JavaScript Introduction

What Is JavaScript?

JavaScript (JS) is the programming language of the web. It runs in every major browser and on servers with Node.js, adding interactivity to HTML and CSS pages—and powering APIs, tools, and apps beyond the browser.

Web trio:
  HTML  → structure
  CSS   → presentation
  JS    → behavior

Standard: ECMAScript (ES)
Engines: V8, SpiderMonkey, JavaScriptCore

Brendan Eich created JavaScript in 1995 for Netscape. Today it is an open ECMAScript standard used for front ends, backends, mobile apps, and more.

Why Use JavaScript?

HTML and CSS alone cannot react to clicks, fetch data, or update the page dynamically. JavaScript is the language that makes pages interactive.

How JavaScript Runs

An engine reads your code and runs it step by step:

Hello, World!

Print a message to the console—in the browser or with Node:

console.log("Hello, World!");

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Hello World</title>
</head>
<body>
  <h1>Hello World demo</h1>
  <p>Open DevTools Console (F12) to see the output.</p>
  <script>
    console.log("Hello, World!");
  </script>
</body>
</html>

Hello, World!

(Browser: DevTools → Console
 Node: node demo.js)

How It Works

1. Script. The <script> block (or .js file) contains JavaScript statements.

2. Call. console.log(...) sends a value to the console.

3. Run. Open the page (F12 → Console) or run node demo.js in a terminal.

node demo.js

JavaScript vs HTML, CSS & Java

Clear up the most common mix-ups:

JavaScript vs Java

Where to run it

Three Ways to Add JS

Pick the method that fits the job. External files are best for real sites.

<script src="app.js"></script>

Place scripts near the end of <body> (or use defer on external scripts) so the HTML is ready before your code runs.

Core Building Blocks

These pieces show up in almost every beginner program:

Where Is JavaScript Used?

JS shows up far beyond simple webpage scripts:

Key Takeaways