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Duplex

Duplex streams are streams that implement both the Readable and Writable interfaces.

Examples of Duplex streams include:

  • TCP sockets
  • zlib streams
  • crypto streams

Since​

v0.9.4

Implements​

Properties​

allowHalfOpenbooleanRequired
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 existingDuplex stream instance, but must be changed before the 'end' event is emitted.
closedbooleanRequired
Is true after 'close' has been emitted.
destroyedbooleanRequired
Is true after readable.destroy() has been called.
errorednull | ErrorRequired
Returns error if the stream has been destroyed with an error.
readablebooleanRequired
Is true if it is safe to call readable.read(), which means the stream has not been destroyed or emitted 'error' or 'end'.
readableAbortedbooleanRequired
Returns whether the stream was destroyed or errored before emitting 'end'.
readableDidReadbooleanRequired
Returns whether 'data' has been emitted.
readableEncodingnull | BufferEncodingRequired
Getter for the property encoding of a given Readable stream. The encodingproperty can be set using the readable.setEncoding() method.
readableEndedbooleanRequired
Becomes true when 'end' event is emitted.
readableFlowingnull | booleanRequired
This property reflects the current state of a Readable stream as described in the Three states section.
readableHighWaterMarknumberRequired
Returns the value of highWaterMark passed when creating this Readable.
readableLengthnumberRequired
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.
readableObjectModebooleanRequired
Getter for the property objectMode of a given Readable stream.
writablebooleanRequired
Is true if it is safe to call writable.write(), which means the stream has not been destroyed, errored, or ended.
writableCorkednumberRequired
Number of times writable.uncork() needs to be called in order to fully uncork the stream.
writableEndedbooleanRequired
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.
writableFinishedbooleanRequired
Is set to true immediately before the 'finish' event is emitted.
writableHighWaterMarknumberRequired
Return the value of highWaterMark passed when creating this Writable.
writableLengthnumberRequired
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.
writableNeedDrainbooleanRequired
Is true if the stream's buffer has been full and stream will emit 'drain'.
writableObjectModebooleanRequired
Getter for the property objectMode of a given Writable stream.
captureRejectionSymboltypeof captureRejectionSymbolRequired
Value: Symbol.for('nodejs.rejection') See how to write a custom rejection handler.
captureRejectionsbooleanRequired
Value: boolean Change the default captureRejections option on all new EventEmitter objects.
defaultMaxListenersnumberRequired
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 _all_EventEmitter instances, the events.defaultMaxListenersproperty can be used. If this value is not a positive number, a RangeErroris 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 singleEventEmitter, the emitter.getMaxListeners() and emitter.setMaxListeners()methods can be used to temporarily avoid this warning: js 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'.
errorMonitortypeof errorMonitorRequired
This symbol shall be used to install a listener for only monitoring 'error'events. Listeners installed using this symbol are called before the regular'error' listeners are called. Installing a listener using this symbol does not change the behavior once an'error' event is emitted. Therefore, the process will still crash if no regular 'error' listener is installed.

Methods​

[asyncDispose]​

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

Returns​
PromisePromise<void>Required

Since​

v20.4.0

[asyncIterator]​

Returns​
AsyncIterableIteratorAsyncIterableIterator<any>Required

[captureRejectionSymbol]​

Parameters​
errorErrorRequired
eventstringRequired
argsany[]Required
Returns​
voidvoid

_construct​

Parameters​
callback(error?: null | Error) => voidRequired
Returns​
voidvoid

_destroy​

Parameters​
errornull | ErrorRequired
callback(error: null | Error) => voidRequired
Returns​
voidvoid

_final​

Parameters​
callback(error?: null | Error) => voidRequired
Returns​
voidvoid

_read​

Parameters​
sizenumberRequired
Returns​
voidvoid

_write​

Parameters​
chunkanyRequired
encodingBufferEncodingRequired
callback(error?: null | Error) => voidRequired
Returns​
voidvoid

