The JavaScript this Keyword
Example
const person = {
firstName: "John",
lastName : "Doe",
id : 5566,
fullName : function() {
return this.firstName + " " + this.lastName;
}
};
Try it Yourself »
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.
fullName : function() {
return this.firstName + " " + this.lastName;
}
Try it Yourself »
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]:
In strict mode, when used alone, this also refers to the global object:
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]:
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.
this in Event Handlers
In HTML event handlers, this refers to the HTML element that received the
event:
Object Method Binding
In these examples, this is the person object:
Example
const person = {
firstName : "John",
lastName : "Doe",
id : 5566,
myFunction : function() {
return this;
}
};
Try it Yourself »
Example
const person = {
firstName: "John",
lastName : "Doe",
id : 5566,
fullName : function() {
return this.firstName + " " +
this.lastName;
}
};
Try it Yourself »
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:
Example
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:
Example
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);
Try it Yourself »
This Precedence
To determine which object this refers to; use the following precedence of order.
| Precedence | Object |
| 1 | bind() |
| 2 | apply() and call() |
| 3 | Object method |
| 4 | Global scope |
Is this in a function being called using bind()?
Is this in a function being called using apply()?
Is this in a function being called using call()?
Is this in an object function (method)?
Is this in a function in the global scope.
