...args) => void

Returns​

this

Inherited from​

Transform.once

pause()​

pause(): 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.

Returns​

this

Since​

v0.9.4

Inherited from​

Transform.pause

pipe()​

pipe<T>(destination, options?): T

Type Parameters​

T​

T extends WritableStream

Parameters​

destination​

T

options?​
end?​

boolean

Returns​

T

Inherited from​

Transform.pipe

prependListener()​

Call Signature​

prependListener(event, listener): this

Adds the listener function to the beginning of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.

server.prependListener('connection', (stream) => {
  console.log('someone connected!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

Parameters​
event​

"close"

listener​

() => void

The callback function

Returns​

this

Since​

v6.0.0

Inherited from​

Transform.prependListener

Call Signature​

prependListener(event, listener): this

Parameters​
event​

"data"

listener​

(chunk) => void

Returns​

this

Inherited from​

Transform.prependListener

Call Signature​

prependListener(event, listener): this

Parameters​
event​

"drain"

listener​

() => void

Returns​

this

Inherited from​

Transform.prependListener

Call Signature​

prependListener(event, listener): this

Parameters​
event​

"end"

listener​

() => void

Returns​

this

Inherited from​

Transform.prependListener

Call Signature​

prependListener(event, listener): this

Parameters​
event​

"error"

listener​

(err) => void

Returns​

this

Inherited from​

Transform.prependListener

Call Signature​

prependListener(event, listener): this

Parameters​
event​

"finish"

listener​

() => void

Returns​

this

Inherited from​

Transform.prependListener

Call Signature​

prependListener(event, listener): this

Parameters​
event​

"pause"

listener​

() => void

Returns​

this

Inherited from​

Transform.prependListener

Call Signature​

prependListener(event, listener): this

Parameters​
event​

"pipe"

listener​

(src) => void

Returns​

this

Inherited from​

Transform.prependListener

Call Signature​

prependListener(event, listener): this

Parameters​
event​

"readable"

listener​

() => void

Returns​

this

Inherited from​

Transform.prependListener

Call Signature​

prependListener(event, listener): this

Parameters​
event​

"resume"

listener​

() => void

Returns​

this

Inherited from​

Transform.prependListener

Call Signature​

prependListener(event, listener): this

Parameters​
event​

"unpipe"

listener​

(src) => void

Returns​

this

Inherited from​

Transform.prependListener

Call Signature​

prependListener(event, listener): this

Parameters​
event​

string | symbol

listener​

(...args) => void

Returns​

this

Inherited from​

Transform.prependListener

prependOnceListener()​

Call Signature​

prependOnceListener(event, listener): this

Adds a one-timelistener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.

server.prependOnceListener('connection', (stream) => {
  console.log('Ah, we have our first user!');
});

Returns a reference to the EventEmitter, so that calls can be chained.

Parameters​
event​

"close"

listener​

() => void

The callback function

Returns​

this

Since​

v6.0.0

Inherited from​

Transform.prependOnceListener

Call Signature​

prependOnceListener(event, listener): this

Parameters​
event​

"data"

listener​

(chunk) => void

Returns​

this

Inherited from​

Transform.prependOnceListener

Call Signature​

prependOnceListener(event, listener): this

Parameters​
event​

"drain"

listener​

() => void

Returns​

this

Inherited from​

Transform.prependOnceListener

Call Signature​

prependOnceListener(event, listener): this

Parameters​
event​

"end"

listener​

() => void

Returns​

this

Inherited from​

Transform.prependOnceListener

Call Signature​

prependOnceListener(event, listener): this

Parameters​
event​

"error"

listener​

(err) => void

Returns​

this

Inherited from​

Transform.prependOnceListener

Call Signature​

prependOnceListener(event, listener): this

Parameters​
event​

"finish"

listener​

() => void

Returns​

this

Inherited from​

Transform.prependOnceListener

Call Signature​

prependOnceListener(event, listener): this

Parameters​
event​

"pause"

