The signature of your callback function, including its arguments and return type.
Create a new Thread to run tasks on a separate Bun worker thread.
The callback function to be executed in parallel upon calling the asynchronous run method.
Argument types must be serializable using the structuredClone() algorithm.
Callback functions can not be closures or rely upon top level imports, as they do not have access to variables or imports outside of their isolated worker thread environment.
They can however use dynamic imports using the const myPackage = await import('some_package') syntax.
Optionaloptions: ThreadOptionsa ThreadOptions configuration object for the thread.
ReadonlyfnThe callback function to be executed in parallel upon calling the asychronous run method.
StaticcaptureValue: boolean
Change the default captureRejections option on all new EventEmitter objects.
Static ReadonlycaptureValue: Symbol.for('nodejs.rejection')
See how to write a custom rejection handler.
StaticdefaultBy 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 ReadonlyerrorThis 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.
Whether the Thread is currently busy running a task or not. It is possible the check this while a task is still running.
The status is stored on the main thread while the task is performed on the underlying worker. To wait until the Thread is not busy, await the idle property.
Whether the Thread's underlying worker is currently instantiated or not.
A unique integer identifier for the referenced Thread. May be undefined if the underlying worker is currently closed.
A promise that resolves once the Thread has finished its task and reached an idle state. Resolves immediately if the Thread is not busy. Used internally by the ThreadPool class.
import { Thread } from "bun-threads";
const countUp = new Thread((countUpTo: number) => {
let current: number = 0
for (let i = 0; i <= countUpTo; i++) {
current = i
}
return current
})
const countDown = new Thread((countDownFrom: number) => {
let current: number = countDownFrom
for (let i = countDownFrom; i >= 0; i--) {
current = i
}
return current
})
countUp.run([1_000_000])
countDown.run([1_000_000])
// you can use the idle property to get the **thread** that finishes first, not the result
Promise.race([countUp.idle, countDown.idle]).then((winner) => {
// do it again
winner.run([1_000_000]).then(async (value: number) => {
if (value === 0) {
console.log('countDown was the winner')
}
else {
console.log('countUp was the winner')
}
}).then(() => {
countUp.close()
countDown.close()
})
})
How long (in milliseconds) to keep the Thread active after completing a task before terminating it.
Keeping the Thread open will decrease repeat startup times, but will cause the program to hang and not exit if the close method is not called.
Default is 0 (close immediately). Set to Infinity to keep the Thread open until closed manually.
Changing this value will restart the Thread's internal timer.
How long (in milliseconds) to keep the Thread active after completing a task before terminating it.
Keeping the Thread open will decrease repeat startup times, but will cause the program to hang and not exit if the close method is not called.
Default is 0 (close immediately). Set to Infinity to keep the Thread open until closed manually.
Changing this value will restart the Thread's internal timer.
How many tasks are currently waiting to use the thread.
Every time you call the run method, this value is incremented by 1.
Every time the run method resolves to a value, this value is decremented by 1.
Optional[captureTerminate the underlying worker. It is safe to call this method more than once, as subsequent calls result in a no-op.
Optionalforce: boolean = falseThis method will wait for the Thread to finish its queued tasks unless force is set to true. Default is false.
A Promise<boolean> that resolves to whether the underlying worker was actually terminated. true if the worker was terminated, false if the worker was already terminated (a no-op).
import { Thread } from "bun-threads";
const waitThenReturn = async (str: string) => {
await Bun.sleep(100)
return str
}
// this code will wait for the thread to finish its operation before closing, printing 'hello'
const threadOne = new Thread(waitThenReturn)
threadOne.run(['hello']).then((result) => console.log(result))
threadOne.close() // force defaults to false
// this code will force the thread to close without waiting for it to finish its operation, 'world' never gets printed
const threadTwo = new Thread(waitThenReturn)
threadTwo.run(['world']).then((result) => console.log(result))
threadTwo.close(true)
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 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 the current max listener value for the EventEmitter which is either
set by emitter.setMaxListeners(n) or defaults to EventEmitter.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
Optionallistener: 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] ]
Adds the listener function to the end of the listeners array for the idle event. This event fires every time a thread has completed all of its pending tasks.
No checks are made to see if the listener has already been added.
Multiple calls passing the same combination of idle and listener will result in the listener being added, and called, multiple times.
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.
A reference to the EventEmitter, so that calls can be chained.
import { Thread } from "bun-threads";
const helloWorld = new Thread(() => {
return 'hello world'
})
const add = new Thread((a: number, b: number) => {
return a + b
})
const idleHandler = (thread: Thread<any>) => {
console.log(`Thread ${thread.id} is now idle.`)
}
helloWorld.on('idle', idleHandler)
add.on('idle', idleHandler)
helloWorld.run([])
add.run([1, 2])
helloWorld.close()
add.close()
Adds the listener function to the end of the listeners array for the busy event. This event fires every time a thread has switched from an idle state to working on a task.
No checks are made to see if the listener has already been added.
Multiple calls passing the same combination of busy and listener will result in the listener being added, and called, multiple times.
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.
