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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 ❯


                                const myObject = {<br />
  firstName:"John",<br />
  lastName: "Doe",<br />
  fullName: function () {<br />
    return this.firstName + " " + this.lastName;<br />
  }<br />
}<br />
<br />
// This will return "John Doe":<br />
myObject.fullName();

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.

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 person2:

EXAMPLE ❯


                                const person = {<br />
  fullName: function() {<br />
    return this.firstName + " " + this.lastName;<br />
  }<br />
}<br />
const person1 = {<br />
  firstName:"John",<br />
  lastName: "Doe"<br />
}<br />
const person2 = {<br />
  firstName:"Mary",<br />
  lastName: "Doe"<br />
}<br />
<br />
// This will return "Mary Doe"<br />
person.fullName.call(person2);

The call() Method with Arguments

The call() method can accept arguments:

EXAMPLE ❯


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

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