listener​

() => void

Returns​

this

Inherited from​

Transform.prependOnceListener

Call Signature​

prependOnceListener(event, listener): this

Parameters​
event​

"pipe"

listener​

(src) => void

Returns​

this

Inherited from​

Transform.prependOnceListener

Call Signature​

prependOnceListener(event, listener): this

Parameters​
event​

"readable"

listener​

() => void

Returns​

this

Inherited from​

Transform.prependOnceListener

Call Signature​

prependOnceListener(event, listener): this

Parameters​
event​

"resume"

listener​

() => void

Returns​

this

Inherited from​

Transform.prependOnceListener

Call Signature​

prependOnceListener(event, listener): this

Parameters​
event​

"unpipe"

listener​

(src) => void

Returns​

this

Inherited from​

Transform.prependOnceListener

Call Signature​

prependOnceListener(event, listener): this

Parameters​
event​

string | symbol

listener​

(...args) => void

Returns​

this

Inherited from​

Transform.prependOnceListener

push()​

push(chunk, encoding?): boolean

Parameters​

chunk​

any

encoding?​

BufferEncoding

Returns​

boolean

Inherited from​

Transform.push

rawListeners()​

rawListeners<K>(eventName): Function[]

Returns a copy of the array of listeners for the event named eventName, including any wrappers (such as those created by .once()).

import { EventEmitter } from 'node:events';
const emitter = new EventEmitter();
emitter.once('log', () => console.log('log once'));

// Returns a new Array with a function `onceWrapper` which has a property
// `listener` which contains the original listener bound above
const listeners = emitter.rawListeners('log');
const logFnWrapper = listeners[0];

// Logs "log once" to the console and does not unbind the `once` event
logFnWrapper.listener();

// Logs "log once" to the console and removes the listener
logFnWrapper();

emitter.on('log', () => console.log('log persistently'));
// Will return a new Array with a single function bound by `.on()` above
const newListeners = emitter.rawListeners('log');

// Logs "log persistently" twice
newListeners[0]();
emitter.emit('log');

Type Parameters​

K​

K

Parameters​

eventName​

string | symbol

Returns​

Function[]

Since​

v9.4.0

Inherited from​

Transform.rawListeners

read()​

read(size?): 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 read after the 'end' event has been emitted will return null. No runtime error will be raised.

Parameters​

size?​

number

Optional argument to specify how much data to read.

Returns​

any

Since​

v0.9.4

Inherited from​

Transform.read

reduce()​

Call Signature​

reduce<T>(fn, initial?, options?): Promise<T>

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.

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.

Type Parameters​
T​

T = any

Parameters​
fn​

(previous, data, options?) => T

a reducer function to call over every chunk in the stream. Async or not.

initial?​

undefined

the initial value to use in the reduction.

options?​

Pick<ArrayOptions, "signal">

Returns​

Promise<T>

a promise for the final value of the reduction.

Since​

v17.5.0

Inherited from​

Transform.reduce

Call Signature​

reduce<T>(fn, initial, options?): Promise<T>

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.

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.

Type Parameters​
T​

T = any

Parameters​
fn​

(previous, data, options?) => T

a reducer function to call over every chunk in the stream. Async or not.

initial​

T

the initial value to use in the reduction.

options?​

Pick<ArrayOptions, "signal">

Returns​

Promise<T>

a promise for the final value of the reduction.

Since​

v17.5.0

Inherited from​

Transform.reduce

removeAllListeners()​

removeAllListeners(eventName?): this

Removes all listeners, or those of the specified eventName.

It is bad practice to remove listeners added elsewhere in the code, particularly when the EventEmitter instance was created by some other component or module (e.g. sockets or file streams).

Returns a reference to the EventEmitter, so that calls can be chained.

