Author: MDBootstrap
var person = {
firstName: "John",
lastName: "Doe",
id: 5566,
fullName: function () {
// "this" keyword refers to the object it belongs to - in this example it's "person" object
return this.firstName + " " + this.lastName;
}
};
console.log(person.fullName()); // John Doe
What is this ?
The JavaScript this keyword refers to the object it belongs
to.
It has different values depending on where it is used:
- In a method,
thisrefers to the owner object. - Alone,
thisrefers to the global object. - In a function,
thisrefers to the global object. - In a function, in strict mode,
thisisundefined. - In an event,
thisrefers to the element that received the event. - Methods like
call(), andapply()can referthisto any object.
this in a Method
Note: Methods are functions stored as object properties.
In an object method, this refers to the "owner" of the
method.
In the example on the top of this page, this refers to the person
object.
The person object is the owner of the fullName method.
fullName : function() {
return this.firstName + " " + this.lastName;
}
this Alone
When used alone, the owner is the Global object, so this refers to
the Global object.
In a browser window the Global object is [object Window]:
var x = this; // Now value of x is [object Window]
In strict mode, when used alone, this also refers to the Global object
[object Window]:
"use strict";
var x = this; // Now value of x is [object Window]
this in a Function (Default)
In a JavaScript function, the owner of the function is the default binding
for this.
So, in a function, this refers to the Global object [object Window].
function myFunction() {
return this; // [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.
"use strict";
function myFunction() {
return this; // undefined
}
this in Event Handlers
In HTML event handlers, this refers to the HTML element that received the
event:
<button onclick="this.style.display='none'">
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</button>
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Object Method Binding
In the example below, this is the person object
(The person object is the "owner" of the function):
var person = {
firstName: "John",
lastName : "Doe",
id : 5566,
fullName : function() {
return this.firstName + " " + this.lastName;
}
};
In other words: this.firstName means the firstName property of this (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.
Note: You wii learn more about call() and apply()
later in this tutorial.
In the example below, when calling person1.fullName with person2 as argument, this
will refer to person2, even if it is a method of person1:
var person1 = {
fullName: function() {
return this.firstName + " " + this.lastName;
}
}
var person2 = {
firstName:"John",
lastName: "Doe",
}
person1.fullName.call(person2); // Will return "John Doe"
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