Understanding this keyword in JavaScript

this is a special keyword in JavaScript that refers to the object that is executing the current function.

In simple terms, this refers to "who called the function."

1. Global Scope

    var myVar = 100;

    function WhoIsThis() {
    var myVar = 200;
    alert(myVar); // 200
    alert(this.myVar); // 100
    }

    WhoIsThis(); // window.WhoIsThis()

In this case, this refers to the global window object.

2. In Strict Mode

    "use strict";
    function test() {
    alert(this); // undefined
    }
    test();

3. Inside Nested Functions

    var myVar = 100;

    function outer() {
    function inner() {
        var myVar = 200;
        alert(myVar); // 200
        alert(this.myVar); // 100
    }

    inner();
    }

    outer();

4. Inside an Object Method

    var obj = {
    myVar: 300,
    whoIsThis: function () {
        var myVar = 200;
        alert(myVar); // 200
        alert(this.myVar); // 300
    }
    };

    obj.whoIsThis();

5. Using Constructor Functions

    function WhoIsThis() {
    this.myVar = 200;
    }

    var obj1 = new WhoIsThis();
    var obj2 = new WhoIsThis();
    obj2.myVar = 300;

    alert(obj1.myVar); // 200
    alert(obj2.myVar); // 300

6. Using `call()` and `apply()`

    var myVar = 100;

    function WhoIsThis() {
    alert(this.myVar);
    }

    var obj1 = { myVar: 200 };
    var obj2 = { myVar: 300 };

    WhoIsThis();              // 100
    WhoIsThis.call(obj1);     // 200
    WhoIsThis.apply(obj2);    // 300

7. Using `bind()`

    function SomeFunction(callback) {
    callback();
    }

    var obj = {
    myVar: 300,
    whoIsThis: function () {
        alert(this.myVar);
    }
    };

    SomeFunction(obj.whoIsThis); // 100 (window)
    SomeFunction(obj.whoIsThis.bind(obj)); // 300

Precedence Order of `this`

bind()
call() and apply()
Object method (dot notation)
Global scope

Summary Table

Context	`this` refers to
Global function	`window` (or `undefined` in strict mode)
Object method	The object itself
Constructor function	The new object
`call()` / `apply()`	The object passed as argument
`bind()`	The bound object
Arrow function	Lexical scope (outer `this`)

Tip: Always ask yourself: “Who is calling this function?” That’s your this!