JavaScript — Lesson 1

Introduction to JavaScript

What JavaScript is and how to use it.

By Majed Qandeel · Jul 26, 2026 · Lesson 1 of 12

What is JavaScript?

JavaScript (JS) is a programming language that makes web pages interactive. It runs in the browser and can respond to user actions, manipulate HTML/CSS, and communicate with servers.

Where JavaScript Runs

  • Client-side: In the browser (Chrome, Firefox, Safari, Edge)
  • Server-side: On the server with Node.js (advanced)

Adding JavaScript to a Page

1. Inline Script

<script> console.log("Hello, World!"); </script>

2. External File (Recommended)

<!-- In HTML, usually before </body> --> <script src="script.js"></script>

Your First JavaScript

<!DOCTYPE html>
<html>
<body>
  <h1 id="demo">Hello, World!</h1>
  <button onclick="sayHello()">Click Me</button>
  <script>
    function sayHello() {
      document.getElementById('demo').innerHTML = 'Hello from JavaScript!';
    }
  </script>
</body>
</html>

Statements

JavaScript instructions are called statements. Each statement ends with a semicolon ; (optional but recommended).

// Three statements let x = 5; let y = 10; let sum = x + y;

Comments

// Single-line comment /* Multi-line comment */

Try It

Open the Deoit Editor, create a JS file, and type: console.log("Hello!"); then click Run. Check the Console panel to see the output!

Tip: Use console.log() often - it's the most important debugging tool in JavaScript!

Practice what you learned in the Deoit Editor — a free, browser-based code editor.

Open Editor →