The JavaScript this Keyword

By | August 27, 2022
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Example:

<!DOCTYPE html>
<html>
<body>
<h1>The JavaScript <i>this</i> Keyword</h1>
<p>In this example, <b>this</b> refers to the <b>person</b> object.</p>
<p>Because <b>fullName</b> is a method of the person object.</p>
<p id="demo"></p>
<script>
// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  id: 5566,
  fullName : function() {
    return this.firstName + " " + this.lastName;
  }
};
// Display data from the object:
document.getElementById("demo").innerHTML = person.fullName();
</script>
​</body>
</html>

Result:

The JavaScript this Keyword

In this example, this refers to the person object.

Because fullName is a method of the person object.

John Doe

What is this?

In JavaScript, the this keyword refers to an object.

Which object depends on how this is being invoked (used or called).

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

In an object method, this refers to the object.
Alone, this refers to the global object.
In a function, this refers to the global object.
In a function, in strict mode, this is undefined.
In an event, this refers to the element that received the event.
Methods like call(), apply(), and bind() can refer this to any object.

Note

this is not a variable. It is a keyword. You cannot change the value of this.

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.

Example:

<!DOCTYPE html>
<html>
<body>
<h1>The JavaScript <i>this</i> Keyword</h1>
<p>In this example, <b>this</b> refers to the <b>person</b> object.</p>
<p>Because <b>fullName</b> is a method of the person object.</p>
<p id="demo"></p>
<script>
// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  id: 5566,
  fullName : function() {
    return this.firstName + " " + this.lastName;
  }
};
// Display data from the object:
document.getElementById("demo").innerHTML = person.fullName();
</script>
​</body>
</html>

Result:

The JavaScript this Keyword

In this example, this refers to the person object.

Because fullName is a method of the person object.

John Doe

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]:

Example

<!DOCTYPE html>
<html>
<body>
<h1>The JavaScript <i>this</i> Keyword</h1>
<p>In this example, <b>this</b> refers to the window object:</p>
<p id="demo"></p>
​<script>
let x = this;
document.getElementById("demo").innerHTML = x;
</script>
</body>
</html>

Result:

The JavaScript this Keyword

In this example, this refers to the window object:

[object Window]

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

Example

<!DOCTYPE html>
<html>
<body>
<h1>The JavaScript <i>this</i> Keyword</h1>
<p>In this example, <b>this</b> refers to the window object:</p>
<p id="demo"></p>​
<script>
"use strict";
let x = this;
document.getElementById("demo").innerHTML = x;
</script>
</body>
</html>

Result:

The JavaScript this Keyword

In this example, this refers to the window object:

[object Window]

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]:

Example

<!DOCTYPE html>
<html>
<body>
<h1>The JavaScript <i>this</i> Keyword</h1>
<p>In this example, <b>this</b> refers to the the window object:</p>
<p id="demo"></p>
<script>
document.getElementById("demo").innerHTML = myFunction();
​function myFunction() {
  return this;
}
</script>
​</body>
</html>

Result:

The JavaScript this Keyword

In this example, this refers to the the window object:

[object Window]

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.

Example

<!DOCTYPE html>
<html>
<body>
<h1>The JavaScript <i>this</i> Keyword</h1>
<p>In a function, by default, <b>this</b> refers to the global object.</p>
<p>Strict mode does not allow default binding, so <b>this</b> is:</p>
<p id="demo"></p>
<script>
"use strict";
document.getElementById("demo").innerHTML = myFunction();
​function myFunction() {
  return this;
}
</script>
​</body>
</html>

Result:

The JavaScript this Keyword

In a function, by default, this refers to the global object.

Strict mode does not allow default binding, so this is:

undefined

this in Event Handlers

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

Example

<!DOCTYPE html>
<html>
<body>
<h1>The JavaScript <i>this</i> Keyword</h1>
​<button onclick="this.style.display='none'">Click to Remove Me!</button>
​</body>
</html>

Result:

Result Shown in your browser.

Object Method Binding

In these examples, this is the person object:

Example

<!DOCTYPE html>
<html>
<body>
<h1>The JavaScript <i>this</i> Keyword</h1>
<p>In this example, <b>this</b> refers to the <b>person object</b>.</p>
<p id="demo"></p>
<script>
// Create an object:
const person = {
  firstName  : "John",
  lastName   : "Doe",
  id     : 5566,
  myFunction : function() {
    return this;
  }
};
// Display data from the object:
document.getElementById("demo").innerHTML = person.myFunction();
</script>
​</body>
</html>

Result:

The JavaScript this Keyword

In this example, this refers to the person object.

[object Object]

Example

<!DOCTYPE html>
<html>
<body>
<h1>The JavaScript <i>this</i> Keyword</h1>
<p>In this example, <b>this</b> refers to the <b>person</b> object.</p>
<p>Because <b>fullName</b> is a method of the person object.</p>
<p id="demo"></p>
<script>
// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  id: 5566,
  fullName : function() {
    return this.firstName + " " + this.lastName;
  }
};
// Display data from the object:
document.getElementById("demo").innerHTML = person.fullName();
</script>
</body>
</html>

Result:

The JavaScript this Keyword

In this example, this refers to the person object.

Because fullName is a method of the person object.

John Doe

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.

See Also:

The Function call() Method

The Function apply() Method

The Function bind() Method

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

Example

<!DOCTYPE html>
<html>
<body>
<h1>The JavaScript <i>this</i> Keyword</h1>
<p>In this example <strong>this</strong> refers to person2, even if it is a method of person1:</p>
<p id="demo"></p>
<script>
const person1 = {
  fullName: function() {
    return this.firstName + " " + this.lastName;
  }
}
​const person2 = {
  firstName:"John",
  lastName: "Doe",
}
let x = person1.fullName.call(person2); 
document.getElementById("demo").innerHTML = x; 
</script>
</body>
</html>​

Result:

The JavaScript this Keyword

In this example this refers to person2, even if it is a method of person1:

John Doe

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:

Example:

<!DOCTYPE html>
<html>
<body>
<h1>JavaScript Function bind()</h1>
<p>This example creates 2 objects (person and member).</p>
<p>The member object borrows the fullname method from person:</p> 
<p id="demo"></p>
<script>
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);
​document.getElementById("demo").innerHTML = fullName();
</script>
</body>
</html>

Result:

JavaScript Function bind()

This example creates 2 objects (person and member).

The member object borrows the fullname method from person:

Hege Nilsen

This Precedence

To determine which object this refers to; Use the following precedence of order.

PrecedenceObject
1bind()
2apply() and call()
3Object method
4Global scope

Is this in a function being called using bind()?

Is this in a function is being called using apply()?

Is this in a function is being called using call()?

Is this in an object function (method)?

Is this in a function in the global scope.

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