Parameters​

eventName?​

string | symbol

Returns​

this

Since​

v0.1.26

Inherited from​

Transform.removeAllListeners

removeListener()​

Call Signature​

removeListener(event, listener): this

Removes the specified listener from the listener array for the event named eventName.

const callback = (stream) => {
  console.log('someone connected!');
};
server.on('connection', callback);
// ...
server.removeListener('connection', callback);

removeListener() will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified eventName, then removeListener() must be called multiple times to remove each instance.

Once an event is emitted, all listeners attached to it at the time of emitting are called in order. This implies that any removeListener() or removeAllListeners() calls after emitting and before the last listener finishes execution will not remove them fromemit() in progress. Subsequent events behave as expected.

import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();

const callbackA = () => {
  console.log('A');
  myEmitter.removeListener('event', callbackB);
};

const callbackB = () => {
  console.log('B');
};

myEmitter.on('event', callbackA);

myEmitter.on('event', callbackB);

// callbackA removes listener callbackB but it will still be called.
// Internal listener array at time of emit [callbackA, callbackB]
myEmitter.emit('event');
// Prints:
//   A
//   B

// callbackB is now removed.
// Internal listener array [callbackA]
myEmitter.emit('event');
// Prints:
//   A

Because listeners are managed using an internal array, calling this will change the position indices of any listener registered after the listener being removed. This will not impact the order in which listeners are called, but it means that any copies of the listener array as returned by the emitter.listeners() method will need to be recreated.

When a single function has been added as a handler multiple times for a single event (as in the example below), removeListener() will remove the most recently added instance. In the example the once('ping') listener is removed:

import { EventEmitter } from 'node:events';
const ee = new EventEmitter();

function pong() {
  console.log('pong');
}

ee.on('ping', pong);
ee.once('ping', pong);
ee.removeListener('ping', pong);

ee.emit('ping');
ee.emit('ping');

Returns a reference to the EventEmitter, so that calls can be chained.

Parameters​
event​

"close"

listener​

() => void

Returns​

this

Since​

v0.1.26

Inherited from​

Transform.removeListener

Call Signature​

removeListener(event, listener): this

Parameters​
event​

"data"

listener​

(chunk) => void

Returns​

this

Inherited from​

Transform.removeListener

Call Signature​

removeListener(event, listener): this

Parameters​
event​

"drain"

listener​

() => void

Returns​

this

Inherited from​

Transform.removeListener

Call Signature​

removeListener(event, listener): this

Parameters​
event​

"end"

listener​

() => void

Returns​

this

Inherited from​

Transform.removeListener

Call Signature​

removeListener(event, listener): this

Parameters​
event​

"error"

listener​

(err) => void

Returns​

this

Inherited from​

Transform.removeListener

Call Signature​

removeListener(event, listener): this

Parameters​
event​

"finish"

listener​

() => void

Returns​

this

Inherited from​

Transform.removeListener

Call Signature​

removeListener(event, listener): this

Parameters​
event​

"pause"

listener​

() => void

Returns​

this

Inherited from​

Transform.removeListener

Call Signature​

removeListener(event, listener): this

Parameters​
event​

"pipe"

listener​

(src) => void

Returns​

this

Inherited from​

Transform.removeListener

Call Signature​

removeListener(event, listener): this

Parameters​
event​

"readable"

listener​

() => void

Returns​

this

Inherited from​

Transform.removeListener

Call Signature​

removeListener(event, listener): this

Parameters​
event​

"resume"

listener​

() => void

Returns​

this

Inherited from​

Transform.removeListener

Call Signature​

removeListener(event, listener): this

Parameters​
event​

"unpipe"

listener​

(src) => void

Returns​

this

Inherited from​

Transform.removeListener

Call Signature​

removeListener(event, listener): this

Parameters​
event​

string | symbol

listener​

(...args) => void

Returns​

this

Inherited from​

Transform.removeListener

resume()​

resume(): 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.