_writev​

Parameters​
chunks{ chunk: any ; encoding: [BufferEncoding](/references/js-client/internal/types/internal.BufferEncoding) }[]Required
callback(error?: null | Error) => voidRequired
Returns​
voidvoid

addListener​

**addListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

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"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**addListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"data"Required
listener(chunk: any) => voidRequired
Returns​
DuplexDuplexRequired

**addListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"drain"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**addListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"end"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**addListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"error"Required
listener(err: Error) => voidRequired
Returns​
DuplexDuplexRequired

**addListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"finish"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**addListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"pause"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**addListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"pipe"Required
listener(src: Readable) => voidRequired
Returns​
DuplexDuplexRequired

**addListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"readable"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**addListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"resume"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**addListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"unpipe"Required
listener(src: Readable) => voidRequired
Returns​
DuplexDuplexRequired

**addListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
eventstring | symbolRequired
listener(...args: any[]) => voidRequired
Returns​
DuplexDuplexRequired

asIndexedPairs​

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​
optionsPick<ArrayOptions, "signal">
Returns​
ReadableReadableRequired
a stream of indexed pairs.

Since​

v17.5.0

compose​

Parameters​
streamComposeFnParam | T | Iterable<T> | AsyncIterable<T>Required
optionsobject
options.signalAbortSignalRequired
Returns​
TTRequired

cork​

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 implementingwritable._writev() may have an adverse effect on throughput.

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

Returns​
voidvoid

Since​

v0.11.2

destroy​

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​
errorError
Error which will be passed as payload in 'error' event
Returns​
DuplexDuplexRequired

Since​

v8.0.0

drop​

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

Parameters​
limitnumberRequired
the number of chunks to drop from the readable.
optionsPick<ArrayOptions, "signal">
Returns​
ReadableReadableRequired
a stream with limit chunks dropped from the start.

Since​

v17.5.0

emit​

**emit**(event): boolean

Synchronously calls each of the listeners registered for the event namedeventName, 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"Required
Returns​
booleanboolean

Since​

v0.1.26

**emit**(event, chunk): boolean

Parameters​
event"data"Required
chunkanyRequired
Returns​
booleanboolean

**emit**(event): boolean

Parameters​
event"drain"Required
Returns​
booleanboolean

**emit**(event): boolean

Parameters​
event"end"Required
Returns​
booleanboolean

**emit**(event, err): boolean

Parameters​
event"error"Required
errErrorRequired
Returns​
booleanboolean

**emit**(event): boolean

Parameters​
event"finish"Required
Returns​
booleanboolean

**emit**(event): boolean

Parameters​
event"pause"Required
Returns​
booleanboolean

**emit**(event, src): boolean

Parameters​
event"pipe"Required
srcReadableRequired
Returns​
booleanboolean

**emit**(event): boolean

Parameters​
event"readable"Required
Returns​
booleanboolean

**emit**(event): boolean

Parameters​
event"resume"Required
Returns​
booleanboolean

**emit**(event, src): boolean

Parameters​
event"unpipe"Required
srcReadableRequired
Returns​
booleanboolean

**emit**(event, ...args): boolean

Parameters​
eventstring | symbolRequired
argsany[]Required
Returns​
booleanboolean

end​

**end**(cb?): [Duplex](/references/js-client/internal/classes/internal.Duplex)

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!'.
const fs = require('node:fs');
const file = fs.createWriteStream('example.txt');
file.write('hello, ');
file.end('world!');
// Writing more now is not allowed!
Parameters​
cb() => void
Returns​
DuplexDuplexRequired

Since​

v0.9.4

**end**(chunk, cb?): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
chunkanyRequired
cb() => void
Returns​
DuplexDuplexRequired

**end**(chunk, encoding?, cb?): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
chunkanyRequired
encodingBufferEncoding
cb() => void
Returns​
DuplexDuplexRequired

eventNames​

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)[](string | symbol)[]Required

Since​

v6.0.0

every​

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: any, options?: Pick<ArrayOptions, "signal">) => boolean | Promise<boolean>Required
a function to call on each chunk of the stream. Async or not.
optionsArrayOptions
Returns​
PromisePromise<boolean>Required
a promise evaluating to true if fn returned a truthy value for every one of the chunks.

