Lesson 1Beginner 3 min read

Introduction to JavaScript

What is JavaScript?

JavaScript is the programming language of the web. It's the only language that runs natively in every browser, and it's what makes websites interactive — responding to clicks, validating forms, fetching data, and updating pages without reloading.

If HTML is the skeleton and CSS is the skin, JavaScript is the brain and muscles.

Where JavaScript runs

JavaScript runs in two places:

The browser — every browser has a JavaScript engine built in (V8 in Chrome, SpiderMonkey in Firefox). This is where most beginners start.
The server — Node.js lets you run JavaScript outside the browser to build servers, CLI tools, and more.

For this course, we'll focus on browser JavaScript first.

Your first JavaScript code

Open your browser, press F12 (or right-click → Inspect), and go to the Console tab. Type this and press Enter:

JavaScript

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

You should see Hello, JavaScript! printed below. console.log() is JavaScript's way of printing output — you'll use it constantly for debugging.

Adding JavaScript to an HTML page

You can add JS to your page using a <script> tag:

HTML

<!DOCTYPE html>
<html>
<head>
  <title>JS Demo</title>
</head>
<body>
  <h1>My Page</h1>

  <script>
    console.log("Page loaded!");
    document.querySelector("h1").textContent = "Hello from JS!";
  </script>
</body>
</html>

For real projects, use an external file:

HTML

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

Place it at the end of <body> or use the defer attribute in <head> so the HTML loads before the script runs.

What you'll learn

In the upcoming lessons, we'll cover:

Variables — storing and naming values
Functions — reusable blocks of code
Arrays and objects — organizing data
DOM manipulation — changing the page with code
Events — responding to user actions

Each concept builds on the previous one, so take them in order.

Strict mode and modern JavaScript

Scripts run in strict mode by default in ES modules (<script type="module">) and in classes. Strict mode catches silent errors (assigning to undeclared variables, duplicate parameter names) and disables some unsafe legacy features.

For classic scripts, enable strict mode at the top of a file:

JavaScript

"use strict";

Modern tutorials and frameworks assume strict behavior — another reason to avoid outdated var patterns.

Statements, expressions, and the console

JavaScript code is made of statements (instructions like const x = 1;) and expressions (values like 2 + 2). The console is your sandbox: try arithmetic, string concatenation, and calling built-in functions:

JavaScript

console.log(typeof 42);        // "number"
console.log("Hello".length);   // 5
console.log(Boolean(""));      // false

typeof helps you inspect values while debugging — you'll use it often before you fully trust what a variable holds.

Module scripts vs classic scripts

HTML

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

Module scripts are deferred by default, run in strict mode, and support import/export between files. They are the standard for new projects. Classic <script src="app.js" defer> still works for simple pages without a bundler.

What makes JavaScript different

JavaScript is dynamically typed (variables don't declare a fixed type) and single-threaded in the browser (one call stack, with async callbacks for waiting on network and timers). It shares syntax ideas with Java, C, and Python but has its own quirks — which the rest of this track covers honestly, including pitfalls like type coercion and asynchronous timing.

Key takeaway

JavaScript is essential for web development. You already have everything you need to start — just open your browser's console and begin experimenting.