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JavaScript Function Call


Method Reuse

With the call() method, you can write a method that can be used on different objects.


All Functions are Methods

In JavaScript all functions are object methods.

If a function is not a method of a JavaScript object, it is a function of the global object (see previous chapter).

The example below creates an object with 3 properties, firstName, lastName, fullName.

Example

var person = {
    firstName:"John",
    lastName: "Doe",
    fullName: function () {
        return this.firstName + " " + this.lastName;
    }
}
person.fullName();         // Will return "John Doe"
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The this Keyword

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

In the example above, this is the person object that "owns" the fullName function.

In other words, this.firstName means the firstName property of this object.

Read more about the this keyword at JS this Keyword.



The JavaScript call() Method

The call() method is a predefined JavaScript method.

It can be used to invoke (call) a method with an owner object as an argument (parameter).

With call(), an object can use a method belonging to another object.

This example calls the fullName method of person, using it on person1:

Example

var person = {
    fullName: function() {
        return this.firstName + " " + this.lastName;
    }
}
var person1 = {
    firstName:"John",
    lastName: "Doe",
}
var person2 = {
    firstName:"Mary",
    lastName: "Doe",
}
person.fullName.call(person1);  // Will return "John Doe"

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This example calls the fullName method of person, using it on person2:

Example

var person = {
    fullName: function() {
        return this.firstName + " " + this.lastName;
    }
}
var person1 = {
    firstName:"John",
    lastName: "Doe",
}
var person2 = {
    firstName:"Mary",
    lastName: "Doe",
}
person.fullName.call(person2);  // Will return "Mary Doe"

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The call() Method with Arguments

The call() method can accept arguments:

Example

var person = {
    fullName: function(city, country) {
        return this.firstName + " " + this.lastName + "," + city + "," + country;
    }
}
var person1 = {
    firstName:"John",
    lastName: "Doe",
}
person.fullName.call(person1, "Oslo", "Norway");

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