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The JavaScript this Keyword

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.

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>

Output

The JavaScript this Keyword

In this example, this refers to the person object.

Because fullName is a method of the person object.

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>

Output

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>

Output

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>

Output

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>

Output

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>

Output

The JavaScript this Keyword

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>

Output

The JavaScript this Keyword

In this example, this refers to the person object.

[object 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:

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>

Output

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() {

turn this.firstName + ” ” + this.lastName;

  }

}

const member = {

  firstName:”web”,

  lastName: “host”,

}

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

document.getElementById(“demo”).innerHTML = fullName();

</script>

</body>

</html>

Output

JavaScript Function bind()

This example creates 2 objects (person and member).

The member object borrows the fullname method from person:

web host


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