The reason to use JS .call() method?

Question

I\'m interested what\'s the reason to have call() method in JS. It seems it duplicates usual method of calling this.

For example, I have a code with cal

Answer 1

2017 Update

All functions by way of Function.prototype have the .call method. The reason to use .call() is to specify what the variable "this" refers to.

MDN specifies:

The call() method calls a function with a given this value and arguments provided individually.

Consider the following:

function x() {
    return this;
}

x()

In strict mode x() returns undefined in non strict mode it returns the Global object, Window in a browser context.

Example with .call() we tell it what "this" refers to:

function x() {
    return this;
}

var obj = {
    myName      : 'Robert',
    myLocation  : 'Earth'
}

x.call(obj);

Result: {myName: "Robert", myLocation: "Earth"}. In the above example we are specifying the obj object as the value of this inside the function x()

It can be used to emulate inheritance in OOP.

Example:

var Robert = {
    name: "Robert Rocha",
    age: 12,
    height: "5,1",
    sex: "male",
    describe: function() {
        return "This is me " + this.name + " " + this.age + " " + this.height + " " + this.sex;
    }
};

Lets say that the above is a master object(prototype) and you want to inherit the function describe in another object:

var Richard = {
    name: "Richard Sash",
    age: 25,
    height: "6,4",
    sex: "male",
}

The Richard object does not have the describe function and you want to simply inherit ,so to speak, the function. You would do it like so:

console.log( Robert.describe.call( Richard ) );

Output: This is me Richard Sash 25 6,4 male