Instances of the readline.Interface
class are constructed using the readline.createInterface()
method. Every instance is associated with a single input
Readable stream and a single output
Writable stream. The output
stream is used to print prompts for user input that arrives on, and is read from, the input
stream.
class
readline.Interface
class Interface
- readonly cursor: number
The cursor position relative to
rl.line
.This will track where the current cursor lands in the input string, when reading input from a TTY stream. The position of cursor determines the portion of the input string that will be modified as input is processed, as well as the column where the terminal caret will be rendered.
- readonly line: string
The current input data being processed by node.
This can be used when collecting input from a TTY stream to retrieve the current value that has been processed thus far, prior to the
line
event being emitted. Once theline
event has been emitted, this property will be an empty string.Be aware that modifying the value during the instance runtime may have unintended consequences if
rl.cursor
is not also controlled.If not using a TTY stream for input, use the
'line'
event.One possible use case would be as follows:
const values = ['lorem ipsum', 'dolor sit amet']; const rl = readline.createInterface(process.stdin); const showResults = debounce(() => { console.log( '\n', values.filter((val) => val.startsWith(rl.line)).join(' '), ); }, 300); process.stdin.on('keypress', (c, k) => { showResults(); });
- static captureRejections: boolean
Value: boolean
Change the default
captureRejections
option on all newEventEmitter
objects. - readonly static captureRejectionSymbol: typeof captureRejectionSymbol
Value:
Symbol.for('nodejs.rejection')
See how to write a custom
rejection handler
. - static defaultMaxListeners: number
By default, a maximum of
10
listeners can be registered for any single event. This limit can be changed for individualEventEmitter
instances using theemitter.setMaxListeners(n)
method. To change the default for allEventEmitter
instances, theevents.defaultMaxListeners
property can be used. If this value is not a positive number, aRangeError
is thrown.Take caution when setting the
events.defaultMaxListeners
because the change affects allEventEmitter
instances, including those created before the change is made. However, callingemitter.setMaxListeners(n)
still has precedence overevents.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
, theemitter.getMaxListeners()
andemitter.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 additionalemitter
,type
, andcount
properties, referring to the event emitter instance, the event's name and the number of attached listeners, respectively. Itsname
property is set to'MaxListenersExceededWarning'
. - readonly static errorMonitor: typeof errorMonitor
This symbol shall be used to install a listener for only monitoring
'error'
events. Listeners installed using this symbol are called before the regular'error'
listeners are called.Installing a listener using this symbol does not change the behavior once an
'error'
event is emitted. Therefore, the process will still crash if no regular'error'
listener is installed. Alias for
rl.close()
.- event: string,listener: (...args: any[]) => void): this;
events.EventEmitter
- close
- line
- pause
- resume
- SIGCONT
- SIGINT
- SIGTSTP
- history
event: 'close',listener: () => void): this;events.EventEmitter
- close
- line
- pause
- resume
- SIGCONT
- SIGINT
- SIGTSTP
- history
event: 'line',listener: (input: string) => void): this;events.EventEmitter
- close
- line
- pause
- resume
- SIGCONT
- SIGINT
- SIGTSTP
- history
event: 'pause',listener: () => void): this;events.EventEmitter
- close
- line
- pause
- resume
- SIGCONT
- SIGINT
- SIGTSTP
- history
event: 'resume',listener: () => void): this;events.EventEmitter
- close
- line
- pause
- resume
- SIGCONT
- SIGINT
- SIGTSTP
- history
event: 'SIGCONT',listener: () => void): this;events.EventEmitter
- close
- line
- pause
- resume
- SIGCONT
- SIGINT
- SIGTSTP
- history
event: 'SIGINT',listener: () => void): this;events.EventEmitter
- close
- line
- pause
- resume
- SIGCONT
- SIGINT
- SIGTSTP
- history
event: 'SIGTSTP',listener: () => void): this;events.EventEmitter
- close
- line
- pause
- resume
- SIGCONT
- SIGINT
- SIGTSTP
- history
event: 'history',listener: (history: string[]) => void): this;events.EventEmitter
- close
- line
- pause
- resume
- SIGCONT
- SIGINT
- SIGTSTP
