JavaScript 'this' illustrated: Learn it once and for all!

#javascript #tutorial #webdev #codenewbie

Hey There 👋🏾

In today's post I wanted to ask you all a question.

How many of you really understand the this keyword in JavaScript? 🤔

I myself have conducted several JavaScript interviews so far and sometimes I ask this question as a way to get things started and the kind of answers that I get are:

It points to the Object (what object?)
It points to the function (which function?)
It points to itself (consciousness?)

Although there is a trend growing up which considers the need to use this is JavaScript as a bad practice and suggests to move away from it.

That might be correct, but knowing about how something works is always good, whether we decide to use it or not!

So let's do it today! 🙌🏾

Let us see how the this keyword in JavaScript really functions.

The actual answer

Well, most of the times the answer that we provide to that question is wrong, because the way the this keyword works, differs based on where the this keyword is being used.

And that is the key to understanding it. Break it down into scenarios and look at it from a per scenario basis.

Inside a function

Inside any function in the global scope, the this keyword points to the global object in non-strict mode and is undefined in strict mode.

As a method on an object

When invoked as a method on an object, like person.getName(), the this keyword refers to the object on which the method is being invoked.

When invoking with call

When the function is invoked using call, like getName.call(animal), the this keyword refers to the object that is being passed to the call function.

When invoked with the `new` keyword

In this case, when we are in the process of creating a new object by invoking a function with the new keyword in front of it, like this:

let bruno = new Dog();

Then inside the Dog function, all the this keywords would point to the object that gets newly allocated and returned.

Here's the same explanation in an illustrated format for the visual learners reading this post:

And that's it. Those are the cases that would cover more than 95 percent of the scenarios.

Now you know how to answer that tricky question.

See you in the next one, Cheers! 🙌🏾

PS:
If you liked the illustration above, you might like my 🎊FREE🎊 ebook that I recently released which covers several other JavaScript concepts. Feel free to get a copy by clicking on the image below:

Top comments (7)

Robin Schambach • Aug 15 '21

Thanks for sharing this however I think you really miss a lot of use cases. Your sketch does not cover 95% of use cases. Not even 30%.
Things you might consider to add

difference between ES5 functions and arrow functions
js bind function (even tho you mentioned call)
this in event listeners -> this can be the element in the Dom (you could consider the 2nd example but I think that's not really clear)
difference between this in ES5 like classes and ES6 classes
special use cases in nested closure calls (the reason why many use a self variable, I know you could combine the already known logic but in a guide you should address and combine multiple rules)