Since​

v17.5.0

filter​

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: any, options?: Pick<ArrayOptions, "signal">) => boolean | Promise<boolean>Required
a function to filter chunks from the stream. Async or not.
optionsArrayOptions
Returns​
ReadableReadableRequired
a stream filtered with the predicate fn.

Since​

v17.4.0, v16.14.0

find​

**find**<TypeParameter T>(fn, options?): Promise&#60;undefined \| T&#62;

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: any, options?: Pick<ArrayOptions, "signal">) => data is TRequired
a function to call on each chunk of the stream. Async or not.
optionsArrayOptions
Returns​
PromisePromise<undefined | T>Required
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

**find**(fn, options?): Promise&#60;any&#62;

Parameters​
fn(data: any, options?: Pick<ArrayOptions, "signal">) => boolean | Promise<boolean>Required
optionsArrayOptions
Returns​
PromisePromise<any>Required

flatMap​

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: any, options?: Pick<ArrayOptions, "signal">) => anyRequired
a function to map over every chunk in the stream. May be async. May be a stream or generator.
optionsArrayOptions
Returns​
ReadableReadableRequired
a stream flat-mapped with the function fn.

Since​

v17.5.0

forEach​

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: any, options?: Pick<ArrayOptions, "signal">) => void | Promise<void>Required
a function to call on each chunk of the stream. Async or not.
optionsArrayOptions
Returns​
PromisePromise<void>Required
a promise for when the stream has finished.

Since​

v17.5.0

getMaxListeners​

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

Returns​
numbernumber

Since​

v1.0.0

isPaused​

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

const readable = new stream.Readable();

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

Since​

v0.11.14

iterator​

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​
optionsobject
options.destroyOnReturnboolean
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​
AsyncIterableIteratorAsyncIterableIterator<any>Required

Since​

v16.3.0

listenerCount​

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.

Parameters​
eventNamestring | symbolRequired
The name of the event being listened for
listenerFunction
The event handler function
Returns​
numbernumber

Since​

v3.2.0

listeners​

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] ]
Parameters​
eventNamestring | symbolRequired
Returns​
Function[]Function[]Required

Since​

v0.1.26

map​

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: any, options?: Pick<ArrayOptions, "signal">) => anyRequired
a function to map over every chunk in the stream. Async or not.
optionsArrayOptions
Returns​
ReadableReadableRequired
a stream mapped with the function fn.

Since​

v17.4.0, v16.14.0

off​

Alias for emitter.removeListener().

Parameters​
eventNamestring | symbolRequired
listener(...args: any[]) => voidRequired
Returns​
DuplexDuplexRequired

Since​

v10.0.0

on​

**on**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

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 eventNameand 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. Theemitter.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"Required
The name of the event.
listener() => voidRequired
The callback function
Returns​
DuplexDuplexRequired

Since​

v0.1.101

**on**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"data"Required
listener(chunk: any) => voidRequired
Returns​
DuplexDuplexRequired

**on**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"drain"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**on**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"end"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**on**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"error"Required
listener(err: Error) => voidRequired
Returns​
DuplexDuplexRequired

**on**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"finish"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**on**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"pause"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**on**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"pipe"Required
listener(src: Readable) => voidRequired
Returns​
DuplexDuplexRequired

**on**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"readable"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**on**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"resume"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**on**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"unpipe"Required
listener(src: Readable) => voidRequired
Returns​
DuplexDuplexRequired

**on**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
eventstring | symbolRequired
listener(...args: any[]) => voidRequired
Returns​
DuplexDuplexRequired

once​

**once**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Adds a one-timelistener 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. Theemitter.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"Required
The name of the event.
listener() => voidRequired
The callback function
Returns​
DuplexDuplexRequired

