Event emitter in node js?

Event Emitter is a class that is in event module. Event emitter simply allows you to listen for "events" and assign actions to
run when those event occur. If you are familiar with front end development then you will know about the keyboard and mouse events. These are very similiar, except you can emit events on you own.
For Ex- New record saved in the database, you can emit recordSaved event.

Using EventEmitters

To use Event Emitter in your application you have to first import the events module, like as follows:

var events = require('events');

this events object has a single property, that is EventEmitter class.

Ex:-

var EventEmitter = require('events').EventEmitter;

var ee = new EventEmitter();
ee.on("newConnection", function() {
    console.log("New event trigger");
});

ee.emit("newConnection");

First of we created a new object of EventEmitter class. There are 2 methods that we have used for events: on and emit

on

on method takes two parameters, the event we are listening for and the callback method. Here the event name we are listening for is newConnection.

emit

emit method's first parameter is the name of the event here it is newConnection.

Other method of EventEmitter:

once

It is same as on, but it'll work only once. After being called for the first time, the listner will be removed.

ee.once('newConnection', function() {
    console.log("I am once method of EventEmitter class.");
});

ee.emit('newConnection'); // I am once method of EventEmitter class.
ee.emit('newConnection');

removeAllListeners

It removes all the listeners

ee.removeAllListeners()

It will remove only the specified listener:

ee.removeAllListeners('newConnection');

Example:-

userModel:

 

var util         = require("util");
var EventEmitter = require("events").EventEmitter;

var id = 1;
var database = {
    users: [
        { id: id++, name: "Joe Smith",  occupation: "developer"    },
        { id: id++, name: "Jane Doe",   occupation: "data analyst" },
        { id: id++, name: "John Henry", occupation: "designer"     }
    ]
};

function UserList () {
    EventEmitter.call(this);
}

UserList.prototype.save = function (obj) {
    obj.id = id++;
    database.users.push(obj);
    this.emit("saved-user", obj);
};

UserList.prototype.all = function () {
    return database.users;
};

util.inherits(UserList, EventEmitter);

module.exports = UserList;

userController.js

var UserList = require("./user");
var users = new UserList();

users.on("saved-user", function (user) {
    console.log("saved: " + user.name + " (" + user.id + ")");
    console.log(users.all())
});


/*
@ Login user
*/
exports.login = function(req, res, next) {
    users.save({ name: "Jane Doe", occupation: "manager" });
    users.save({ name: "John Jacob", occupation: "developer" })
}


/*
Results:

[ { id: 1, name: 'Joe Smith', occupation: 'developer' },
  { id: 2, name: 'Jane Doe', occupation: 'data analyst' },
  { id: 3, name: 'John Henry', occupation: 'designer' },
  { name: 'Jane Doe', occupation: 'manager', id: 4 },
  { name: 'John Jacob', occupation: 'developer', id: 5 } ]

  */