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Vinay Rajagopal
Vinay Rajagopal
2,471 Points
Posted by Vinay Rajagopal
Vinay Rajagopal
2,471 Points

What is EventEmitter?

I'm looking at profile.js and I am trying to figure out what EventEmitter is used for. To me, it just looks like another way to do console.log, but can someone please clarify?

Thanks. Here is my code for reference:

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

/**

  • An EventEmitter to get a Treehouse students profile.
  • @param username

  • @constructor */ function Profile(username) {

    EventEmitter.call(this);

    var profileEmitter = this; var url = 'https://teamtreehouse.com/'+ username + '.json';

    var request = https.get(url, function(response) { var body = ""; // aggregates the chunks if (response.statusCode !== 200) { //Status Code Error request.abort(); profileEmitter.emit("error", new Error("There was an error getting the profile for " + username + ". (" + http.STATUS_CODES[response.statusCode] + ")")); } //Read the data response.on('data', function (chunk) { body += chunk; // profileEmitter.emit("data", chunk); }); response.on('end', function () { if(response.statusCode === 200) { try { var profile = JSON.parse(body); //Parse the data profileEmitter.emit("end", profile); } catch (error) { profileEmitter.emit("error", error); } } }).on("error", function(error){ profileEmitter.emit("error", error); }); }); }

util.inherits( Profile, EventEmitter );

module.exports = Profile;

1 Answer

Thomas Nilsen
Thomas Nilsen
14,957 Points
Thomas Nilsen
Thomas Nilsen
14,957 Points

Here is a fairly simple implementation of an EventEmitter:

function EventEmitter() {
    //An object containing all the different events
    this.events = {};
}

EventEmitter.prototype.on = function(type, callback) {
    //We check if the key exists - 
    //If not we create it, and set it equal to an empty array
    this.events[type] = this.events[type] || [];

    //Add our callback to the array
    this.events[type].push(callback);   
}

EventEmitter.prototype.emit = function(type) {
    //Does the key exists
    if(this.events[type]) {
        //Loops over the array and call every function that is stored in it. 
        this.events[type].forEach(function(func) {
            func();
        });
    }
}

var e = new EventEmitter();

//Here we create an event to look out for, and add the function that will be called
e.on('entry', function() {
    console.log('hello');
});


//This outputs 'hello'
e.emit('entry');

Basically you can use it to communicate different events across your app, and react accordingly.