Description

Global context

In the global execution context (outside of any function), this refers to the global object whether in strict mode or not. 

// In web browsers, the window object is also the global object:
console.log(this === window); // true

a = 37;
console.log(window.a); // 37

this.b = "MDN";
console.log(window.b)  // "MDN"
console.log(b)         // "MDN"

Note: You can always easily get the global object using the global globalThis property, regardless of the current context in which your code is running.


Function context

Inside a function, the value of this depends on how the function is called.

Since the following code is not in strict mode, and because the value of this is not set by the call, this will default to the global object, which is window in a browser. 

function f1() {
  return this;
}

// In a browser:
f1() === window; // true

// In Node:
f1() === globalThis; // true

In strict mode, however, if the value of this is not set when entering an execution context, it remains as undefined, as shown in the following example: 

function f2() {
  'use strict'; // see strict mode
  return this;
}

f2() === undefined; // true

Note: In the second example, this should be undefined, because f2 was called directly and not as a method or property of an object (e.g. window.f2()). This feature wasn't implemented in some browsers when they first started to support strict mode. As a result, they incorrectly returned the window object.

To set the value of this to a particular value when calling a function, use call(), or apply() as in the examples below.


Class context

The behavior of this in classes and functions is similar, since classes are functions under the hood. But there are some differences and caveats.

Within a class constructor, this is a regular object. All non-static methods within the class are added to the prototype of this: 

class Example {
  constructor() {
    const proto = Object.getPrototypeOf(this);
    console.log(Object.getOwnPropertyNames(proto));
  }
  first(){}
  second(){}
  static third(){}
}

new Example(); // ['constructor', 'first', 'second']

Note: Static methods are not properties of this. They are properties of the class itself.


Derived classes

Unlike base class constructors, derived constructors have no initial this binding. Calling super() creates a this binding within the constructor and essentially has the effect of evaluating the following line of code, where Base is the inherited class: 

this = new Base();

Warning: Referring to this before calling super() will throw an error.

Derived classes must not return before calling super(), unless they return an Object or have no constructor at all. 

class Base {}
class Good extends Base {}
class AlsoGood extends Base {
  constructor() {
    return {a: 5};
  }
}
class Bad extends Base {
  constructor() {}
}

new Good();
new AlsoGood();
new Bad(); // ReferenceError