Alias for emitter.on(eventName, listener).

Synchronously calls each of the listeners registered for the event namedeventName, in the order they were registered, passing the supplied arguments to each.

Returns true if the event had listeners, false otherwise.

js const EventEmitter = require('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.

js const EventEmitter = require('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 to the event named eventName.

Returns a copy of the array of listeners for the event named eventName.

js 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 eventNameand listener will result in the listener being added, and called, multiple times.

js server.on('connection', (stream) => { console.log('someone connected!'); });

Returns a reference to the EventEmitter, so that calls can be chained.

By default, event listeners are invoked in the order they are added. Theemitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

js const myEE = new EventEmitter(); myEE.on('foo', () => console.log('a')); myEE.prependListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a

Adds a **one-time**listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.

js server.once('connection', (stream) => { console.log('Ah, we have our first user!'); });

Returns a reference to the EventEmitter, so that calls can be chained.

By default, event listeners are invoked in the order they are added. Theemitter.prependOnceListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

js const myEE = new EventEmitter(); myEE.once('foo', () => console.log('a')); myEE.prependOnceListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a

Adds the listener function to the _beginning_ of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventNameand listener will result in the listener being added, and called, multiple times.

js server.prependListener('connection', (stream) => { console.log('someone connected!'); });

Returns a reference to the EventEmitter, so that calls can be chained.

Adds a **one-time**listener 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.

js 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()).

js 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.

Removes the specified listener from the listener array for the event namedeventName.

js const callback = (stream) => { console.log('someone connected!'); }; server.on('connection', callback); // ... server.removeListener('connection', callback);

removeListener() will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified eventName, then removeListener() must be called multiple times to remove each instance.

Once an event is emitted, all listeners attached to it at the time of emitting are called in order. This implies that anyremoveListener() or removeAllListeners() calls _after_ emitting and_before_ the last listener finishes execution will not remove them fromemit() in progress. Subsequent events behave as expected.

js 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:

js 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.

By default EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks. The emitter.setMaxListeners() method allows the limit to be modified for this specific EventEmitter instance. The value can be set toInfinity (or 0) to indicate an unlimited number of listeners.

Returns a reference to the EventEmitter, so that calls can be chained.