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WritableBase

The EventEmitter class is defined and exposed by the node:events module:

import { EventEmitter } from 'node:events';

All EventEmitters emit the event 'newListener' when new listeners are added and 'removeListener' when existing listeners are removed.

It supports the following option:

Since​

v0.1.26

Implements​

Properties​

closedbooleanRequired
Is true after 'close' has been emitted.
destroyedbooleanRequired
Is true after writable.destroy() has been called.
errorednull | ErrorRequired
Returns error if the stream has been destroyed with an error.
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​

[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

_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): [WritableBase](/references/js-client/internal/classes/internal.WritableBase)

Event emitter The defined events on documents including:

  1. close
  2. drain
  3. error
  4. finish
  5. pipe
  6. unpipe

Parameters​

event"close"Required
listener() => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"drain"Required
listener() => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"error"Required
listener(err: Error) => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"finish"Required
listener() => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"pipe"Required
listener(src: Readable) => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"unpipe"Required
listener(src: Readable) => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

eventstring | symbolRequired
listener(...args: any[]) => voidRequired

Returns​

WritableBaseWritableBaseRequired

compose​

Parameters​

streamComposeFnParam | T | Iterable<T> | AsyncIterable<T>Required
optionsobject
options.signalAbortSignalRequired

Returns​

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 writable stream has ended and subsequent calls to write() or end() will result in an ERR_STREAM_DESTROYED error. This is a destructive and immediate way to destroy a stream. Previous calls towrite() may not have drained, and may trigger an ERR_STREAM_DESTROYED error. Use end() instead of destroy if data should flush before close, or wait for the 'drain' event before destroying the stream.

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

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

Parameters​

errorError
Optional, an error to emit with 'error' event.

Returns​

WritableBaseWritableBaseRequired

Since​

v8.0.0

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): boolean

Parameters​

event"drain"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, src): boolean

Parameters​

event"pipe"Required
srcReadableRequired

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?): [WritableBase](/references/js-client/internal/classes/internal.WritableBase)

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​

WritableBaseWritableBaseRequired

Since​

v0.9.4

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

Parameters​

chunkanyRequired
cb() => void

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

chunkanyRequired
encodingBufferEncodingRequired
cb() => void

Returns​

WritableBaseWritableBaseRequired

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

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

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

off​

Alias for emitter.removeListener().

Parameters​

eventNamestring | symbolRequired
listener(...args: any[]) => voidRequired

Returns​

WritableBaseWritableBaseRequired

Since​

v10.0.0

on​

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

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​

WritableBaseWritableBaseRequired

Since​

v0.1.101

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

Parameters​

event"drain"Required
listener() => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"error"Required
listener(err: Error) => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"finish"Required
listener() => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"pipe"Required
listener(src: Readable) => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"unpipe"Required
listener(src: Readable) => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

eventstring | symbolRequired
listener(...args: any[]) => voidRequired

Returns​

WritableBaseWritableBaseRequired

once​

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

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​

WritableBaseWritableBaseRequired

Since​

v0.3.0

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

Parameters​

event"drain"Required
listener() => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"error"Required
listener(err: Error) => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"finish"Required
listener() => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"pipe"Required
listener(src: Readable) => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"unpipe"Required
listener(src: Readable) => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

eventstring | symbolRequired
listener(...args: any[]) => voidRequired

Returns​

WritableBaseWritableBaseRequired

pipe​

Parameters​

destinationTRequired
optionsobject
options.endboolean

Returns​

prependListener​

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

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​

WritableBaseWritableBaseRequired

Since​

v6.0.0

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

Parameters​

event"drain"Required
listener() => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"error"Required
listener(err: Error) => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"finish"Required
listener() => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"pipe"Required
listener(src: Readable) => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"unpipe"Required
listener(src: Readable) => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

eventstring | symbolRequired
listener(...args: any[]) => voidRequired

Returns​

WritableBaseWritableBaseRequired

prependOnceListener​

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

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​

WritableBaseWritableBaseRequired

Since​

v6.0.0

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

Parameters​

event"drain"Required
listener() => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"error"Required
listener(err: Error) => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"finish"Required
listener() => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"pipe"Required
listener(src: Readable) => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"unpipe"Required
listener(src: Readable) => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

eventstring | symbolRequired
listener(...args: any[]) => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

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​

WritableBaseWritableBaseRequired

Since​

v0.1.26

removeListener​

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

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​

WritableBaseWritableBaseRequired

Since​

v0.1.26

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

Parameters​

event"drain"Required
listener() => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"error"Required
listener(err: Error) => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"finish"Required
listener() => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"pipe"Required
listener(src: Readable) => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

event"unpipe"Required
listener(src: Readable) => voidRequired

Returns​

WritableBaseWritableBaseRequired

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

Parameters​

eventstring | symbolRequired
listener(...args: any[]) => voidRequired

Returns​

WritableBaseWritableBaseRequired

setDefaultEncoding​

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

Parameters​

encodingBufferEncodingRequired
The new default encoding

Returns​

WritableBaseWritableBaseRequired

Since​

v0.11.15

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​

WritableBaseWritableBaseRequired

Since​

v0.3.5

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

write​

**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 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.
callback(error: undefined | null | Error) => void
Callback for when this chunk of data is flushed.

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, encoding, callback?): boolean

Parameters​

chunkanyRequired
encodingBufferEncodingRequired
callback(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

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

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

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