externalLogger
Hierarchy
- EventEmitter
- Logger
Index
Constructors
Properties
Methods
- [captureRejectionSymbol]
- _log
- _outputWithConsole
- addListener
- emit
- eventNames
- getMaxListeners
- getOptions
- listenerCount
- listeners
- log
- off
- on
- once
- prependListener
- prependOnceListener
- rawListeners
- removeAllListeners
- removeListener
- setMaxListeners
- setOptions
- addAbortListener
- getEventListeners
- getMaxListeners
- listenerCount
- on
- once
- setMaxListeners
Constructors
externalconstructor
Parameters
externaloptions: Record<string, any>
Returns Logger
Properties
staticexternalreadonlycaptureRejectionSymbol
Value: Symbol.for('nodejs.rejection')
See how to write a custom rejection handler.
staticexternalcaptureRejections
Value: boolean
Change the default captureRejections option on all new EventEmitter objects.
staticexternaldefaultMaxListeners
By default, a maximum of 10 listeners can be registered for any single
event. This limit can be changed for individual EventEmitter instances
using the emitter.setMaxListeners(n) method. To change the default
for allEventEmitter instances, the events.defaultMaxListenersproperty can be used. If this value is not a positive number, a RangeErroris thrown.
Take caution when setting the events.defaultMaxListeners because the
change affects allEventEmitter instances, including those created before
the change is made. However, calling emitter.setMaxListeners(n) still has
precedence over events.defaultMaxListeners.
This is not a hard limit. The EventEmitter instance will allow
more listeners to be added but will output a trace warning to stderr indicating
that a "possible EventEmitter memory leak" has been detected. For any singleEventEmitter, the emitter.getMaxListeners() and emitter.setMaxListeners()methods can be used to
temporarily avoid this warning:
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'.
staticexternalreadonlyerrorMonitor
This symbol shall be used to install a listener for only monitoring 'error'events. Listeners installed using this symbol are called before the regular'error' listeners are called.
Installing a listener using this symbol does not change the behavior once an'error' event is emitted. Therefore, the process will still crash if no
regular 'error' listener is installed.
Methods
externaloptional[captureRejectionSymbol]
Parameters
externalerror: Error
externalevent: string
externalrest...args: any[]
Returns void
external_log
Parameters
externallevel: LogLevel
externalmessage: string
externaloptionaldata: any
externaloptionalexception: Exception
externaloptionalopts: Record<string, any>
Returns void
external_outputWithConsole
Parameters
externallevel: LogLevel
externalline: string
Returns void
externaladdListener
Alias for
emitter.on(eventName, listener).Parameters
externaleventName: string | symbol
externallistener: (...args: any[]) => void
Returns Logger
externalemit
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
trueif the event had listeners,falseotherwise.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 listenerParameters
externaleventName: string | symbol
externalrest...args: any[]
Returns boolean
externaleventNames
Returns an array listing the events for which the emitter has registered listeners. The values in the array are strings or
Symbols.import { EventEmitter } from 'node:events';
const myEE = new EventEmitter();
myEE.on('foo', () => {});
myEE.on('bar', () => {});
const sym = Symbol('symbol');
myEE.on(sym, () => {});
console.log(myEE.eventNames());
// Prints: [ 'foo', 'bar', Symbol(symbol) ]Returns (string | symbol)[]
externalgetMaxListeners
Returns the current max listener value for the
EventEmitterwhich is either set byemitter.setMaxListeners(n)or defaults to defaultMaxListeners.Returns number
externalgetOptions
Returns Record<string, any>
externallistenerCount
Returns the number of listeners listening for the event named
eventName. Iflisteneris provided, it will return how many times the listener is found in the list of the listeners of the event.Parameters
externaleventName: string | symbol
The name of the event being listened for
externaloptionallistener: Function
The event handler function
Returns number
externallisteners
Returns a copy of the array of listeners for the event named
eventName.server.on('connection', (stream) => {
console.log('someone connected!');
});
console.log(util.inspect(server.listeners('connection')));
// Prints: [ [Function] ]Parameters
externaleventName: string | symbol
Returns Function[]
externallog
Parameters
externallevel: LogLevel
externalmessage: string
externalrest...args: any[]
Returns void
externaloff
Alias for
emitter.removeListener().Parameters
externaleventName: string | symbol
externallistener: (...args: any[]) => void
Returns Logger
externalon
Adds the
listenerfunction to the end of the listeners array for the event namedeventName. No checks are made to see if thelistenerhas already been added. Multiple calls passing the same combination ofeventNameandlistenerwill result in thelistenerbeing 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
// aParameters
externaleventName: string | symbol
The name of the event.
