Understanding the window Object, Global Execution Context, and this in JavaScript

What is the window Object?

→ In the browser, when JavaScript code runs, the JavaScript engine automatically creates a global object and in browsers, this object is called the window.

It represents the global scope and provides access to browser APIs, such as alert(), setTimeout(), console, document, and more.

2. Global Execution Context (GEC)

When the JavaScript engine starts running code, it creates the Global Execution Context: this happens only once when the page first loads.

Two key things are created during this:

1. The global object (window in the browser).
2. The this keyword, which points to the global object in the global scope.

So, at the global level:

console.log(this === window); // true

3. The Global Space

Anything that is not inside a function or block is considered part of the global space.

When you declare variables or functions in this space using var or a function declaration, they automatically become properties of the global object (window).

Example:

var a = 10;
function greet() {
  return "Hello!";
}

you can access above variable and function in three different ways:

console.log(a);          // 10
console.log(window.a);   // 10
console.log(this.a);     // 10

console.log(greet());        // Hello!
console.log(window.greet()); // Hello!
console.log(this.greet());   // Hello!

4. Let and Const Are Different

It’s important to note that let and const do not attach variables to the window object:

let b = 20;
const c = 30;

console.log(window.b); // undefined
console.log(window.c); // undefined

Only var and function declarations get attached to the window object in the global context.

Summary

• The browser creates the window object when a script starts running.
• The global execution context also defines the this keyword to point to the window object.
• Variables/functions defined with var and function declarations in the global space are added as properties to window.
• You can access these using window.variableName, this.variableName, or just variableName.
• let and const do not attach to the window object.

Thanks…