Node.js - Events

In Node.js, events play a crucial role in creating asynchronous, event-driven applications.

Every action on a computer can be considered an event. For example, when a connection is established or a file is opened, these are events.

In Node.js, objects can trigger events. For instance, the readStream object emits events when opening and closing a file. Events allow you to respond to specific occurrences in your application.

EventEmitter Class

Following are the methods of the EventEmitter class −

Sr.No. Methods & Description
1

emitter.getMaxListeners()

It returns the count of listener functions listening to the event named eventName.
2

emitter.setMaxListeners()

This method returns a reference to the EventEmitter so that calls can be chained.
3

emitter.listeners()

It returns a copy of the array of listeners for the event (eventName) which we pass as a parameter.
4

emitter.listenerCount()

(integer) This method will return the count of listener function(s) which are listening to the event (eventName) that we pass as a parameter.
5

emitter.prependOnceListener()

It returns a reference to the EventEmitter so that calls can be chained
6

emitter.prependListener()

It returns a reference to the EventEmitter so that calls can be chained.
7

emitter.emit()

(Boolean) This method returns true if the event had listeners and false if the event doesnt have listeners.
8

emitter.eventNames()

The events for which the emitter has registered listeners are returned in an array list by this method.
9

emitter.off()

It returns a reference to the EventEmitter so that calls can be chained
10

emitter.rawListeners()

It returns a copy of the listeners array for a specific event named eventName. It also returns a copy of the array of listeners for any wrappers which are created by the emitter.once() method.

EventEmitter Class: Event Listeners Methods

Following are the event listener methods of the EventEmitter class −

Sr.No. Methods & Description
1

emitter.addlistener(event, listener)

It returns the instance of the EventEmitter to which the listener was added.
2

emitter.on(event, listener)

It returns an event listener for a specific event.
3

emitter.once(event, listener)

It returns an event listener for a specific event. When the specified event is emitted, the provided callback function is executed.

EventEmitter Class: Methods to Remove Listeners

Following are the methods to remove listeners −

Sr.No. Methods & Description
1

emitter.removeListener()

It returns a reference to the EventEmitter so that calls can be chained.
2

emitter.removeAllListeners()

It returns emitter, so calls can be chained.

EventTarget Class

Following are the methods of the EventTarget class −

Sr.No. Methods & Description
1

eventTarget.addEventListener()

It returns a reference to the EventEmitter so that calls can be chained.
2

eventTarget.dispatchEvent()

This methods return value is false if at least one of the event handlers which are handling that specific event called Event.preventDefault() method else, it returns true.
3

eventTarget.removeEventListener()

It returns a reference to the EventEmitter so that calls can be chained.
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