Node js EventEmitter

Node js is event driven program. An event handler is called when some event is occurred. This event is same like a javascript event, like - event called on mouse click or on key press.

The eventEmitter class is used to bind event and event listener. Whenever an event get fired, there is a listener function start for executing. An instance of EventEmitter is created to listen and fire an event.

Multiple in-built events are available in Node js through the event module.

Example of EventEmitter

// get the events module
var events = require('events');

//create an object of EventEmitter class
var eventemitter = new EventEmitter();

//Assign the event handler to an event
eventemitter.on("eventname", function () {
    console.log("event has occurred.");
});

//fire the event
eventemitter.emit("eventname");
console.log("Program Ended.");  

List of all the important methods of EventEmitter class.

addListener(event, listener)	This method adds a listener at the end of the listeners array for the specified event. There is no check made to see if the listener has already been added.
on(event, listener)	This method adds a listener at the end of the listeners array for the specified event. This method is an alias for addListener(event, listener).
once(event, listener)	This method adds a one time listener to the event. The next time, when the event is triggered, this listener is removed and then invoked. It returns a reference to the EventEmitter, so that calls can be chained.
removeListener(event, listener)	It removes a listener from the listener array for the specified event. The 'removeListener' event is emitted after the listener is removed.
removeAllListeners([event])	It removes all listeners from the specified event. It returns a reference to the EventEmitter, so that calls can be chained.
setMaxListeners(n)	The setMaxListeners() method allows the limit of listeners to be modified for this specific EventEmitter instance, because by default, EventEmitters will print a warning if more than 10 listeners are added to a particular event. For unlimited number of listeners, this value can be set to '0' (Infinity). It returns a reference to the EventEmitter, so that calls can be chained.
getMaxListeners()	The returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or default value.
listeners(event)	It returns a copy of the array of listeners for the specified event.
listenerCount(type)	It returns the number of listeners listening to the specified event.
emit(event[, arg1][,..])	It calls the each of the listeners in order with the supplied arguments. It returns true if the event had listeners, false otherwise.