Event Emitter pattern


In the TCP server code, we can see some evernts are registered, eg: socket.on('data', cb); so there the cb (callback) function gets invoked when data comes in from the client. So we can say that data is an event which is emitted by the client and when this event occurrs then we execute the cb(callback) function.

In node we have a code module called events, using which we can create custom events too.

Example 1:

var events = require('events'); // require events
var eventEmitter = new events.EventEmitter(); // create a new instance

var homeRun = function()
  console.log('Home Run!!');

// Register an event for 'swing'
eventEmitter.on('swing',homeRun); // yay!!

// Ball pitched.. So lets emit a 'swing'
eventEmitter.emit('swing'); // causes the homeRun method to run.

Output: Home Run!!

In the above program we registered an event (‘swing’) passing the callback ( homeRun()). And in the last line we Emitted the event (‘ swing’). So eventEmitter.emit is responsible for executing the callback function passed in .on method.

To know more about events, visit here.

Implementing event emitter in a class.

Example 2:

var events = require('events');

function Batter(name) {
  this.name = name;

//Invoking the EventEmitter's constructor with Batter.
  this.swing = function()

Batter.prototype = events.EventEmitter.prototype; // Inheriting EventEmitters methods into Batter. ex: 'on', as user below
var batter = new Batter('Babe Ruth');

batter.on('swing', function() {
    console.log('It is a Strrikkkeee!!!!');

Output: It is a Strrikkkeee!!!!