- history
The
rl.close()
method closes theInterface
instance and relinquishes control over theinput
andoutput
streams. When called, the'close'
event will be emitted.Calling
rl.close()
does not immediately stop other events (including'line'
) from being emitted by theInterface
instance.- emit(event: string | symbol,...args: any[]): boolean;
Synchronously calls each of the listeners registered for the event named
eventName
, in the order they were registered, passing the supplied arguments to each.Returns
true
if the event had listeners,false
otherwise.import { EventEmitter } from 'node:events'; const myEmitter = new EventEmitter(); // First listener myEmitter.on('event', function firstListener() { console.log('Helloooo! first listener'); }); // Second listener myEmitter.on('event', function secondListener(arg1, arg2) { console.log(`event with parameters ${arg1}, ${arg2} in second listener`); }); // Third listener myEmitter.on('event', function thirdListener(...args) { const parameters = args.join(', '); console.log(`event with parameters ${parameters} in third listener`); }); console.log(myEmitter.listeners('event')); myEmitter.emit('event', 1, 2, 3, 4, 5); // Prints: // [ // [Function: firstListener], // [Function: secondListener], // [Function: thirdListener] // ] // Helloooo! first listener // event with parameters 1, 2 in second listener // event with parameters 1, 2, 3, 4, 5 in third listener
Returns 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 real position of the cursor in relation to the input prompt + string. Long input (wrapping) strings, as well as multiple line prompts are included in the calculations.
Returns the current max listener value for the
EventEmitter
which is either set byemitter.setMaxListeners(n)
or defaults to EventEmitter.defaultMaxListeners.The
rl.getPrompt()
method returns the current prompt used byrl.prompt()
.@returnsthe current prompt string
- eventName: string | symbol,listener?: Function): number;
Returns the number of listeners listening for the event named
eventName
. Iflistener
is provided, it will return how many times the listener is found in the list of the listeners of the event.@param eventNameThe name of the event being listened for
@param listenerThe event handler function
- eventName: string | symbol): 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] ]
- eventName: string | symbol,listener: (...args: any[]) => void): this;
Alias for
emitter.removeListener()
. - on(event: string,listener: (...args: any[]) => void): this;
Adds the
listener
function to the end of the listeners array for the event namedeventName
. No checks are made to see if thelistener
has already been added. Multiple calls passing the same combination ofeventName
andlistener
will result in thelistener
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
@param listenerThe callback function
- once(event: string,listener: (...args: any[]) => void): this;
Adds a one-time
listener
function for the event namedeventName
. The next timeeventName
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
@param listenerThe callback function
The
rl.pause()
method pauses theinput
stream, allowing it to be resumed later if necessary.Calling
rl.pause()
does not immediately pause other events (including'line'
) from being emitted by theInterface
instance.- event: string,listener: (...args: any[]) => void): this;
Adds the
listener
function to the beginning of the listeners array for the event namedeventName
. No checks are made to see if thelistener
has already been added. Multiple calls passing the same combination ofeventName
andlistener
will result in thelistener
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.@param listenerThe callback function
- event: string,listener: (...args: any[]) => void): this;
Adds a one-time
listener
function for the event namedeventName
to the beginning of the listeners array. The next timeeventName
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.@param listenerThe callback function
- preserveCursor?: boolean): void;
The
rl.prompt()
method writes theInterface
instances configuredprompt
to a new line inoutput
in order to provide a user with a new location at which to provide input.When called,
rl.prompt()
will resume theinput
stream if it has been paused.If the
Interface
was created withoutput
set tonull
orundefined
the prompt is not written.@param preserveCursorIf
true
, prevents the cursor placement from being reset to0
. - query: string,callback: (answer: string) => void): void;
