this in JavaScript

this is a keyword that has different values depending on where it’s used.

Not knowing this tiny detail of JavaScript can cause a lot of headaches, so it’s worth taking 5 minutes to learn all the tricks.

this in strict mode

Outside any object, this in strict mode is always undefined.

Notice I mentioned strict mode. If strict mode is disabled (the default state if you don’t explicitly add 'use strict' on top of your file ), you are in the so-called sloppy mode, and this - unless some specific cases mentioned here below - has the value of the global object.

Which means window in a browser context.

this in methods

A method is a function attached to an object.

You can see it in various forms.

Here’s one:

const car = {
  maker: 'Ford',
  model: 'Fiesta',

  drive() {
    console.log(`Driving a ${this.maker} ${this.model} car!`)
  }
}

car.drive()
//Driving a Ford Fiesta car!

In this case, using a regular function, this is automatically bound to the object.

Note: the above method declaration is the same as drive: function() {…, but shorter:

const car = {
  maker: 'Ford',
  model: 'Fiesta',

  drive: function() {
    console.log(`Driving a ${this.maker} ${this.model} car!`)
  }
}

The same works in this example:

const car = {
  maker: 'Ford',
  model: 'Fiesta'
}

car.drive = function() {
  console.log(`Driving a ${this.maker} ${this.model} car!`)
}

car.drive()
//Driving a Ford Fiesta car!

An arrow function does not work in the same way, as it’s lexically bound:

const car = {
  maker: 'Ford',
  model: 'Fiesta',

  drive: () => {
    console.log(`Driving a ${this.maker} ${this.