node.js: Exploring events module

Events module provide support for asynchronous events. Events module provides EventEmitter class, it implements pub-sub design pattern.

All the node.js objects that emit events are the instance of EventEmitter claas.

Following are the basic steps in event handling.

a.   An EventEmitter instance register listeners to an event

b.   An eventEmitter instance emit an event, once the event is emitted, correspondent listeners are called synchronously.

Let me explain with an example.

Step 1: Create an instance of EventEmitter.

var events = require('events');

var emitter = new events.EventEmitter();

Step 2: Register to an event

emitter.on('myCustomEvent', function(){

         console.log("Custom Event emitted");

});

Step 3: Emit the event.

emitter.emit('myCustomEvent');

When the event is emitted, the function registered in step 2 is called.

HelloWorld.js

var events = require('events');

var emitter = new events.EventEmitter();

/* Register an event name 'myCustomEvent' */
emitter.on('myCustomEvent', function(){
 console.log("Custom Event emitted");
});

/* Emit myCustomEvent */
emitter.emit('myCustomEvent');

Output

Custom Event emitted

A listener function can take multiple arguments.

emitter.on('myCustomEvent', function(eventId, message) {

    console.log(`${eventId} : ${message}`);

});

You can pass the arguments to listener function when emitting the event.

emitter.emit('myCustomEvent', "demoEvent123", "Demo Application event");

HelloWorld.js

var events = require('events');

var emitter = new events.EventEmitter();

/* Register an event name 'myCustomEvent' */
emitter.on('myCustomEvent', function(eventId, message) {
    console.log(`${eventId} : ${message}`);
});

/* Emit myCustomEvent */
emitter.emit('myCustomEvent', "demoEvent123", "Demo Application event");

Output

demoEvent123 : Demo Application event

You can add multiple listeners to an event using addListener method.

emitter.addListener('myCustomEvent', (eventId, message) =>{

         console.log(`Listener2: ${eventId} : ${message}`);

        

});

HelloWorld.js

var events = require('events');

var emitter = new events.EventEmitter();

/* Register an event name 'myCustomEvent' */
emitter.on('myCustomEvent', function(eventId, message) {
    console.log(`Listener1: ${eventId} : ${message}`);
});

emitter.addListener('myCustomEvent', (eventId, message) =>{
 console.log(`Listener2: ${eventId} : ${message}`);
 
});

emitter.addListener('myCustomEvent', (eventId, message) =>{
 console.log(`Listener3: ${eventId} : ${message}`);
 
});

/* Emit myCustomEvent */
emitter.emit('myCustomEvent', "demoEvent123", "Demo Application event");

Output

Listener1: demoEvent123 : Demo Application event

Listener2: demoEvent123 : Demo Application event

Listener3: demoEvent123 : Demo Application event

Previous                                                 Next                                                 Home