Optional
opts: WritableOptionsReadonly
closedIs true
after 'close'
has been emitted.
Is true
after writable.destroy()
has been called.
Readonly
erroredReturns error if the stream has been destroyed with an error.
Readonly
writableIs true
if it is safe to call writable.write()
, which means
the stream has not been destroyed, errored, or ended.
Readonly
writableNumber of times writable.uncork()
needs to be
called in order to fully uncork the stream.
Readonly
writableIs true
after writable.end()
has been called. This property
does not indicate whether the data has been flushed, for this use writable.writableFinished
instead.
Readonly
writableIs set to true
immediately before the 'finish'
event is emitted.
Readonly
writableReturn the value of highWaterMark
passed when creating this Writable
.
Readonly
writableThis 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
.
Readonly
writableIs true
if the stream's buffer has been full and stream will emit 'drain'
.
Readonly
writableGetter for the property objectMode
of a given Writable
stream.
Static
captureValue: boolean
Change the default captureRejections
option on all new EventEmitter
objects.
Static
Readonly
captureValue: Symbol.for('nodejs.rejection')
See how to write a custom rejection handler
.
Static
defaultBy default, a maximum of 10
listeners can be registered for any single
event. This limit can be changed for individual EventEmitter
instances
using the emitter.setMaxListeners(n)
method. To change the default
for allEventEmitter
instances, the events.defaultMaxListeners
property
can be used. If this value is not a positive number, a RangeError
is thrown.
Take caution when setting the events.defaultMaxListeners
because the
change affects all EventEmitter
instances, including those created before
the change is made. However, calling emitter.setMaxListeners(n)
still has
precedence over events.defaultMaxListeners
.
This is not a hard limit. The EventEmitter
instance will allow
more listeners to be added but will output a trace warning to stderr indicating
that a "possible EventEmitter memory leak" has been detected. For any single
EventEmitter
, the emitter.getMaxListeners()
and emitter.setMaxListeners()
methods can be used to
temporarily avoid this warning:
import { EventEmitter } from 'node:events';
const emitter = new EventEmitter();
emitter.setMaxListeners(emitter.getMaxListeners() + 1);
emitter.once('event', () => {
// do stuff
emitter.setMaxListeners(Math.max(emitter.getMaxListeners() - 1, 0));
});
The --trace-warnings
command-line flag can be used to display the
stack trace for such warnings.
The emitted warning can be inspected with process.on('warning')
and will
have the additional emitter
, type
, and count
properties, referring to
the event emitter instance, the event's name and the number of attached
listeners, respectively.
Its name
property is set to 'MaxListenersExceededWarning'
.
Static
Readonly
errorThis 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.
Optional
_constructOptional
_writevOptional
[captureEvent emitter The defined events on documents including:
Event emitter The defined events on documents including:
Event emitter The defined events on documents including:
Event emitter The defined events on documents including:
Event emitter The defined events on documents including:
Event emitter The defined events on documents including:
Event emitter The defined events on documents including:
Optional
options: { signal: AbortSignal }The writable.cork()
method forces all written data to be buffered in memory.
The buffered data will be flushed when either the uncork or end methods are called.
The primary intent of writable.cork()
is to accommodate a situation in which
several small chunks are written to the stream in rapid succession. Instead of
immediately forwarding them to the underlying destination, writable.cork()
buffers all the chunks until writable.uncork()
is called, which will pass them
all to writable._writev()
, if present. This prevents a head-of-line blocking
situation where data is being buffered while waiting for the first small chunk
to be processed. However, use of writable.cork()
without implementing writable._writev()
may have an adverse effect on throughput.
See also: writable.uncork()
, writable._writev()
.
Destroy the stream. Optionally emit an 'error'
event, and emit a 'close'
event (unless emitClose
is set to false
). After this call, the writable
stream has ended and subsequent calls to write()
or end()
will result in
an ERR_STREAM_DESTROYED
error.
This is a destructive and immediate way to destroy a stream. Previous calls to write()
may not have drained, and may trigger an ERR_STREAM_DESTROYED
error.
Use end()
instead of destroy if data should flush before close, or wait for
the 'drain'
event before destroying the stream.
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()
.
Optional
error: ErrorOptional, an error to emit with 'error'
event.
Synchronously calls each of the listeners registered for the event named eventName
, in the order they were registered, passing the supplied arguments
to each.
