How Javascript Works?

JavaScript was created in 1995 by Brendan Eich.

• Earlier: used only for small tasks like alerts
• Today: used everywhere — websites, mobile apps, games, servers, even AI tools

What Can JavaScript Do?

1️⃣ Client Side (In Your Browser)

Client side means inside your browser (Chrome, Edge, Firefox).

JavaScript can:

• Update content without reloading the page
• Show interactive maps
• Play videos
• Validate forms (check email, password, etc.)
• Show notifications and animations

Example: You like a post on Instagram and the count increases instantly — JavaScript did that.

2️⃣ Server Side (Behind the Scenes)

With tools like Node.js, JavaScript also works on the server.

It can:

• Connect to databases
• Save user data
• Handle login & signup
• Control the backend logic of websites

Example: When you log in and your data is verified — JavaScript works on the server.

3️⃣ DOM Manipulation (Changing the Page)

DOM means the structure of a web page.

JavaScript can:

• Change text
• Change colors
• Hide or show elements
• Add new content dynamically

Example: Click a button → menu opens → background color changes That instant change is called DOM Manipulation.

How JavaScript Understands Your Code?

You write JavaScript as text, but the computer understands only machine language (1s and 0s).

So JavaScript code goes through three main steps.

Phase 1: Tokenizing (Breaking into Pieces)

First, the browser breaks your code into small parts called tokens.

Example line:

const age = 25;
        

It becomes:

• const
• age
• =
• 25
• ;

👉 Think of it like: Breaking a sentence into words and symbols so the computer can read it properly.

Phase 2: Abstract Syntax Tree (AST) – The Blueprint

Now the browser organizes those tokens into a structured form called an AST.

What is an AST?

An AST is like a blueprint or map of your code.

Example analogy:

• You want to build a house
• The blueprint shows where everything belongs
• AST shows how your code is connected

If you:

• Miss a bracket
• Make a spelling mistake

The AST cannot be built → Syntax Error occurs.

Phase 3: Compilation (Making It Fast)

Once the AST is ready:

• The JavaScript Engine (like Chrome’s V8) converts it into machine code
• Machine code = 1s and 0s
• This happens very fast, just before running the code

This process is called JIT (Just-In-Time) Compilation.

Meaning: JavaScript is compiled at runtime to make it super fast.