The ‘this' keyword in Javascript

The keyword this in Javascript can be a little slippery to wrap your had around at first.

TL;DR When you refer to this in the global context, it refers to the global variable (which is Window in browsers). When you create a new object with this refers to the object's scope.

What Does this Do?

When you call this on the root of a document, it refers to the document's root object, in the case of our browser this refers to Window.

So if we create a function and add a variable to the global scope then we can call it using this

At first hello() returns undefined because saySomething is undefined on window, but once saySomething is defined we get a proper greeting.

So what do we do if we want this to refer to something other than the global scope?

Our first approach is to use call or apply to specify the scope.

This in Objects

Now let's try this in a different way. Let's see what happens when we use this in a function that's been set as the property of an object. In this case, this will refer to the local scope of the adventure object.

In the above case we created the example in an object literal, but what happens when we wrap the example in a constructor function? When we instantiate it with the new keyword, an empty object is created and this will refer to that context.

You can instantiate multiple versions of our Adventure class and this will refer to the local scope of each instance.

Getting Tricky with 'this'

It's been simple enough to follow the context of ‘this' up to this point, but here's where it can start to get tricky. Take the following example:

function Adventure() {
  console.log("Your this is: ", this)
  this.hasAdventure = false
  this.waterslide = function(numGoSlide) {
    if(numGoSlide > 0) {
    this.hasAdventure = true
    celebrate()
    }
  }
}
function celebrate() { 
  console.log("Your this is: ", this)
  if(this.hasAdventure) {
    alert("Yay, you've been on a soggy and thrilling adventure!")
  }
}

You might assume that we'd get the alert here, but when run we don't get much of anything. The reason is that celebrate() was defined on the global scope and so its this refers to the global scope. We can see that when we run this in the console, we're referring to the global scope, global scope doesn't have the attribute hasAdventure, so we don't get the alert.

To fix this conundrum, we can pass Adventure's local this into celebrate and modify celebrate to take an argument. When we update celebrate to take an argument you can't use the reserved word this, so pick another variable to hold that place.

function Adventure() {
  console.log("Your this is: ", this)
  this.hasAdventure = false
  this.waterslide = function(numGoSlide) {
    if(numGoSlide > 0) {
    this.hasAdventure = true
    celebrate(this)
    }
  }
}
function celebrate(you) { 
  if(you.hasAdventure) {
    alert("Yay, you've been on a soggy and thrilling adventure!")
  }
}

Now when we call the function we get the alert!

In conclusion, this is a powerful tool that helps developers easily refer to properties within specific contexts, but can get tricky when passing through different levels of scope.

Good Luck & Godspeed.

Comments

[Madara Uchiha]

You didn't really touch the tricky parts of this. Like for example, what happens when you try to pass around a function that refers to this:

const o = {
  f: function() { console.log(this); }
};

o.f(); // prints o, no problems!

const g = o.f; // passed around the function instance, this could be passing to an event handler or some thing

g(); // prints window, oops. 

The main problem with this isn't that you need to pass it around in a parameter or that you might accidentally define things outside the scope.

The main problem with this is that this is determine when the function is called and how the function is called, in the example here, this changed because I no longer called f() with o behind the dot operator. So it defaults to the global object.

[Guid75]

And then, for a most advanced part, one could talk about arrow functions special rules and the bindfunction.

[Wesley Handy]

Helpful! I tutor students learning to code and will save 'this' 😉 post and use liberally. Well-done and fun to read!

[The Grumpy Scot]

Excellent! Cheers :)

[Necmettin Begiter]

This doesn't make any sense. The comments have hearts, but the content doesn't. (See what I did there?)

[Nonso Biose]

Nice one Rachel. Just incase anyone is also curious about the "This" keyword in java,

why not check the link...... medium.com/java-for-absolute-dummi...

[Eoin Murphy]

Thanks! Your article actually cleared this topic right up for me.

[John Askew]

The most important thing to understand about this is that it refers to the caller from the call stack. Not to be confused with lexical scope.