The
rl.question()
method displays thequery
by writing it to theoutput
, waits for user input to be provided oninput
, then invokes thecallback
function passing the provided input as the first argument.When called,
rl.question()
will resume theinput
stream if it has been paused.If the
Interface
was created withoutput
set tonull
orundefined
thequery
is not written.The
callback
function passed torl.question()
does not follow the typical pattern of accepting anError
object ornull
as the first argument. Thecallback
is called with the provided answer as the only argument.An error will be thrown if calling
rl.question()
afterrl.close()
.Example usage:
rl.question('What is your favorite food? ', (answer) => { console.log(`Oh, so your favorite food is ${answer}`); });
Using an
AbortController
to cancel a question.const ac = new AbortController(); const signal = ac.signal; rl.question('What is your favorite food? ', { signal }, (answer) => { console.log(`Oh, so your favorite food is ${answer}`); }); signal.addEventListener('abort', () => { console.log('The food question timed out'); }, { once: true }); setTimeout(() => ac.abort(), 10000);
@param queryA statement or query to write to
output
, prepended to the prompt.@param callbackA callback function that is invoked with the user's input in response to the
query
.query: string,callback: (answer: string) => void): void;The
rl.question()
method displays thequery
by writing it to theoutput
, waits for user input to be provided oninput
, then invokes thecallback
function passing the provided input as the first argument.When called,
rl.question()
will resume theinput
stream if it has been paused.If the
Interface
was created withoutput
set tonull
orundefined
thequery
is not written.The
callback
function passed torl.question()
does not follow the typical pattern of accepting anError
object ornull
as the first argument. Thecallback
is called with the provided answer as the only argument.An error will be thrown if calling
rl.question()
afterrl.close()
.Example usage:
rl.question('What is your favorite food? ', (answer) => { console.log(`Oh, so your favorite food is ${answer}`); });
Using an
AbortController
to cancel a question.const ac = new AbortController(); const signal = ac.signal; rl.question('What is your favorite food? ', { signal }, (answer) => { console.log(`Oh, so your favorite food is ${answer}`); }); signal.addEventListener('abort', () => { console.log('The food question timed out'); }, { once: true }); setTimeout(() => ac.abort(), 10000);
@param queryA statement or query to write to
output
, prepended to the prompt.@param callbackA callback function that is invoked with the user's input in response to the
query
. - eventName: string | symbol): 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');
- eventName?: string | symbol): this;
Removes all listeners, or those of the specified
eventName
.It is bad practice to remove listeners added elsewhere in the code, particularly when the
EventEmitter
instance was created by some other component or module (e.g. sockets or file streams).Returns a reference to the
EventEmitter
, so that calls can be chained. - eventName: string | symbol,listener: (...args: any[]) => void): this;
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 specifiedeventName
, thenremoveListener()
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()
orremoveAllListeners()
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 theonce('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
rl.resume()
method resumes theinput
stream if it has been paused.- n: number): this;
By default
EventEmitter
s will print a warning if more than10
listeners are added for a particular event. This is a useful default that helps finding memory leaks. Theemitter.setMaxListeners()
method allows the limit to be modified for this specificEventEmitter
instance. The value can be set toInfinity
(or0
) to indicate an unlimited number of listeners.Returns a reference to the
EventEmitter
, so that calls can be chained. - prompt: string): void;
The
rl.setPrompt()
method sets the prompt that will be written tooutput
wheneverrl.prompt()
is called. - ): void;
The
rl.write()
method will write eitherdata
or a key sequence identified bykey
to theoutput
. Thekey
argument is supported only ifoutput
is aTTY
text terminal. SeeTTY keybindings
for a list of key combinations.If
key
is specified,data
is ignored.When called,
rl.write()
will resume theinput
stream if it has been paused.If the
