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[Javascript, IMO] The `this` keyword

Oh, jeez - this is going to get confusing

(See what I did there?)

We're talking about the this keyword, and it's going to be very confusing. To keep the terms straight, when talking about the keyword I will be using this, in the monospaced, code style. Any other time, the word will just be used like normal. Hopefully, this will help us keep this straight. Alright, everyone good? Let's jump on in!

What is this?

The this keyword is one of the most difficult parts of Javascript. It allows you to access the to the current context. Outside of a function, this refers to the global scope. Inside a function, the this keyword can refer to many different things. By default, a global function will have a this keyword equal to the window function. By default in strict mode, the this keyword will be undefined when called a global function.

console.log(this === window); // true

// strict mode
console.log(this === undefined); // true

Also by default, a functions this binding is that of the scope above it.

function f1() {
return this;
}

console.log(f1() === window); // true

function f1() {
'use strict';
return this;
}

console.log(f1() === undefined); // true

So now we know what this is by default.

How does "this" change?

There are many functions that can change the binding of this in Javascript. Using the .call, .apply and .bind methods on a function, you can change the this of a function to almost anything.

When to use "this"?