Since​

v0.3.0

**once**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"data"Required
listener(chunk: any) => voidRequired
Returns​
DuplexDuplexRequired

**once**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"drain"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**once**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"end"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**once**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"error"Required
listener(err: Error) => voidRequired
Returns​
DuplexDuplexRequired

**once**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"finish"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**once**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"pause"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**once**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"pipe"Required
listener(src: Readable) => voidRequired
Returns​
DuplexDuplexRequired

**once**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"readable"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**once**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"resume"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**once**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"unpipe"Required
listener(src: Readable) => voidRequired
Returns​
DuplexDuplexRequired

**once**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
eventstring | symbolRequired
listener(...args: any[]) => voidRequired
Returns​
DuplexDuplexRequired

pause​

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​
DuplexDuplexRequired

Since​

v0.9.4

pipe​

Parameters​
destinationTRequired
optionsobject
options.endboolean
Returns​
TTRequired

prependListener​

**prependListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

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 eventNameand 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"Required
The name of the event.
listener() => voidRequired
The callback function
Returns​
DuplexDuplexRequired

Since​

v6.0.0

**prependListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"data"Required
listener(chunk: any) => voidRequired
Returns​
DuplexDuplexRequired

**prependListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"drain"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**prependListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"end"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**prependListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"error"Required
listener(err: Error) => voidRequired
Returns​
DuplexDuplexRequired

**prependListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"finish"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**prependListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"pause"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**prependListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"pipe"Required
listener(src: Readable) => voidRequired
Returns​
DuplexDuplexRequired

**prependListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"readable"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**prependListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"resume"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**prependListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"unpipe"Required
listener(src: Readable) => voidRequired
Returns​
DuplexDuplexRequired

**prependListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
eventstring | symbolRequired
listener(...args: any[]) => voidRequired
Returns​
DuplexDuplexRequired

prependOnceListener​

**prependOnceListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

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"Required
The name of the event.
listener() => voidRequired
The callback function
Returns​
DuplexDuplexRequired

Since​

v6.0.0

**prependOnceListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"data"Required
listener(chunk: any) => voidRequired
Returns​
DuplexDuplexRequired

**prependOnceListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"drain"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**prependOnceListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"end"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**prependOnceListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"error"Required
listener(err: Error) => voidRequired
Returns​
DuplexDuplexRequired

**prependOnceListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"finish"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**prependOnceListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"pause"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**prependOnceListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"pipe"Required
listener(src: Readable) => voidRequired
Returns​
DuplexDuplexRequired

**prependOnceListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"readable"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**prependOnceListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"resume"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**prependOnceListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"unpipe"Required
listener(src: Readable) => voidRequired
Returns​
DuplexDuplexRequired

**prependOnceListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
eventstring | symbolRequired
listener(...args: any[]) => voidRequired
Returns​
DuplexDuplexRequired

push​

Parameters​
chunkanyRequired
encodingBufferEncoding
Returns​
booleanboolean

rawListeners​

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');
Parameters​
eventNamestring | symbolRequired
Returns​
Function[]Function[]Required

Since​

v9.4.0

read​

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. Ifsize bytes are not available to be read, null will be returned _unless_the stream has ended, in which case all of the data remaining in the internal buffer will be returned.

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

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

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

const readable = getReadableStreamSomehow();

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

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

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

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

const chunks = [];

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

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

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

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

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

Parameters​
sizenumber
Optional argument to specify how much data to read.
Returns​
anyany

Since​

v0.9.4

reduce​

**reduce**<TypeParameter T>(fn, initial?, options?): Promise&#60;T&#62;

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.

Parameters​
fn(previous: any, data: any, options?: Pick<ArrayOptions, "signal">) => TRequired
a reducer function to call over every chunk in the stream. Async or not.
initialundefined
the initial value to use in the reduction.
optionsPick<ArrayOptions, "signal">
Returns​
PromisePromise<T>Required
a promise for the final value of the reduction.