Returns true
if the event had listeners, false
otherwise.
import { EventEmitter } from 'node:events';
const myEmitter = new EventEmitter();
// First listener
myEmitter.on('event', function firstListener() {
console.log('Helloooo! first listener');
});
// Second listener
myEmitter.on('event', function secondListener(arg1, arg2) {
console.log(`event with parameters ${arg1}, ${arg2} in second listener`);
});
// Third listener
myEmitter.on('event', function thirdListener(...args) {
const parameters = args.join(', ');
console.log(`event with parameters ${parameters} in third listener`);
});
console.log(myEmitter.listeners('event'));
myEmitter.emit('event', 1, 2, 3, 4, 5);
// Prints:
// [
// [Function: firstListener],
// [Function: secondListener],
// [Function: thirdListener]
// ]
// Helloooo! first listener
// event with parameters 1, 2 in second listener
// event with parameters 1, 2, 3, 4, 5 in third listener
Calling the writable.end()
method signals that no more data will be written
to the Writable
. The optional chunk
and encoding
arguments allow one
final additional chunk of data to be written immediately before closing the
stream.
Calling the write method after calling end will raise an error.
// Write 'hello, ' and then end with 'world!'.
import fs from 'node:fs';
const file = fs.createWriteStream('example.txt');
file.write('hello, ');
file.end('world!');
// Writing more now is not allowed!
Optional
cb: () => voidCalling the writable.end()
method signals that no more data will be written
to the Writable
. The optional chunk
and encoding
arguments allow one
final additional chunk of data to be written immediately before closing the
stream.
Calling the write method after calling end will raise an error.
// Write 'hello, ' and then end with 'world!'.
import fs from 'node:fs';
const file = fs.createWriteStream('example.txt');
file.write('hello, ');
file.end('world!');
// Writing more now is not allowed!
Optional data to write. For streams not operating in object mode, chunk
must be a {string}, {Buffer},
{TypedArray} or {DataView}. For object mode streams, chunk
may be any JavaScript value other than null
.
Optional
cb: () => voidCalling the writable.end()
method signals that no more data will be written
to the Writable
. The optional chunk
and encoding
arguments allow one
final additional chunk of data to be written immediately before closing the
stream.
Calling the write method after calling end will raise an error.
// Write 'hello, ' and then end with 'world!'.
import fs from 'node:fs';
const file = fs.createWriteStream('example.txt');
file.write('hello, ');
file.end('world!');
// Writing more now is not allowed!
Optional data to write. For streams not operating in object mode, chunk
must be a {string}, {Buffer},
{TypedArray} or {DataView}. For object mode streams, chunk
may be any JavaScript value other than null
.
The encoding if chunk
is a string
Optional
cb: () => voidReturns an array listing the events for which the emitter has registered
listeners. The values in the array are strings or Symbol
s.
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 the current max listener value for the EventEmitter
which is either
set by emitter.setMaxListeners(n)
or defaults to defaultMaxListeners.
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.
The name of the event being listened for
Optional
listener: FunctionThe event handler function
Returns a copy of the array of listeners for the event named eventName
.
server.on('connection', (stream) => {
console.log('someone connected!');
});
console.log(util.inspect(server.listeners('connection')));
// Prints: [ [Function] ]
Alias for emitter.removeListener()
.
Adds the listener
function to the end of the listeners array for the event
named eventName
. No checks are made to see if the listener
has already
been added. Multiple calls passing the same combination of eventName
and
listener
will result in the listener
being added, and called, multiple times.
server.on('connection', (stream) => {
console.log('someone connected!');
});
Returns a reference to the EventEmitter
, so that calls can be chained.
By default, event listeners are invoked in the order they are added. The emitter.prependListener()
method can be used as an alternative to add the
event listener to the beginning of the listeners array.
import { EventEmitter } from 'node:events';
const myEE = new EventEmitter();
myEE.on('foo', () => console.log('a'));
myEE.prependListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
// b
// a
The callback function
Adds a one-time listener
function for the event named eventName
. The
next time eventName
is triggered, this listener is removed and then invoked.
server.once('connection', (stream) => {
console.log('Ah, we have our first user!');
});
Returns a reference to the EventEmitter
, so that calls can be chained.
By default, event listeners are invoked in the order they are added. The emitter.prependOnceListener()
method can be used as an alternative to add the
event listener to the beginning of the listeners array.
import { EventEmitter } from 'node:events';
const myEE = new EventEmitter();
myEE.once('foo', () => console.log('a'));
myEE.prependOnceListener('foo', () => console.log('b'));
myEE.emit('foo');
// Prints:
// b
// a
The callback function
Optional
options: { end?: boolean }Adds the listener
function to the beginning of the listeners array for the
event named eventName
. No checks are made to see if the listener
has
already been added. Multiple calls passing the same combination of eventName
and listener
will result in the listener
being added, and called, multiple times.
server.prependListener('connection', (stream) => {
console.log('someone connected!');
});
Returns a reference to the EventEmitter
, so that calls can be chained.
