Value of "this" in a JS constructor function returns "undefined"

Question

So i have a constructor function

    var APP = function(name){

        this.appName = name

    }

And a prototype function

    APP.prototype.test = function(){

        console.log(this.appName)

    }

Then i create a new APP() and try out the test function.

    var app = new APP("ieps")
    var testing = app.test

    console.log(app.test()) // returns "ieps"
    console.log(testing()) // returns undefined

Why is it that testing() is returning undefined? testing() should do the same thing as app.test() since i'm just referencing app.test.

The value of this as well as a lot of other things is explained here: stackoverflow.com/questions/16063394/… — HMR
– HMR, Commented Nov 28, 2014 at 0:37

Renato Gama · Accepted Answer · 2014-11-27 19:32:50Z

2

You would have to bind the object to the function

var app = new APP("ieps");
var testing = app.test.bind(app);

console.log(testing());

http://jsbin.com/kiyiyutili/2/edit

EDIT: From the MDN docs for .bind:

"The bind() method creates a new function that, when called, has its this keyword set to the provided value, with a given sequence of arguments preceding any provided when the new function is called."

edited Nov 27, 2014 at 19:32

answered Nov 27, 2014 at 19:18

Renato Gama

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2 Comments

kevinius Over a year ago

Can you explain 'why' you need to bind it?

Renato Gama Over a year ago

Have a look at the first example from the MDN docs: " A common mistake for new JavaScript programmers is to extract a method from an object, then to later call that function and expect it to use the original object as its this (e.g. by using that method in callback-based code). Without special care, however, the original object is usually lost."

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