Since​

v17.5.0

**reduce**<TypeParameter T>(fn, initial, options?): Promise&#60;T&#62;

Parameters​
fn(previous: T, data: any, options?: Pick<ArrayOptions, "signal">) => TRequired
initialTRequired
optionsPick<ArrayOptions, "signal">
Returns​
PromisePromise<T>Required

removeAllListeners​

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​
eventstring | symbol
Returns​
DuplexDuplexRequired

Since​

v0.1.26

removeListener​

**removeListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Removes the specified listener from the listener array for the event namedeventName.

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 anyremoveListener() 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"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

Since​

v0.1.26

**removeListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"data"Required
listener(chunk: any) => voidRequired
Returns​
DuplexDuplexRequired

**removeListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"drain"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**removeListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"end"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**removeListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"error"Required
listener(err: Error) => voidRequired
Returns​
DuplexDuplexRequired

**removeListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"finish"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**removeListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"pause"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**removeListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"pipe"Required
listener(src: Readable) => voidRequired
Returns​
DuplexDuplexRequired

**removeListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"readable"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**removeListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"resume"Required
listener() => voidRequired
Returns​
DuplexDuplexRequired

**removeListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
event"unpipe"Required
listener(src: Readable) => voidRequired
Returns​
DuplexDuplexRequired

**removeListener**(event, listener): [Duplex](/references/js-client/internal/classes/internal.Duplex)

Parameters​
eventstring | symbolRequired
listener(...args: any[]) => voidRequired
Returns​
DuplexDuplexRequired

resume​

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​
DuplexDuplexRequired

Since​

v0.9.4

setDefaultEncoding​

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

Parameters​
encodingBufferEncodingRequired
The new default encoding
Returns​
DuplexDuplexRequired

Since​

v0.11.15

setEncoding​

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

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

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

const readable = getReadableStreamSomehow();
readable.setEncoding('utf8');
readable.on('data', (chunk) => {
  assert.equal(typeof chunk, 'string');
  console.log('Got %d characters of string data:', chunk.length);
});
Parameters​
encodingBufferEncodingRequired
The encoding to use.
Returns​
DuplexDuplexRequired

Since​

v0.9.4

setMaxListeners​

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

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

Parameters​
nnumberRequired
Returns​
DuplexDuplexRequired

Since​

v0.3.5

some​

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: any, options?: Pick<ArrayOptions, "signal">) => boolean | Promise<boolean>Required
a function to call on each chunk of the stream. Async or not.
optionsArrayOptions
Returns​
PromisePromise<boolean>Required
a promise evaluating to true if fn returned a truthy value for at least one of the chunks.

Since​

v17.5.0

take​

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

Parameters​
limitnumberRequired
the number of chunks to take from the readable.
optionsPick<ArrayOptions, "signal">
Returns​
ReadableReadableRequired
a stream with limit chunks taken.

Since​

v17.5.0

toArray​

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​
optionsPick<ArrayOptions, "signal">
Returns​
PromisePromise<any[]>Required
a promise containing an array with the contents of the stream.

Since​

v17.5.0

uncork​

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() usingprocess.nextTick(). Doing so allows batching of allwritable.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​
voidvoid

Since​

v0.11.2

unpipe​

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.

