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C # tutorial
In JavaScript, the this keyword refers to an object
In JavaScript, the this
keyword refers to an object.
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
is not a variable. It is a keyword. You cannot change the value of this
.
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;
}
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;
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;
}
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;
}
In HTML event handlers, this
refers to the HTML element that received the
event:
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).
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);
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);
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 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.