this.emit is not working but self.emit is working. why?

Question

The following node.js script is not working

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

var TickE = function() {    
}

util.inherits(TickE, EventEmitter); //TickE.prototype.__proto__ = EventEmitter.prototype;

TickE.prototype.ticker = function() {
    var self = this;
    setInterval (function () {
         self.emit('tick');      
    }, 1000);
};

var t = new TickE ();

//console.log (util.inspect(t));

t.on('tick', function() { console.log ('Tick...');});

t.ticker();

It is not working if I call the emit method like below

TickE.prototype.ticker = function() {
    //var self = this; // commented this line
    setInterval (function () {
         this.emit('tick'); // using this in place of self
    }, 1000);
};

self is just a variable holding reference of this and why this is throwing error ?

This has been asked so very often before. Please search for `this` and `javascript` on Google. You should get *tons* of results. — Golo Roden, May 03 '13 at 13:04
@Golo: that might be the problem - can you suggest a good and *correct* reading? — Bergi, May 03 '13 at 13:08
E.g., see http://stackoverflow.com/a/134149/1333873 ... section "As a function" — Golo Roden, May 03 '13 at 13:15
you can solve this problem by using new ES2015 arrow functions. — Kishore Relangi, Nov 23 '16 at 10:54

score 6 · Answer 1 · answered May 03 '13 at 13:05

Because the this keyword has a different value in the function that is invoked by setInterval.

You already know the solution with the self variable in a closure, a different (and shorter) solution would be binding the emit method:

setInterval(this.emit.bind(this, "tick"), 1000);

this.emit is not working but self.emit is working. why?

1 Answers1