externallistener: (...args: any[]) => void
The callback function
Returns Logger
externalonce
Adds a one-time
listenerfunction for the event namedeventName. The next timeeventNameis 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
// aParameters
externaleventName: string | symbol
The name of the event.
externallistener: (...args: any[]) => void
The callback function
Returns Logger
externalprependListener
Adds the
listenerfunction to the beginning of the listeners array for the event namedeventName. No checks are made to see if thelistenerhas already been added. Multiple calls passing the same combination ofeventNameandlistenerwill result in thelistenerbeing added, and called, multiple times.server.prependListener('connection', (stream) => {
console.log('someone connected!');
});Returns a reference to the
EventEmitter, so that calls can be chained.Parameters
externaleventName: string | symbol
The name of the event.
externallistener: (...args: any[]) => void
The callback function
Returns Logger
externalprependOnceListener
Adds a one-time
listenerfunction for the event namedeventNameto the beginning of the listeners array. The next timeeventNameis triggered, this listener is removed, and then invoked.server.prependOnceListener('connection', (stream) => {
console.log('Ah, we have our first user!');
});Returns a reference to the
EventEmitter, so that calls can be chained.Parameters
externaleventName: string | symbol
The name of the event.
externallistener: (...args: any[]) => void
The callback function
Returns Logger
externalrawListeners
Returns a copy of the array of listeners for the event named
eventName, including any wrappers (such as those created by.once()).import { EventEmitter } from 'node:events';
const emitter = new EventEmitter();
emitter.once('log', () => console.log('log once'));
// Returns a new Array with a function `onceWrapper` which has a property
// `listener` which contains the original listener bound above
const listeners = emitter.rawListeners('log');
const logFnWrapper = listeners[0];
// Logs "log once" to the console and does not unbind the `once` event
logFnWrapper.listener();
// Logs "log once" to the console and removes the listener
logFnWrapper();
emitter.on('log', () => console.log('log persistently'));
// Will return a new Array with a single function bound by `.on()` above
const newListeners = emitter.rawListeners('log');
// Logs "log persistently" twice
newListeners[0]();
emitter.emit('log');Parameters
externaleventName: string | symbol
Returns Function[]
externalremoveAllListeners
Removes all listeners, or those of the specified
eventName.It is bad practice to remove listeners added elsewhere in the code, particularly when the
EventEmitterinstance was created by some other component or module (e.g. sockets or file streams).Returns a reference to the
EventEmitter, so that calls can be chained.Parameters
externaloptionalevent: string | symbol
Returns Logger
externalremoveListener
Removes the specified
listenerfrom 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:
// ABecause 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.Parameters
externaleventName: string | symbol
externallistener: (...args: any[]) => void
Returns Logger
externalsetMaxListeners
By default
EventEmitters will print a warning if more than10listeners 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 specificEventEmitterinstance. 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.Parameters
externaln: number
Returns Logger
externalsetOptions
Parameters
externaloptions: Record<string, any>
Returns void
staticexternaladdAbortListener
- experimental
Listens once to the
abortevent on the providedsignal.Listening to the
abortevent 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
AbortSignals in Node.js APIs by solving these two issues by listening to the event such thatstopImmediatePropagationdoes not prevent the listener from running.Returns a disposable so that it may be unsubscribed from more easily.
import { addAbortListener } from 'node:events';
function example(signal) {
let disposable;
try {
signal.addEventListener('abort', (e) => e.stopImmediatePropagation());
disposable = addAbortListener(signal, (e) => {
// Do something when signal is aborted.
