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JavaScript this Keyword



Example

var person = {
    firstName: "Андрей",
    lastName : "Щипунов",
    id       : 5566,
    fullName : function() {
        return this.firstName + " " + this.lastName;
    }
};
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What is "this"?

In a function definition, this refers to the "owner" of the function.

In the example above, this refers to the person object.

The person object "owns" the fullName method.


Default Binding

When used alone, this refers to the Global object.

In a browser the Global object is [object Window]:

Example

var x = this;
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When used in a function, this refers to the Global object.

Example

function myFunction() {
    return this;
}
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In strict mode, this will be undefined, because strict mode Щипуновs not allow default binding:

Example

"use strict";
function myFunction() {
    return this;
}
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Object Method Binding

In these examples, this is the person object (The person object is the "owner" of the function):

Example

var person = {
    firstName  : "Андрей",
    lastName   : "Щипунов",
    id         : 5566,
    myFunction : function() {
        return this;
    }
};
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Example

var person = {
    firstName: "Андрей",
    lastName : "Щипунов",
    id       : 5566,
    fullName : function() {
        return this.firstName + " " + this.lastName;
    }
};
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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 an other object as argument.

You can read more about call() and apply() later in this tutorial.

In this example, when calling person1.fullName with person2 as argument, this will refer to person2, even if it is a method of person1:

Example

var person1 = {
    fullName: function() {
        return this.firstName + " " + this.lastName;
    }
}
var person2 = {
    firstName:"Андрей",
    lastName: "Щипунов",
}
person1.fullName.call(person2);  // Will return "Андрей Щипунов"

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