this in an Object Method
Example
const
person = {
firstName: "John", lastName : "Doe", id : 5566, fullName : function() {
return this .firstName + " " + this .lastName;
}
};When used in an object method, this refers to the object .
In the example above, this refers to the person object .
Because the fullName method is a method of the person object.
Object Method Binding
In these examples, this is the person object :
Example
const
person = {
firstName : "John", lastName : "Doe", id : 5566, myFunction : function() {
return this ;
}
};Example
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).
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
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);What is this ?
In JavaScript, the this keyword refers to an object .
The this keyword refers to different objects depending on how it is used:
| 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 object method, this refers to the object . |
| In an event, this refers to the element that received the event. |
| In methods like call() , apply() and bind() , this can refer to any object . |
this is not a variable.
this is a keyword.
You cannot change the value of this .
This Precedence
Use the following precedence of order to determine which object this refers to:
| Order | Object | Because |
|---|---|---|
| 1 | bind() | this is in a function being called using bind() |
| 2 | apply() | this is in a function being called using apply() |
| 2 | call() | this is in a function being called using call() |
| 3 | Object method | this is in an object function (method) |
| 4 | Global scope | this is in a function in the global scope |
Advanced Chapters
JavaScript Object Definitions
JavaScript Object Advanced this
JavaScript Object Iterations
JavaScript Object Getters & Setters
JavaScript Object Management
JavaScript Object Protection
JavaScript Object Prototypes
JavaScript Object Reference