Introduction to JavaScript

What is JavaScript?

JavaScript (JS) is a lightweight, interpreted, dynamic scripting language used to make web pages interactive. It runs in the browser and also on the server (via Node.js).

Key Features

• Interpreted (no compilation)
• Dynamically typed
• Object-based (prototype inheritance)
• Event-driven and asynchronous
• Runs in every browser

Including JavaScript in HTML

<!-- Inline JS (not recommended) -->
<button onclick="alert('Hello!')">Click</button>

<!-- Internal script -->
<script>
  console.log('Hello from internal script');
</script>

<!-- External script (recommended) -->
<script src="app.js" defer></script>
<!-- defer: runs after HTML is parsed -->
<!-- async: runs as soon as downloaded -->

JavaScript Output

console.log("Debug message");       // Browser console
console.warn("Warning");
console.error("Error message");
alert("Popup message");             // Alert box
document.write("Write to page");    // Directly to page
document.getElementById("out").textContent = "Hello!"; // DOM

Hello World Example

// Single line comment

/*
  Multi-line comment
*/

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

// Variables
let name = "Alice";
const PI = 3.14159;
var old = "avoid var";

// Basic operations
let sum = 10 + 5;
let greeting = "Hello, " + name + "!";
console.log(greeting); // Hello, Alice!

JavaScript vs Other Languages

JavaScript Versions (ECMAScript)

Developer Tools

Open browser DevTools with F12 or Right-click → Inspect:

• Console: Run JS, see errors/logs
• Sources: Debug JS code
• Network: Monitor HTTP requests
• Elements: Inspect HTML/CSS

Key Takeaway: JavaScript is the language of the Web, making pages dynamic and interactive. Use external scripts with defer for best performance and console.log for debugging.