The callback function
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.
The callback function
Returns a copy of the array of listeners for the event named eventName
,
including any wrappers (such as those created by .once()
).
import { EventEmitter } from 'node:events';
const emitter = new EventEmitter();
emitter.once('log', () => console.log('log once'));
// Returns a new Array with a function `onceWrapper` which has a property
// `listener` which contains the original listener bound above
const listeners = emitter.rawListeners('log');
const logFnWrapper = listeners[0];
// Logs "log once" to the console and does not unbind the `once` event
logFnWrapper.listener();
// Logs "log once" to the console and removes the listener
logFnWrapper();
emitter.on('log', () => console.log('log persistently'));
// Will return a new Array with a single function bound by `.on()` above
const newListeners = emitter.rawListeners('log');
// Logs "log persistently" twice
newListeners[0]();
emitter.emit('log');
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.
Optional
eventName: string | symbolRemoves the specified listener
from the listener array for the event named eventName
.
const callback = (stream) => {
console.log('someone connected!');
};
server.on('connection', callback);
// ...
server.removeListener('connection', callback);
removeListener()
will remove, at most, one instance of a listener from the
listener array. If any single listener has been added multiple times to the
listener array for the specified eventName
, then removeListener()
must be
called multiple times to remove each instance.
Once an event is emitted, all listeners attached to it at the
time of emitting are called in order. This implies that any removeListener()
or removeAllListeners()
calls after emitting and before the last listener finishes execution
will not remove them fromemit()
in progress. Subsequent events behave as expected.
import { EventEmitter } from 'node:events';
class MyEmitter extends EventEmitter {}
const myEmitter = new MyEmitter();
const callbackA = () => {
console.log('A');
myEmitter.removeListener('event', callbackB);
};
const callbackB = () => {
console.log('B');
};
myEmitter.on('event', callbackA);
myEmitter.on('event', callbackB);
// callbackA removes listener callbackB but it will still be called.
// Internal listener array at time of emit [callbackA, callbackB]
myEmitter.emit('event');
// Prints:
// A
// B
// callbackB is now removed.
// Internal listener array [callbackA]
myEmitter.emit('event');
// Prints:
// A
Because listeners are managed using an internal array, calling this will
change the position indices of any listener registered after the listener
being removed. This will not impact the order in which listeners are called,
but it means that any copies of the listener array as returned by
the emitter.listeners()
method will need to be recreated.
When a single function has been added as a handler multiple times for a single
event (as in the example below), removeListener()
will remove the most
recently added instance. In the example the once('ping')
listener is removed:
import { EventEmitter } from 'node:events';
const ee = new EventEmitter();
function pong() {
console.log('pong');
}
ee.on('ping', pong);
ee.once('ping', pong);
ee.removeListener('ping', pong);
ee.emit('ping');
ee.emit('ping');
Returns a reference to the EventEmitter
, so that calls can be chained.
The writable.setDefaultEncoding()
method sets the default encoding
for a Writable
stream.
The new default encoding
By default EventEmitter
s will print a warning if more than 10
listeners are
added for a particular event. This is a useful default that helps finding
memory leaks. The emitter.setMaxListeners()
method allows the limit to be
modified for this specific EventEmitter
instance. The value can be set to Infinity
(or 0
) to indicate an unlimited number of listeners.
Returns a reference to the EventEmitter
, so that calls can be chained.
The writable.uncork()
method flushes all data buffered since cork was called.
When using writable.cork()
and writable.uncork()
to manage the buffering
of writes to a stream, defer calls to writable.uncork()
using process.nextTick()
. Doing so allows batching of all writable.write()
calls that occur within a given Node.js event
loop phase.
stream.cork();
stream.write('some ');
stream.write('data ');
process.nextTick(() => stream.uncork());
If the writable.cork()
method is called multiple times on a stream, the
same number of calls to writable.uncork()
must be called to flush the buffered
data.
stream.cork();
stream.write('some ');
stream.cork();
stream.write('data ');
process.nextTick(() => {
stream.uncork();
// The data will not be flushed until uncork() is called a second time.
stream.uncork();
});
See also: writable.cork()
.
The writable.write()
method writes some data to the stream, and calls the
supplied callback
once the data has been fully handled. If an error
occurs, the callback
will be called with the error as its
first argument. The callback
is called asynchronously and before 'error'
is
emitted.