});
} finally {
disposable?.[Symbol.dispose]();
}
}Parameters
externalsignal: AbortSignal
externalresource: (event: Event) => void
Returns Disposable
Disposable that removes the
abortlistener.
staticexternalgetEventListeners
Returns a copy of the array of listeners for the event named
eventName.For
EventEmitters this behaves exactly the same as calling.listenerson the emitter.For
EventTargets this is the only way to get the event listeners for the event target. This is useful for debugging and diagnostic purposes.import { getEventListeners, EventEmitter } from 'node:events';
{
const ee = new EventEmitter();
const listener = () => console.log('Events are fun');
ee.on('foo', listener);
console.log(getEventListeners(ee, 'foo')); // [ [Function: listener] ]
}
{
const et = new EventTarget();
const listener = () => console.log('Events are fun');
et.addEventListener('foo', listener);
console.log(getEventListeners(et, 'foo')); // [ [Function: listener] ]
}Parameters
externalemitter: EventEmitter | _DOMEventTarget
externalname: string | symbol
Returns Function[]
staticexternalgetMaxListeners
Returns the currently set max amount of listeners.
For
EventEmitters this behaves exactly the same as calling.getMaxListenerson the emitter.For
EventTargets this is the only way to get the max event listeners for the event target. If the number of event handlers on a single EventTarget exceeds the max set, the EventTarget will print a warning.import { getMaxListeners, setMaxListeners, EventEmitter } from 'node:events';
{
const ee = new EventEmitter();
console.log(getMaxListeners(ee)); // 10
setMaxListeners(11, ee);
console.log(getMaxListeners(ee)); // 11
}
{
const et = new EventTarget();
console.log(getMaxListeners(et)); // 10
setMaxListeners(11, et);
console.log(getMaxListeners(et)); // 11
}Parameters
externalemitter: EventEmitter | _DOMEventTarget
Returns number
staticexternallistenerCount
A class method that returns the number of listeners for the given
eventNameregistered on the givenemitter.import { EventEmitter, listenerCount } from 'node:events';
const myEmitter = new EventEmitter();
myEmitter.on('event', () => {});
myEmitter.on('event', () => {});
console.log(listenerCount(myEmitter, 'event'));
// Prints: 2Parameters
externalemitter: EventEmitter
The emitter to query
externaleventName: string | symbol
The event name
Returns number
staticexternalon
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 hereReturns an
AsyncIteratorthat iterateseventNameevents. It will throw if theEventEmitteremits'error'. It removes all listeners when exiting the loop. Thevaluereturned by each iteration is an array composed of the emitted event arguments.An
AbortSignalcan be used to cancel waiting on events:import { on, EventEmitter } from 'node:events';
import process from 'node:process';
const ac = new AbortController();
(async () => {
const ee = new EventEmitter();
// Emit later on
process.nextTick(() => {
ee.emit('foo', 'bar');
ee.emit('foo', 42);
});
for await (const event of on(ee, 'foo', { signal: ac.signal })) {
// The execution of this inner block is synchronous and it
// processes one event at a time (even with await). Do not use
// if concurrent execution is required.
console.log(event); // prints ['bar'] [42]
}
// Unreachable here
})();
process.nextTick(() => ac.abort());Parameters
externalemitter: EventEmitter
externaleventName: string
The name of the event being listened for
externaloptionaloptions: StaticEventEmitterOptions
Returns AsyncIterableIterator<any>
that iterates
eventNameevents emitted by theemitter
staticexternalonce
Creates a
Promisethat is fulfilled when theEventEmitteremits the given event or that is rejected if theEventEmitteremits'error'while waiting. ThePromisewill 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 boomAn
AbortSignalcan be used to cancel waiting for the event:import { EventEmitter, once } from 'node:events';
const ee = new EventEmitter();
const ac = new AbortController();
async function foo(emitter, event, signal) {
try {
await once(emitter, event, { signal });
console.log('event emitted!');
} catch (error) {
if (error.name === 'AbortError') {
console.error('Waiting for the event was canceled!');
} else {
console.error('There was an error', error.message);
}
}
}
foo(ee, 'foo', ac.signal);
ac.abort(); // Abort waiting for the event
ee.emit('foo'); // Prints: Waiting for the event was canceled!Parameters
externalemitter: _NodeEventTarget
externaleventName: string | symbol
externaloptionaloptions: StaticEventEmitterOptions
Returns Promise<any[]>
staticexternalsetMaxListeners
import { setMaxListeners, EventEmitter } from 'node:events';
const target = new EventTarget();
const emitter = new EventEmitter();
setMaxListeners(5, target, emitter);Parameters
externaloptionaln: number
A non-negative number. The maximum number of listeners per
EventTargetevent.externalrest...eventTargets: (EventEmitter | _DOMEventTarget)[]
Returns void
This is an abstract class that should be extended by custom logger classes.
this._log() method must be implemented by them.