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Blog et astuces d'un développeur freelance full-stack, Ruby, Rails et frameworks Javascript

About this and the lost of the context

As a javascript developer you will at one time or another find the common problem where you have something undefined because the this refers to the global window object.

Let see an example:

    var MyObject = {
     name: "Loic",
     hello: function() {
     alert("Hello I'm " %2B this.name);
     getName: function () {
      return this.name;

    MyObject.hello();       // alert "Hello I'm Loic"

    var hello = MyObject.hello;
    hello();   // alert "Hello I'm "

    setTimeout(MyObject.hello, 1000); // alert "Hello I'm "

So what we see is that the this.name is not known in the last two run. Why that? This is because this refers to the context the function is run on, it is determined at the time the function is executed.

So when you call a function using MyObject.hello(), ‘this’ is the MyObject object.

MyObject.hello();       // this is directly the MyObject

But you loose the context of the original object if you take a reference to the function.

    var hello = MyObject.hello;
    hello();   // hello is a reference to the function so it will loose the original context

    setTimeout(MyObject.hello, 1000); // inside the setTimout function, the function is a reference variable too

The solution is to manually tell javascript that we want the this to refer to a particular object.

    var hello = MyObject.hello;
    hello.apply(MyObject);   // "apply" run the function at his left with the this referring to his first argument.

    setTimeout(MyObject.hello.bind(MyObject), 1000); // "bind" returns the function but during execution, it will apply the context of the object passed in his first parameter.

I know about this and I usually correct the error directly when it occurs, but I had to write it down to be able to explain it and put it in words. Now that this is explained, I shouldn’t wait for the error to come up and fix but see directly when I will loose the context and fix it straight away.

Loic Ginoux

Développeur freelance full-stack avec 9 ans d'expérience, spécialisé sur les applications web, e-commerce, Ruby on Rails et frameworks Javascript tels que React ou Angular.

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