The return value is true
if the internal buffer is less than the highWaterMark
configured when the stream was created after admitting chunk
.
If false
is returned, further attempts to write data to the stream should
stop until the 'drain'
event is emitted.
While a stream is not draining, calls to write()
will buffer chunk
, and
return false. Once all currently buffered chunks are drained (accepted for
delivery by the operating system), the 'drain'
event will be emitted.
Once write()
returns false, do not write more chunks
until the 'drain'
event is emitted. While calling write()
on a stream that
is not draining is allowed, Node.js will buffer all written chunks until
maximum memory usage occurs, at which point it will abort unconditionally.
Even before it aborts, high memory usage will cause poor garbage collector
performance and high RSS (which is not typically released back to the system,
even after the memory is no longer required). Since TCP sockets may never
drain if the remote peer does not read the data, writing a socket that is
not draining may lead to a remotely exploitable vulnerability.
Writing data while the stream is not draining is particularly
problematic for a Transform
, because the Transform
streams are paused
by default until they are piped or a 'data'
or 'readable'
event handler
is added.
If the data to be written can be generated or fetched on demand, it is
recommended to encapsulate the logic into a Readable
and use pipe. However, if calling write()
is preferred, it is
possible to respect backpressure and avoid memory issues using the 'drain'
event:
function write(data, cb) {
if (!stream.write(data)) {
stream.once('drain', cb);
} else {
process.nextTick(cb);
}
}
// Wait for cb to be called before doing any other write.
write('hello', () => {
console.log('Write completed, do more writes now.');
});
A Writable
stream in object mode will always ignore the encoding
argument.
Optional data to write. For streams not operating in object mode, chunk
must be a {string}, {Buffer},
{TypedArray} or {DataView}. For object mode streams, chunk
may be any JavaScript value other than null
.
Optional
callback: (error: undefined | null | Error) => voidCallback for when this chunk of data is flushed.
false
if the stream wishes for the calling code to wait for the 'drain'
event to be emitted before continuing to write additional data; otherwise true
.
The writable.write()
method writes some data to the stream, and calls the
supplied callback
once the data has been fully handled. If an error
occurs, the callback
will be called with the error as its
first argument. The callback
is called asynchronously and before 'error'
is
emitted.
The return value is true
if the internal buffer is less than the highWaterMark
configured when the stream was created after admitting chunk
.
If false
is returned, further attempts to write data to the stream should
stop until the 'drain'
event is emitted.
While a stream is not draining, calls to write()
will buffer chunk
, and
return false. Once all currently buffered chunks are drained (accepted for
delivery by the operating system), the 'drain'
event will be emitted.
Once write()
returns false, do not write more chunks
until the 'drain'
event is emitted. While calling write()
on a stream that
is not draining is allowed, Node.js will buffer all written chunks until
maximum memory usage occurs, at which point it will abort unconditionally.
Even before it aborts, high memory usage will cause poor garbage collector
performance and high RSS (which is not typically released back to the system,
even after the memory is no longer required). Since TCP sockets may never
drain if the remote peer does not read the data, writing a socket that is
not draining may lead to a remotely exploitable vulnerability.
Writing data while the stream is not draining is particularly
problematic for a Transform
, because the Transform
streams are paused
by default until they are piped or a 'data'
or 'readable'
event handler
is added.
If the data to be written can be generated or fetched on demand, it is
recommended to encapsulate the logic into a Readable
and use 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.
Optional data to write. For streams not operating in object mode, chunk
must be a {string}, {Buffer},
{TypedArray} or {DataView}. For object mode streams, chunk
may be any JavaScript value other than null
.
The encoding, if chunk
is a string.
Optional
callback: (error: undefined | null | Error) => voidCallback for when this chunk of data is flushed.
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
.
Static
addExperimental
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 AbortSignal
s 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]();
}
}
Disposable that removes the abort
listener.
Static
getReturns a copy of the array of listeners for the event named eventName
.
For EventEmitter
s this behaves exactly the same as calling .listeners
on
the emitter.
For EventTarget
s 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] ]
}
Static
getReturns the currently set max amount of listeners.
For EventEmitter
s this behaves exactly the same as calling .getMaxListeners
on
the emitter.
