JavaScript this Keyword

const person = {
  firstName: "John",
  lastName : "Doe",
  id       : 5566,
  fullName : function() {
    return this.firstName + " " + this.lastName;
  }
};

What is this?

In JavaScript, the this keyword refers to an object.

The this keyword refers to different objects depending on how it is used:

this in a Method

When used in an object method, this refers to the object.

In the example on top of this page, this refers to the person object.

Because the fullName method is a method of the person object.

fullName : function() {
  return this.firstName + " " + this.lastName;
}

this Alone

When used alone, this refers to the global object.

Because this is running in the global scope.

In a browser window the global object is [object Window]:

let x = this;

 In strict mode, when used alone, this also refers to the global object:

"use strict";
let x = this;

this in a Function (Default)

In a function, the global object is the default binding for this.

In a browser window the global object is [object Window]:

function myFunction() {
  return this;
}

this in a Function (Strict)

JavaScript strict mode does not allow default binding.

So, when used in a function, in strict mode, this is undefined.

"use strict";
function myFunction() {
  return this;
}

this in Event Handlers

In HTML event handlers, this refers to the HTML element that received the event:

<button onclick="this.style.display='none'">
  Click to Remove Me!
</button>

Object Method Binding

In these examples, this is the person object:

const person = {
  firstName  : "John",
  lastName   : "Doe",
  id         : 5566,
  myFunction : function() {
    return this;
  }
};

const person = {
  firstName: "John",
  lastName : "Doe",
  id       : 5566,
  fullName : function() {
    return this.firstName + " " + this.lastName;
  }
};

i.e. this.firstName is the firstName property of this (the person object).

Explicit Function Binding

The call() and apply() methods are predefined JavaScript methods.

They can both be used to call an object method with another object as argument.

The example below calls person1.fullName with person2 as an argument, this refers to person2, even if fullName is a method of person1:

const person1 = {
  fullName: function() {
    return this.firstName + " " + this.lastName;
  }
}

const person2 = {
  firstName:"John",
  lastName: "Doe",
}

// Return "John Doe":
person1.fullName.call(person2);

Function Borrowing

With the bind() method, an object can borrow a method from another object.

This example creates 2 objects (person and member).

The member object borrows the fullname method from the person object:

const person = {
  firstName:"John",
  lastName: "Doe",
  fullName: function () {
    return this.firstName + " " + this.lastName;
  }
}

const member = {
  firstName:"Hege",
  lastName: "Nilsen",
}

let fullName = person.fullName.bind(member);