Returns​

this

Since​

v0.9.4

Inherited from​

Transform.resume

setDefaultEncoding()​

setDefaultEncoding(encoding): this

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

Parameters​

encoding​

BufferEncoding

The new default encoding

Returns​

this

Since​

v0.11.15

Inherited from​

Transform.setDefaultEncoding

setEncoding()​

setEncoding(encoding): this

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);
});

Parameters​

encoding​

BufferEncoding

The encoding to use.

Returns​

this

Since​

v0.9.4

Inherited from​

Transform.setEncoding

setMaxListeners()​

setMaxListeners(n): this

By default EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks. The emitter.setMaxListeners() method allows the limit to be modified for this specific EventEmitter instance. The value can be set to Infinity (or 0) to indicate an unlimited number of listeners.

Returns a reference to the EventEmitter, so that calls can be chained.

Parameters​

n​

number

Returns​

this

Since​

v0.3.5

Inherited from​

Transform.setMaxListeners

some()​

some(fn, options?): Promise<boolean>

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.

Parameters​

fn​

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

a function to call on each chunk of the stream. Async or not.

options?​

ArrayOptions

Returns​

Promise<boolean>

a promise evaluating to true if fn returned a truthy value for at least one of the chunks.

Since​

v17.5.0

Inherited from​

Transform.some

take()​

take(limit, options?): Readable

This method returns a new stream with the first limit chunks.

Parameters​

limit​

number

the number of chunks to take from the readable.

options?​

Pick<ArrayOptions, "signal">

Returns​

Readable

a stream with limit chunks taken.

Since​

v17.5.0

Inherited from​

Transform.take

toArray()​

toArray(options?): Promise<any[]>

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.

Parameters​

options?​

Pick<ArrayOptions, "signal">

Returns​

Promise<any[]>

a promise containing an array with the contents of the stream.

Since​

v17.5.0

Inherited from​

Transform.toArray

uncork()​

uncork(): void

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

When using writable.cork() and writable.uncork() to manage the buffering of writes to a stream, defer calls to writable.uncork() 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().

Returns​

void

Since​

v0.11.2

Inherited from​

Transform.uncork

unpipe()​

unpipe(destination?): this

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

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

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

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

Parameters​

destination?​

WritableStream

Optional specific stream to unpipe

Returns​

this

Since​

v0.9.4

Inherited from​

Transform.unpipe

unshift()​

unshift(chunk, encoding?): void

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

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

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

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

// Pull off a header delimited by \n\n.
// Use unshift() if we get too much.
// Call the callback with (error, header, stream).
import { StringDecoder } from 'node:string_decoder';
function parseHeader(stream, callback) {
  stream.on('error', callback);
  stream.on('readable', onReadable);
  const decoder = new StringDecoder('utf8');
  let header = '';
  function onReadable() {
    let chunk;
    while (null !== (chunk = stream.read())) {
      const str = decoder.write(chunk);
      if (str.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 push, stream.unshift(chunk) will not end the reading process by resetting the internal reading state of the stream. This can cause unexpected results if readable.unshift() is called during a read (i.e. from within a _read implementation on a custom stream). Following the call to readable.unshift() with an immediate push will reset the reading state appropriately, however it is best to simply avoid calling readable.unshift() while in the process of performing a read.

Parameters​

chunk​

any

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

encoding?​

BufferEncoding

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

Returns​

void

Since​

v0.9.11

Inherited from​

Transform.unshift

wrap()​

wrap(stream): this

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 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.

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

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

Parameters​

stream​

ReadableStream

An "old style" readable stream

Returns​

this

Since​

v0.9.4

Inherited from​

Transform.wrap

write()​

Call Signature​

write(chunk, callback?): boolean

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 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.

Parameters​
chunk​

any

Optional data to write. For streams not operating in object mode, chunk must be a {string}, {Buffer}, {TypedArray} or {DataView}. For object mode streams, chunk may be any JavaScript value other than null.

callback?​

(error) => void

Callback for when this chunk of data is flushed.