A reference to the EventEmitter, so that calls can be chained.
import { Thread } from "bun-threads";
const countOccurences = new Thread((char: string, inString: string) => {
let occurences: number = 0
for (let i = 0; i < inString.length; i++) {
if (inString[i] === char) {
occurences++
}
}
return occurences
})
countOccurences.on('busy', () => {
console.log('Begun counting occurences in a separate thread.')
})
console.log(await countOccurences.run(['o', 'hello world']))
console.log(await countOccurences.run(['e', 'Answer to the Ultimate Question of Life, The Universe, and Everything']))
countOccurences.close()
Adds the listener function to the end of the listeners array for the close event. This event fires when a thread has closed its underlying worker object.
A thread can still be reused by calling run() again, but will have longer startup times vs. not closing it before calling run() again, as a worker has to be created again after closing.
No checks are made to see if the listener has already been added.
Multiple calls passing the same combination of close and listener will result in the listener being added, and called, multiple times.
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.
A reference to the EventEmitter, so that calls can be chained.
import { Thread } from "bun-threads";
const scramble = new Thread((toScramble: string) => {
const randomNumber = (min: number, max: number) => {
return Math.random() * (max - min) + min;
}
const oldArr: string[] = toScramble.split('')
const newArr: string[] = []
while (oldArr.length > 0) {
const rand: number = randomNumber(0, oldArr.length)
newArr.push(oldArr.splice(rand, 1)[0]!)
}
return newArr.join('')
}, { idleTimeout: 500 })
scramble.on('close', () => {
console.log(`Scramble thread has completed its work and has closed after its idleTimeout of ${scramble.idleTimeout} milliseconds.`)
})
console.log(await scramble.run(['hello world'])) // outputs a randomly rearranged 'hello world'
Adds a one-time listener function for the event named idle. The next time idle is triggered, this listener is removed and then invoked.
This event fires every time a thread has completed all of its pending tasks.
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.
A reference to the EventEmitter, so that calls can be chained.
import { Thread } from "bun-threads";
const reverse = new Thread((longStringtoReverse: string) => {
return longStringtoReverse.split('').toReversed().join('')
})
reverse.once('idle', () => console.log('Reverse thread is now idling.'))
reverse.run(['Answer to the Ultimate Question of Life, The Universe, and Everything']).then((result) => console.log('Reversed:', result))
console.log('doing some other work in the meantime...')
console.log('working...')
console.log('working...')
reverse.close()
Adds a one-time listener function for the event named busy. The next time busy is triggered, this listener is removed and then invoked.
This event fires every time a thread has switched from an idle state to working on a task.
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.
A reference to the EventEmitter, so that calls can be chained.
import { Thread } from "bun-threads";
const generate = new Thread((length: number, min: number = 0, max: number = 100) => {
const arr: number[] = []
for (let i = 0; i < length; i++) {
arr.push(Math.round(Math.random() * (max - min) + min))
}
return arr
}, { idleTimeout: 0 })
generate.once('busy', () => console.log('Thread is busy generating a random number array...'))
generate.run([100]).then((result: number[]) => console.log(result))
console.log('Doing other work in the meantime...')
Adds a one-time listener function for the event named close. The next time close is triggered, this listener is removed and then invoked.
This event fires once when a thread has closed its underlying worker object.
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.
A reference to the EventEmitter, so that calls can be chained.
import { Thread } from "bun-threads";
const sumThread = new Thread((start: number, end: number) => {
let sum: number = 0
for (let i = start; i <= end; i++) {
sum += i
}
return sum
}, { idleTimeout: 0 })
sumThread.once('close', () => console.log('sumThread has finished operation and is shutting down...'))
sumThread.run([0, 1_000_000]).then((sum: number) => console.log(sum))
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.
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.
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.
OptionaleventName: 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.
Execute the callback that was specified in the constructor in a separate worker thread.
An array of arguments to pass to the callback function. If your callback function does not have arguments, you still must pass an empty array. This is required for TypeScript to be able infer arguments. Argument types must be serializable using the structuredClone() algorithm.
A Promise<ReturnType<T>> that resolves to the return type of your callback function.
By default EventEmitters will print a warning if more than 10 listeners are
added for a particular event. This is a useful default that helps finding
memory leaks. The emitter.setMaxListeners() method allows the limit to be
modified for this specific EventEmitter instance. The value can be set to Infinity (or 0) to indicate an unlimited number of listeners.
Returns a reference to the EventEmitter, so that calls can be chained.
StaticaddExperimentalListens 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]();
}
}
Disposable that removes the abort listener.
StaticgetReturns 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] ]
}
StaticgetReturns 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
}
StaticlistenerA 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
Staticonimport { 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'
Optionaloptions: 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'
Optionaloptions: StaticEventEmitterIteratorOptionsAn AsyncIterator that iterates eventName events emitted by the emitter
StaticonceCreates 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!
Optionaloptions: 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!
Optionaloptions: StaticEventEmitterOptionsStaticsetimport { setMaxListeners, EventEmitter } from 'node:events';
const target = new EventTarget();
const emitter = new EventEmitter();
setMaxListeners(5, target, emitter);
Optionaln: 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.
Abstraction around Bun workers to enable working with them as promises.