YCM Jason

Posted on Jun 15, 2018 • Edited on Jun 18, 2018

Let me explain to you what is `this`. (Javascript)

#javascript #beginners

Original post: https://www.ycmjason.com/blog/2018/06/15.html

this article assumes 'use strict' in all context

this article also assumes some knowledge about functions but still a bit confused

this in Javascript is the probably most magical keyword in the programming world. It's unpredictable nature has reached to an unprecedented level.

However, it is essential to understand it fully if you wish to become a master of Javascript. So let me try to explain you what is this. (if it doesn't work, well, at least I tried.)

Functions

Starting with functions. In this article, I would like to put functions into 3 different categories.

Normal functions
Arrow functions
Bound functions

Normal functions

I define normal functions as any function created with...

// function declaration
function magic() {
    ...
}

// function expression
const magic = function() {
    ...
};

// (or if you hate your life)
// function constructor
const magic = new Function('...');

Arrow functions

Arrow functions are basically the ES6 arrow functions:

const magic = () => {
    ...
};

Bound functions

Bound functions can be created by calling Function.prototype.bind on a normal functions.

// magic is created with function declaration/expression/constructor
const bound = magic.bind(...);

Ways to call a function

Now let's say we have a function f (any category). There are 2 ways to call it.

Implicit (direct) calls
Explicit calls

Implicit (direct) calls

Implicit (direct) calls are boring:

/* f is defined */

// direct call
f();

// or attach it to an object and call it
const obj = {};
obj.fn = f;
obj.fn();

Explicit call

Explicit calls are more interesting. You can call your function with Function.prototype.call or Function.prototype.apply.

/* f is defined */

// Function.prototype.call
f.call(...);

// Function.prototype.apply
f.apply(...);

Quick recap

Let's do a quick recap, we have 3 categories of functions:

Normal functions - created with function declaration/expression/constructor
Arrow functions - () => {...}
Bound functions - created with f.bind(...)

And 2 ways to call a function:

Implicit (direct) calls - f() or obj.f()
Explicit calls - f.call(...) or f.apply(...)

This means we have 6 different scenarios.

Normal functions + Implicit (direct) calls
Normal functions + Explicit calls
Arrow functions + Implicit (direct) calls
Arrow functions + Explicit calls
Bound functions + Implicit (direct) calls
Bound functions + Explicit calls

Don't panic, it is not that scary.

In fact, arrow functions and bound functions do not care about implicit/explicit calls. So this reduces down to only 4 scenarios:

Normal functions + Implicit (direct) calls
Normal functions + Explicit calls
Arrow functions
Bound functions

Procedure to find `this`

Below is the procedure to find the binding of this in function f:

Exercises!

Given magic defined as follows:

'use strict';

const magic = function() {
    // a. what is `this`?
    console.log(this);

    const cool = () => {
        // b. what is `this`?
        console.log(this);
    };
    cool();
};

// QUESTION 1
magic();


// QUESTION 2
const apple = { name: 'apple' };
apple.magic = magic;
apple.magic();

// QUESTION 3
const orange = { name: 'orange' };
magic.call(orange);

QUESTION 1.a

Following flow chart, we want to find this in magic.

Category of magic is normal function
magic is called implicitly (directly)
magic is called with magic()
So this = undefined!!!

QUESTION 1.b

Following flow chart, we want to find this in cool.

Category of cool is arrow function
From QUESTION 1.b, we know magic's this is undefined
cool's definer is magic
So this = magic's this = undefined!

Lazy lecturer

The remaining questions, QUESTION 2.a, 2.b, 3.a and 3.b, are trivial with my flow chart. So I will leave them as an exercise for you all.

Answers

https://repl.it/@ycmjason/What-is-this

Click run and you will see the answer in order (1.a, 1.b, 2.a, 2.b, 3.a, 3.b).

Note

There is no "bound arrow function". (() => {...}).bind(...) is still the original arrow function.
For implicit calling, only the shape (f() or obj.f()) matter. It doesn't matter where f comes from. Consider the following code:

const f = obj.f; // obj.f is a normal function
f(); // `this` in the body of `f` is `undefined`!!! not `obj`!!!