Returns​

boolean

false if the stream wishes for the calling code to wait for the 'drain' event to be emitted before continuing to write additional data; otherwise true.

Since​

v0.9.4

Inherited from​

Transform.write

Call Signature​

write(chunk, encoding, callback?): boolean

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 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.

Parameters​
chunk​

any

Optional data to write. For streams not operating in object mode, chunk must be a {string}, {Buffer}, {TypedArray} or {DataView}. For object mode streams, chunk may be any JavaScript value other than null.

encoding​

BufferEncoding

The encoding, if chunk is a string.

callback?​

(error) => void

Callback for when this chunk of data is flushed.

Returns​

boolean

false if the stream wishes for the calling code to wait for the 'drain' event to be emitted before continuing to write additional data; otherwise true.

Since​

v0.9.4

Inherited from​

Transform.write

addAbortListener()​

static addAbortListener(signal, resource): Disposable

Experimental

Listens once to the abort event on the provided signal.

Listening to the abort event on abort signals is unsafe and may lead to resource leaks since another third party with the signal can call e.stopImmediatePropagation(). Unfortunately Node.js cannot change this since it would violate the web standard. Additionally, the original API makes it easy to forget to remove listeners.

This API allows safely using AbortSignals in Node.js APIs by solving these two issues by listening to the event such that stopImmediatePropagation does not prevent the listener from running.

Returns a disposable so that it may be unsubscribed from more easily.

import { addAbortListener } from 'node:events';

function example(signal) {
  let disposable;
  try {
    signal.addEventListener('abort', (e) => e.stopImmediatePropagation());
    disposable = addAbortListener(signal, (e) => {
      // Do something when signal is aborted.
    });
  } finally {
    disposable?.[Symbol.dispose]();
  }
}

Parameters​

signal​

AbortSignal

resource​

(event) => void

Returns​

Disposable

Disposable that removes the abort listener.

Since​

v20.5.0

Inherited from​

Transform.addAbortListener

from()​

static from(src): Duplex

A utility method for creating duplex streams.

  • Stream converts writable stream into writable Duplex and readable stream to Duplex.
  • Blob converts into readable Duplex.
  • string converts into readable Duplex.
  • ArrayBuffer converts into readable Duplex.
  • AsyncIterable converts into a readable Duplex. Cannot yield null.
  • AsyncGeneratorFunction converts into a readable/writable transform Duplex. Must take a source AsyncIterable as first parameter. Cannot yield null.
  • AsyncFunction converts into a writable Duplex. Must return either null or undefined
  • Object ({ writable, readable }) converts readable and writable into Stream and then combines them into Duplex where the Duplex will write to the writable and read from the readable.
  • Promise converts into readable Duplex. Value null is ignored.

Parameters​

src​

string | Object | Promise<any> | Stream | ArrayBuffer | Blob | Iterable<any, any, any> | AsyncIterable<any, any, any> | AsyncGeneratorFunction

Returns​

Duplex

Since​

v16.8.0

Inherited from​

Transform.from

fromWeb()​

static fromWeb(duplexStream, options?): Duplex

Experimental

A utility method for creating a Duplex from a web ReadableStream and WritableStream.

Parameters​

duplexStream​
readable​

ReadableStream

writable​

WritableStream

options?​

Pick<DuplexOptions<Duplex>, "signal" | "allowHalfOpen" | "decodeStrings" | "encoding" | "highWaterMark" | "objectMode">

Returns​

Duplex

Since​

v17.0.0

Inherited from​

Transform.fromWeb

getEventListeners()​

static getEventListeners(emitter, name): Function[]

Returns a copy of the array of listeners for the event named eventName.

For EventEmitters this behaves exactly the same as calling .listeners on the emitter.