Interface
was created withoutput
set tonull
orundefined
thedata
andkey
are not written.rl.write('Delete this!'); // Simulate Ctrl+U to delete the line written previously rl.write(null, { ctrl: true, name: 'u' });
The
rl.write()
method will write the data to thereadline
Interface
'sinput
as if it were provided by the user.): void;The
rl.write()
method will write eitherdata
or a key sequence identified bykey
to theoutput
. Thekey
argument is supported only ifoutput
is aTTY
text terminal. SeeTTY keybindings
for a list of key combinations.If
key
is specified,data
is ignored.When called,
rl.write()
will resume theinput
stream if it has been paused.If the
Interface
was created withoutput
set tonull
orundefined
thedata
andkey
are not written.rl.write('Delete this!'); // Simulate Ctrl+U to delete the line written previously rl.write(null, { ctrl: true, name: 'u' });
The
rl.write()
method will write the data to thereadline
Interface
'sinput
as if it were provided by the user. - ): Disposable;
Listens once to the
abort
event on the providedsignal
.Listening to the
abort
event on abort signals is unsafe and may lead to resource leaks since another third party with the signal can calle.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 thatstopImmediatePropagation
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](); } }
@returnsDisposable that removes the
abort
listener. - name: string | symbol): Function[];
Returns 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] ] }
- ): number;
Returns 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 }
- emitter: EventEmitter,eventName: string | symbol,options?: StaticEventEmitterIteratorOptions): AsyncIterator<any[]>;
import { on, EventEmitter } from 'node:events'; import process from 'node:process'; const ee = new EventEmitter(); // Emit later on process.nextTick(() => { ee.emit('foo', 'bar'); ee.emit('foo', 42); }); for await (const event of on(ee, 'foo')) { // The execution of this inner block is synchronous and it // processes one event at a time (even with await). Do not use // if concurrent execution is required. console.log(event); // prints ['bar'] [42] } // Unreachable here
Returns an
AsyncIterator
that iterateseventName
events. It will throw if theEventEmitter
emits'error'
. It removes all listeners when exiting the loop. Thevalue
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'
@returnsAn
AsyncIterator
that iterateseventName
events emitted by theemitter
eventName: string,options?: StaticEventEmitterIteratorOptions): AsyncIterator<any[]>;import { on, EventEmitter } from 'node:events'; import process from 'node:process'; const ee = new EventEmitter(); // Emit later on process.nextTick(() => { ee.emit('foo', 'bar'); ee.emit('foo', 42); }); for await (const event of on(ee, 'foo')) { // The execution of this inner block is synchronous and it // processes one event at a time (even with await). Do not use // if concurrent execution is required. console.log(event); // prints ['bar'] [42] } // Unreachable here
Returns an
AsyncIterator
that iterateseventName
events. It will throw if theEventEmitter
emits'error'
. It removes all listeners when exiting the loop. Thevalue
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'
@returnsAn
AsyncIterator
that iterateseventName
events emitted by theemitter
- emitter: EventEmitter,eventName: string | symbol,options?: StaticEventEmitterOptions): Promise<any[]>;
Creates a
Promise
that is fulfilled when theEventEmitter
emits the given event or that is rejected if theEventEmitter
emits'error'
while waiting. ThePromise
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 whenevents.once()
is used to wait for another event. Ifevents.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!
eventName: string,options?: StaticEventEmitterOptions): Promise<any[]>;Creates a
Promise
that is fulfilled when theEventEmitter
emits the given event or that is rejected if theEventEmitter
emits'error'
while waiting. ThePromise
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 whenevents.once()
is used to wait for another event. Ifevents.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!
- n?: number,): void;
import { setMaxListeners, EventEmitter } from 'node:events'; const target = new EventTarget(); const emitter = new EventEmitter(); setMaxListeners(5, target, emitter);
@param nA non-negative number. The maximum number of listeners per
EventTarget
event.@param eventTargetsZero 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.