For EventTarget
s 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
}
Static
listenerA class method that returns the number of listeners for the given eventName
registered on the given emitter
.
import { EventEmitter, listenerCount } from 'node:events';
const myEmitter = new EventEmitter();
myEmitter.on('event', () => {});
myEmitter.on('event', () => {});
console.log(listenerCount(myEmitter, 'event'));
// Prints: 2
The emitter to query
The event name
Static
onimport { on, EventEmitter } from 'node:events';
import process from 'node:process';
const ee = new EventEmitter();
// Emit later on
process.nextTick(() => {
ee.emit('foo', 'bar');
ee.emit('foo', 42);
});
for await (const event of on(ee, 'foo')) {
// The execution of this inner block is synchronous and it
// processes one event at a time (even with await). Do not use
// if concurrent execution is required.
console.log(event); // prints ['bar'] [42]
}
// Unreachable here
Returns an AsyncIterator
that iterates eventName
events. It will throw
if the EventEmitter
emits 'error'
. It removes all listeners when
exiting the loop. The value
returned by each iteration is an array
composed of the emitted event arguments.
An AbortSignal
can be used to cancel waiting on events:
import { on, EventEmitter } from 'node:events';
import process from 'node:process';
const ac = new AbortController();
(async () => {
const ee = new EventEmitter();
// Emit later on
process.nextTick(() => {
ee.emit('foo', 'bar');
ee.emit('foo', 42);
});
for await (const event of on(ee, 'foo', { signal: ac.signal })) {
// The execution of this inner block is synchronous and it
// processes one event at a time (even with await). Do not use
// if concurrent execution is required.
console.log(event); // prints ['bar'] [42]
}
// Unreachable here
})();
process.nextTick(() => ac.abort());
Use the close
option to specify an array of event names that will end the iteration:
import { on, EventEmitter } from 'node:events';
import process from 'node:process';
const ee = new EventEmitter();
// Emit later on
process.nextTick(() => {
ee.emit('foo', 'bar');
ee.emit('foo', 42);
ee.emit('close');
});
for await (const event of on(ee, 'foo', { close: ['close'] })) {
console.log(event); // prints ['bar'] [42]
}
// the loop will exit after 'close' is emitted
console.log('done'); // prints 'done'
Optional
options: StaticEventEmitterIteratorOptionsAn AsyncIterator
that iterates eventName
events emitted by the emitter
import { on, EventEmitter } from 'node:events';
import process from 'node:process';
const ee = new EventEmitter();
// Emit later on
process.nextTick(() => {
ee.emit('foo', 'bar');
ee.emit('foo', 42);
});
for await (const event of on(ee, 'foo')) {
// The execution of this inner block is synchronous and it
// processes one event at a time (even with await). Do not use
// if concurrent execution is required.
console.log(event); // prints ['bar'] [42]
}
// Unreachable here
Returns an AsyncIterator
that iterates eventName
events. It will throw
if the EventEmitter
emits 'error'
. It removes all listeners when
exiting the loop. The value
returned by each iteration is an array
composed of the emitted event arguments.
An AbortSignal
can be used to cancel waiting on events:
import { on, EventEmitter } from 'node:events';
import process from 'node:process';
const ac = new AbortController();
(async () => {
const ee = new EventEmitter();
// Emit later on
process.nextTick(() => {
ee.emit('foo', 'bar');
ee.emit('foo', 42);
});
for await (const event of on(ee, 'foo', { signal: ac.signal })) {
// The execution of this inner block is synchronous and it
// processes one event at a time (even with await). Do not use
// if concurrent execution is required.
console.log(event); // prints ['bar'] [42]
}
// Unreachable here
})();
process.nextTick(() => ac.abort());
Use the close
option to specify an array of event names that will end the iteration:
import { on, EventEmitter } from 'node:events';
import process from 'node:process';
const ee = new EventEmitter();
// Emit later on
process.nextTick(() => {
ee.emit('foo', 'bar');
ee.emit('foo', 42);
ee.emit('close');
});
for await (const event of on(ee, 'foo', { close: ['close'] })) {
console.log(event); // prints ['bar'] [42]
}
// the loop will exit after 'close' is emitted
console.log('done'); // prints 'done'
Optional
options: StaticEventEmitterIteratorOptionsAn AsyncIterator
that iterates eventName
events emitted by the emitter
Static
onceCreates 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!
Optional
options: StaticEventEmitterOptionsCreates 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!
Optional
options: StaticEventEmitterOptionsStatic
setimport { setMaxListeners, EventEmitter } from 'node:events';
const target = new EventTarget();
const emitter = new EventEmitter();
setMaxListeners(5, target, emitter);
Optional
n: numberA non-negative number. The maximum number of listeners per EventTarget
event.
Zero or more {EventTarget} or {EventEmitter} instances. If none are specified, n
is set as the default max for all newly created {EventTarget} and {EventEmitter}
objects.
The
EventEmitter
class is defined and exposed by thenode:events
module:All
EventEmitter
s 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