For EventTargets this is the only way to get the event listeners for the event target. This is useful for debugging and diagnostic purposes.

import { getEventListeners, EventEmitter } from 'node:events';

{
  const ee = new EventEmitter();
  const listener = () => console.log('Events are fun');
  ee.on('foo', listener);
  console.log(getEventListeners(ee, 'foo')); // [ [Function: listener] ]
}
{
  const et = new EventTarget();
  const listener = () => console.log('Events are fun');
  et.addEventListener('foo', listener);
  console.log(getEventListeners(et, 'foo')); // [ [Function: listener] ]
}

Parameters​

emitter​

EventEmitter<DefaultEventMap> | EventTarget

name​

string | symbol

Returns​

Function[]

Since​

v15.2.0, v14.17.0

Inherited from​

Transform.getEventListeners

getMaxListeners()​

static getMaxListeners(emitter): number

Returns the currently set max amount of listeners.

For EventEmitters this behaves exactly the same as calling .getMaxListeners on the emitter.

For EventTargets this is the only way to get the max event listeners for the event target. If the number of event handlers on a single EventTarget exceeds the max set, the EventTarget will print a warning.

import { getMaxListeners, setMaxListeners, EventEmitter } from 'node:events';

{
  const ee = new EventEmitter();
  console.log(getMaxListeners(ee)); // 10
  setMaxListeners(11, ee);
  console.log(getMaxListeners(ee)); // 11
}
{
  const et = new EventTarget();
  console.log(getMaxListeners(et)); // 10
  setMaxListeners(11, et);
  console.log(getMaxListeners(et)); // 11
}

Parameters​

emitter​

EventEmitter<DefaultEventMap> | EventTarget

Returns​

number

Since​

v19.9.0

Inherited from​

Transform.getMaxListeners

listenerCount()​

static listenerCount(emitter, eventName): number

A class method that returns the number of listeners for the given eventName registered on the given emitter.

import { EventEmitter, listenerCount } from 'node:events';

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

Parameters​

emitter​

EventEmitter

The emitter to query

eventName​

string | symbol

The event name

Returns​

number

Since​

v0.9.12

Deprecated​

Since v3.2.0 - Use listenerCount instead.

Inherited from​

Transform.listenerCount

on()​

Call Signature​

static on(emitter, eventName, options?): AsyncIterator<any[]>

import { on, EventEmitter } from 'node:events';
import process from 'node:process';

const ee = new EventEmitter();

// Emit later on
process.nextTick(() => {
  ee.emit('foo', 'bar');
  ee.emit('foo', 42);
});

for await (const event of on(ee, 'foo')) {
  // The execution of this inner block is synchronous and it
  // processes one event at a time (even with await). Do not use
  // if concurrent execution is required.
  console.log(event); // prints ['bar'] [42]
}
// Unreachable here

Returns an AsyncIterator that iterates eventName events. It will throw if the EventEmitter emits 'error'. It removes all listeners when exiting the loop. The value returned by each iteration is an array composed of the emitted event arguments.

An AbortSignal can be used to cancel waiting on events:

import { on, EventEmitter } from 'node:events';
import process from 'node:process';

const ac = new AbortController();

(async () => {
  const ee = new EventEmitter();

  // Emit later on
  process.nextTick(() => {
    ee.emit('foo', 'bar');
    ee.emit('foo', 42);
  });

  for await (const event of on(ee, 'foo', { signal: ac.signal })) {
    // The execution of this inner block is synchronous and it
    // processes one event at a time (even with await). Do not use
    // if concurrent execution is required.
    console.log(event); // prints ['bar'] [42]
  }
  // Unreachable here
})();

process.nextTick(() => ac.abort());

Use the close option to specify an array of event names that will end the iteration:

import { on, EventEmitter } from 'node:events';
import process from 'node:process';

const ee = new EventEmitter();

// Emit later on
process.nextTick(() => {
  ee.emit('foo', 'bar');
  ee.emit('foo', 42);
  ee.emit('close');
});

for await (const event of on(ee, 'foo', { close: ['close'] })) {
  console.log(event); // prints ['bar'] [42]
}
// the loop will exit after 'close' is emitted
console.log('done'); // prints 'done'
Parameters​
emitter​