Updates:

16 July 2018: Thanks to @joshcheek for reminding me the correct binding of this of arrow functions!

18 June 2018: Thanks to Yong for pointing out typo!

Top comments (29)

Elarcis • Jun 16 '18

this in the body of f is undefined!!! not obj!!!

Why does it happen though, that this code ran on Firefox (and I suppose, any other browser):

function f() { console.log(this); }

f();

const obj = {};
obj.fn = f;
obj.fn();

prints respectively the Window object and the obj one?

YCM Jason • Jun 16 '18 • Edited

Great question. You are getting Window because you are not using 'use strict'.

Go to this link and the section called "Securing JavaScript" explains it.

developer.mozilla.org/en-US/docs/W...

Elarcis • Jun 16 '18

Oops. That actually explains a lot, as I used to get could not read property 'foo' of undefined a lot when passing my functions around as references, in strict mode.

Thank you for the link, I had forgot how much use strict mode changes the game!

YCM Jason • Jun 16 '18 • Edited

It makes code a little bit more reasonable.

Babel has implicit strict mode and u don't have to worry about forgetting to add it. :)

Guillaume Martigny • Jun 16 '18

Without specific definition, this is the object wrapping the function. window is the fallback when can't be infer from any context.

yahyaoui-mohamed • Mar 13 '22

To get undefined as result you have to use the strict mode.

WhistlerIAM • Jun 16 '18

Awesome explanation! The flowchart is by far the most unique thing on the web for this :p

Kat • Jun 19 '18

What this are you referring to? 😁

WhistlerIAM • Jun 20 '18

very droll

YCM Jason • Jun 16 '18

😁😁 that is a huge compliment thanks!

Adrian B.G. • Jun 16 '18

old man voice: And this my children is why you do not want to learn JS as your first language: you have too many (not so obvious) issues, you have to learn too many "JS" specific things, and less computer science/business problems. The result: you spend 90% debugging.

Ofc there are other 1000 reasons why you should learn JS as a first language, just saying :))

YCM Jason • Jun 16 '18

You are absolutely right. I always suggest python as the first language. :)

Adrian B.G. • Jun 16 '18

Is like the difference between jumping into a volcano and entering in a burning building, I think. Presumably the "new dev" use python to learn web dev, he/she also has to learn JS to make one, so is double the overkill.

I usually suggest something simpler like scratch.edu (for programming concepts, non-tech people, or kids), or Java (because the big ecosystem and the fact that OOP is more "natural" for non-tech people), or any strong typed language if you are studying CS, preferable with pointers.

Josh Cheek • Jun 16 '18

This was really good :) In the flow chart, I don't think the arrow function's this is the caller's, I think it's the this at the time of construction. Eg:

$ node -p '({magic: function() { return () => this }}).magic()()'

The this will be bound to our magic object, where the arrow fn was created, not to global, where it was called.

YCM Jason • Jun 16 '18

I have changed "caller" to "definer", which makes more sense.

Thank you so much! :)

YCM Jason • Jun 16 '18

Oh yes you are right, let me update it later. Thanks for the shout! 😁😁😁

Thorsten Hirsch • Jun 16 '18

20 years ago Perl's $_ was probably the most magical keyword in the programming world. :)

Anthony Bouvier • Jun 16 '18

And still easier to grok than JavaScript's this

YCM Jason • Jun 16 '18

I am lucky I was just 3 years old at that time. 😌😌

Kushan Joshi • Jun 16 '18 • Edited

Since you tackled this so well. It would be great if you also added the almighty this inside an object constructor function to the flow chart.

To me more specific I am talking about

function Tree(name) {
  this.name = name;
}

var theTree = new Tree('Redwood');

I understand this is not exactly what your are addressing in your article, but new folks might frequently encounter this (oh the irony when talking about this) in these type of functions.

Cheers 🥂

YCM Jason • Jun 17 '18

I could add this in the note. Not sure where to fit it in the flow chart haha.

kidme • Jun 17 '18

Honestly really good article and explanation of the 'this' keyword after working with javascript for sometime now I still struggle with it. So thank you great work