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);
Parameters​
destinationWritableStream
Optional specific stream to unpipe
Returns​
DuplexDuplexRequired

Since​

v0.9.4

unshift​

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 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​
chunkanyRequired
Chunk of data to unshift onto the read queue. For streams not operating in object mode, chunk must be a string, Buffer, Uint8Array, or null. For object mode streams, chunk may be any JavaScript value.
encodingBufferEncoding
Encoding of string chunks. Must be a valid Buffer encoding, such as 'utf8' or 'ascii'.
Returns​
voidvoid

Since​

v0.9.11

wrap​

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

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

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

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.
});
Parameters​
streamReadableStreamRequired
An "old style" readable stream
Returns​
DuplexDuplexRequired

Since​

v0.9.4

write​

**write**(chunk, encoding?, cb?): 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 thehighWaterMark 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​
chunkanyRequired
Optional data to write. For streams not operating in object mode, chunk must be a string, Buffer or Uint8Array. For object mode streams, chunk may be any JavaScript value other than null.
encodingBufferEncoding
Callback for when this chunk of data is flushed.
cb(error: undefined | null | Error) => void
Returns​
booleanboolean
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

**write**(chunk, cb?): boolean

Parameters​
chunkanyRequired
cb(error: undefined | null | Error) => void
Returns​
booleanboolean

addAbortListener​

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​
signalAbortSignalRequired
resource(event: Event) => voidRequired
Returns​
[dispose]() => voidRequired

Since​

v20.5.0

from​

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​
srcstring | Object | Promise<any> | ArrayBuffer | Stream | Blob | Iterable<any> | AsyncIterable<any> | AsyncGeneratorFunctionRequired
Returns​
DuplexDuplexRequired

Since​

v16.8.0

fromWeb​

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

Parameters​
duplexStreamobjectRequired
duplexStream.readableReadableStream<any>Required
duplexStream.writableWritableStream<any>Required
optionsPick<DuplexOptions, "signal" | "allowHalfOpen" | "decodeStrings" | "encoding" | "highWaterMark" | "objectMode">
Returns​
DuplexDuplexRequired

Since​

v17.0.0

getEventListeners​

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​
emitterEventEmitter | _DOMEventTargetRequired
namestring | symbolRequired
Returns​
Function[]Function[]Required

Since​

v15.2.0, v14.17.0

getMaxListeners​

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​
emitterEventEmitter | _DOMEventTargetRequired
Returns​
numbernumber

Since​

v19.9.0

isDisturbed​

Returns whether the stream has been read from or cancelled.

Parameters​
streamReadableStream | ReadableRequired
Returns​
booleanboolean

Since​

v16.8.0

listenerCount​

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

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

const myEmitter = new EventEmitter();
myEmitter.on('event', () => {});
myEmitter.on('event', () => {});
console.log(listenerCount(myEmitter, 'event'));
// Prints: 2
Parameters​
emitterEventEmitterRequired
The emitter to query
eventNamestring | symbolRequired
The event name
Returns​
numbernumber

Since​

v0.9.12

Deprecated​

Since v3.2.0 - Use listenerCount instead.

on​

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());
Parameters​
emitterEventEmitterRequired
eventNamestringRequired
The name of the event being listened for
optionsStaticEventEmitterOptions
Returns​
AsyncIterableIteratorAsyncIterableIterator<any>Required
that iterates eventName events emitted by the emitter

Since​

v13.6.0, v12.16.0

once​

Static **once**(emitter, eventName, options?): Promise&#60;any[]&#62;

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_NodeEventTargetRequired
eventNamestring | symbolRequired
optionsStaticEventEmitterOptions
Returns​
PromisePromise<any[]>Required

Since​

v11.13.0, v10.16.0

Static **once**(emitter, eventName, options?): Promise&#60;any[]&#62;

Parameters​
emitter_DOMEventTargetRequired
eventNamestringRequired
optionsStaticEventEmitterOptions
Returns​
PromisePromise<any[]>Required

setMaxListeners​

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

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

setMaxListeners(5, target, emitter);
Parameters​
nnumber
A non-negative number. The maximum number of listeners per EventTarget event.
eventTargets(EventEmitter | _DOMEventTarget)[]Required
Returns​
voidvoid

Since​

v15.4.0

toWeb​

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

Parameters​
streamDuplexDuplexRequired
Returns​
objectobject
readableReadableStream<any>Required
writableWritableStream<any>Required

Since​

v17.0.0

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