EventEmitter

eventName​

string | symbol

options?​

StaticEventEmitterIteratorOptions

Returns​

AsyncIterator<any[]>

An AsyncIterator that iterates eventName events emitted by the emitter

Since​

v13.6.0, v12.16.0

Inherited from​

Transform.on

Call Signature​

static on(emitter, eventName, options?): AsyncIterator<any[]>

import { on, EventEmitter } from 'node:events';
import process from 'node:process';

const ee = new EventEmitter();

// Emit later on
process.nextTick(() => {
  ee.emit('foo', 'bar');
  ee.emit('foo', 42);
});

for await (const event of on(ee, 'foo')) {
  // The execution of this inner block is synchronous and it
  // processes one event at a time (even with await). Do not use
  // if concurrent execution is required.
  console.log(event); // prints ['bar'] [42]
}
// Unreachable here

Returns an AsyncIterator that iterates eventName events. It will throw if the EventEmitter emits 'error'. It removes all listeners when exiting the loop. The value returned by each iteration is an array composed of the emitted event arguments.

An AbortSignal can be used to cancel waiting on events:

import { on, EventEmitter } from 'node:events';
import process from 'node:process';

const ac = new AbortController();

(async () => {
  const ee = new EventEmitter();

  // Emit later on
  process.nextTick(() => {
    ee.emit('foo', 'bar');
    ee.emit('foo', 42);
  });

  for await (const event of on(ee, 'foo', { signal: ac.signal })) {
    // The execution of this inner block is synchronous and it
    // processes one event at a time (even with await). Do not use
    // if concurrent execution is required.
    console.log(event); // prints ['bar'] [42]
  }
  // Unreachable here
})();

process.nextTick(() => ac.abort());

Use the close option to specify an array of event names that will end the iteration:

import { on, EventEmitter } from 'node:events';
import process from 'node:process';

const ee = new EventEmitter();

// Emit later on
process.nextTick(() => {
  ee.emit('foo', 'bar');
  ee.emit('foo', 42);
  ee.emit('close');
});

for await (const event of on(ee, 'foo', { close: ['close'] })) {
  console.log(event); // prints ['bar'] [42]
}
// the loop will exit after 'close' is emitted
console.log('done'); // prints 'done'
Parameters​
emitter​

EventTarget

eventName​

string

options?​

StaticEventEmitterIteratorOptions

Returns​

AsyncIterator<any[]>

An AsyncIterator that iterates eventName events emitted by the emitter

Since​

v13.6.0, v12.16.0

Inherited from​

Transform.on

once()​

Call Signature​

static once(emitter, eventName, options?): Promise<any[]>

Creates a Promise that is fulfilled when the EventEmitter emits the given event or that is rejected if the EventEmitter emits 'error' while waiting. The Promise will resolve with an array of all the arguments emitted to the given event.

This method is intentionally generic and works with the web platform EventTarget interface, which has no special'error' event semantics and does not listen to the 'error' event.

import { once, EventEmitter } from 'node:events';
import process from 'node:process';

const ee = new EventEmitter();

process.nextTick(() => {
  ee.emit('myevent', 42);
});

const [value] = await once(ee, 'myevent');
console.log(value);

const err = new Error('kaboom');
process.nextTick(() => {
  ee.emit('error', err);
});

try {
  await once(ee, 'myevent');
} catch (err) {
  console.error('error happened', err);
}

The special handling of the 'error' event is only used when events.once() is used to wait for another event. If events.once() is used to wait for the 'error' event itself, then it is treated as any other kind of event without special handling:

import { EventEmitter, once } from 'node:events';

const ee = new EventEmitter();

once(ee, 'error')
  .then(([err]) => console.log('ok', err.message))
  .catch((err) => console.error('error', err.message));

ee.emit('error', new Error('boom'));

