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11.4 JS 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 1: JS Functions - All Functions are Methods

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 means the firstName property of this.
Same as:
this.firstName means the firstName property of person.

What is this?

In JavaScript, the this keyword refers to an object.
Which object depends on how 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.

Note
this is not a variable. It is a keyword. You cannot change the value of this.

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 2: JS Functions - The JavaScript call() Method part 1

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

Example 3: JS Functions - The JavaScript call() Method part 2

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

// This will return "Mary Doe"
person.fullName.call(person2);

The call() Method with Arguments

The call() method can accept arguments:

Example 4: JS Functions - The JavaScript call() Method with Arguments

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");

JavaScript Fundamentals course home
Module 11. JS Functions

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Example files created in this module:
JS Functions - Function Declaration JS Functions - Function Expressions part 1 JS Functions - Function Expressions part 2 JS Functions - The Function() Constructor part 1 JS Functions - The Function() Constructor part 2 JS Functions - Self-Invoking Functions JS Functions - Functions Can Be Used as Values part 1 JS Functions - Functions Can Be Used as Values part 2 JS Functions are Objects part 1 JS Functions are Objects part 2 JS Arrow Functions part 1 JS Arrow Functions part 2 JS Default Parameters JS Functions - Default Parameter Values JS functions - The Rest Parameter JS Functions - Finding the largest number JS Functions - Sum of all arguments JS Functions - Invoking a Function as a Function part 1 JS Functions - Invoking a Function as a Function part 2 JS Functions - The Global Object JS Functions - Invoking a Function as a Method part 1 JS Functions - Invoking a Function as a Method part 2 JS Functions - Invoking a Function with a Function Constructor JS Functions - All Functions are Methods JS Functions - The JavaScript call() Method part 1 JS Functions - The JavaScript call() Method part 2 JS Functions - The JavaScript call() Method with Arguments JS Functions - The JavaScript apply() Method JS Functions - The apply() Method with Arguments JS Functions - The call() Method with Arguments JS Math.max() part 1 JS Math.max() part 2 JS Math.max() part 3 JS Math.max() part 4 JS Math.max() part 5 JS Function bind() part 1 JS Function bind() part 2 JS Function bind() part 3 JS Function bind() part 4 JS Functions - Global Variables part 1 JS Functions - Global Variables part 2 JS Functions - Global Variables part 3 JS Function Closures - Counter Dilemma part 1 JS Function Closures - Counter Dilemma part 2 JS Function Closures - Counter Dilemma part 3 JS Function Closures - Nested Functions JS Closures