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JavaScript Basic

JSHome

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JS Versions

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JS Objects

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JS Funtions

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JS Classes

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JS Async

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JS HTML DOM

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JS Browser BOM

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JS Web APIs

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JS AJAX

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JS JSON

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JS vs jQuery

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JS Graphics

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Text lesson

Function call

Method Reuse

Using the call() method, you can write a function that can be applied to different objects by specifying a different this value for each call.

All Functions are Methods

In JavaScript, all functions are considered object methods.

If a function is not a method of a specific object, it becomes a function of the global object (as explained in the previous chapter).

The example below creates an object with three properties: firstName, lastName, and fullName.

Example

const person = {
  firstName:“John”,
  lastName: “Doe”,
fullName: function () {
    return this.firstName + ” “ + this.lastName;
  }
}

// This will return “John Doe”:
person.fullName();

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

this.firstName refers to the firstName property of this.

This is equivalent to:

this.firstName referring to the firstName property of person.

What is this?

In JavaScript, the this keyword refers to an object.

The object that this refers to depends on how it is used.

In an object method, this refers to the object that owns the method.

When used alone, this refers to the global object.

In a function, this refers to the global object.

In strict mode, this is undefined inside a function.

In an event handler, this refers to the element that triggered the event.

Methods like call(), apply(), and bind() allow you to set this to any object.

Note:

this is not a variable; it is a keyword. You cannot modify its value.d

The JavaScript call() Method

The call() method is a built-in JavaScript method.

It allows you to invoke (call) a method, with the owner object passed as an argument (parameter).

Using call(), an object can invoke a method that belongs to another object.

This example invokes the fullName method of person, applying it to person1.

Example

const person = {
  fullName: function() {
    return this.firstName + ” “ + this.lastName;
  }
}
const person1 = {
  firstName:“John”,
lastName: “Doe”
}
const person2 = {
firstName:“Mary”,
lastName: “Doe”
}

// This will return “John Doe”:
person.fullName.call(person1);

This example invokes the fullName method of person, applying it to person2.

Example

The call() Method with Arguments

The call() method can take arguments:

Example

const person = {
fullName: function(city, country) {
    return this.firstName + ” “ + this.lastName + “,” + city + “,” + country;
}
}

const person1 = {
  firstName:“John”,
  lastName: “Doe”
}

person.fullName.call(person1, “Oslo”, “Norway”);