// Prints: ok boom

An AbortSignal can be used to cancel waiting for the event:

import { EventEmitter, once } from 'node:events';

const ee = new EventEmitter();
const ac = new AbortController();

async function foo(emitter, event, signal) {
  try {
    await once(emitter, event, { signal });
    console.log('event emitted!');
  } catch (error) {
    if (error.name === 'AbortError') {
      console.error('Waiting for the event was canceled!');
    } else {
      console.error('There was an error', error.message);
    }
  }
}

foo(ee, 'foo', ac.signal);
ac.abort(); // Abort waiting for the event
ee.emit('foo'); // Prints: Waiting for the event was canceled!
Parameters​
emitter​

EventEmitter

eventName​

string | symbol

options?​

StaticEventEmitterOptions

Returns​

Promise<any[]>

Since​

v11.13.0, v10.16.0

Inherited from​

Transform.once

Call Signature​

static once(emitter, eventName, options?): Promise<any[]>

Creates a Promise that is fulfilled when the EventEmitter emits the given event or that is rejected if the EventEmitter emits 'error' while waiting. The Promise will resolve with an array of all the arguments emitted to the given event.

This method is intentionally generic and works with the web platform EventTarget interface, which has no special'error' event semantics and does not listen to the 'error' event.

import { once, EventEmitter } from 'node:events';
import process from 'node:process';

const ee = new EventEmitter();

process.nextTick(() => {
  ee.emit('myevent', 42);
});

const [value] = await once(ee, 'myevent');
console.log(value);

const err = new Error('kaboom');
process.nextTick(() => {
  ee.emit('error', err);
});

try {
  await once(ee, 'myevent');
} catch (err) {
  console.error('error happened', err);
}

The special handling of the 'error' event is only used when events.once() is used to wait for another event. If events.once() is used to wait for the 'error' event itself, then it is treated as any other kind of event without special handling:

import { EventEmitter, once } from 'node:events';

const ee = new EventEmitter();

once(ee, 'error')
  .then(([err]) => console.log('ok', err.message))
  .catch((err) => console.error('error', err.message));

ee.emit('error', new Error('boom'));

// Prints: ok boom

An AbortSignal can be used to cancel waiting for the event:

import { EventEmitter, once } from 'node:events';

const ee = new EventEmitter();
const ac = new AbortController();

async function foo(emitter, event, signal) {
  try {
    await once(emitter, event, { signal });
    console.log('event emitted!');
  } catch (error) {
    if (error.name === 'AbortError') {
      console.error('Waiting for the event was canceled!');
    } else {
      console.error('There was an error', error.message);
    }
  }
}

foo(ee, 'foo', ac.signal);
ac.abort(); // Abort waiting for the event
ee.emit('foo'); // Prints: Waiting for the event was canceled!
Parameters​
emitter​

EventTarget

eventName​

string

options?​

StaticEventEmitterOptions

Returns​

Promise<any[]>

Since​

v11.13.0, v10.16.0

Inherited from​

Transform.once

setMaxListeners()​

static setMaxListeners(n?, ...eventTargets): void

import { setMaxListeners, EventEmitter } from 'node:events';

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

setMaxListeners(5, target, emitter);

Parameters​

n?​

number

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

eventTargets​

...(EventEmitter<DefaultEventMap> | EventTarget)[]

Zero or more {EventTarget} or {EventEmitter} instances. If none are specified, n is set as the default max for all newly created {EventTarget} and {EventEmitter} objects.

Returns​

void

Since​

v15.4.0

Inherited from​

Transform.setMaxListeners

toWeb()​

static toWeb(streamDuplex): object

Experimental

A utility method for creating a web ReadableStream and WritableStream from a Duplex.

Parameters​

streamDuplex​

Duplex

Returns​

object

readable​

readable: ReadableStream

writable​

writable: WritableStream

Since​

v17.0.0

Inherited from​

Transform.toWeb