What is JavaScript?

01 What is JavaScript?

JavaScript is a programming language — a precise, written way of telling a computer what to do. It was built for the web, and every modern browser understands it natively, which means it runs the moment a page loads, with no separate installation.

Two other languages usually sit alongside it: HTML describes the structure of a page, and CSS describes its appearance. JavaScript adds behavior — the part that reacts, calculates, and changes things over time.

It has also grown well beyond the browser. The same language now runs on servers, in command-line tools, and inside mobile apps, using engines that execute JavaScript outside of a web page entirely.

Why does this matter? Before JavaScript existed, web pages were static documents — you could read them, but the page itself couldn't respond to anything you did. JavaScript was created in 1995 (by Brendan Eich, in about ten days, for Netscape Navigator) specifically to close that gap. Every interactive thing you take for granted on the modern web — a form that validates itself as you type, a map you can drag, a feed that loads more posts without reloading the page — exists because JavaScript can run after the page has loaded, in direct response to what the user does.

02 Simple Example

Here is a small piece of JavaScript reacting to a moment in time — the click of a button.

const button = document.querySelector("button");

button.addEventListener("click", () => {
  console.log("The button was clicked.");
});

Nothing happens until the click occurs. JavaScript is comfortable waiting for the world to do something — that reactive quality is central to how it works, and it's a theme this book returns to often.

03 How It Works

When a browser loads a page, it hands any JavaScript to a JavaScript engine built into the browser. The engine reads the code, converts it into a form the machine can run, and executes it — line by line, top to bottom, unless it's told to wait or repeat.

The engine is the bridge between the code you write and what the page actually does.

04 Step-by-Step

1. The browser downloads the HTML page.

2. It finds a <script> tag or linked .js file and hands the code to the JavaScript engine.

3. The engine parses the code, checking that it's written correctly.

4. The engine executes the code from top to bottom.

5. Any part of the code that listens for events — clicks, typing, timers — stays ready to run later, in response to those events.

05 Mental Model

"JavaScript is the stage crew of a web page — invisible until something needs to move."

HTML builds the set, CSS lights and dresses it, and JavaScript is the crew making things move on cue: opening a menu, validating a form, refreshing a feed.

06 Common Mistakes

07 Interview Questions

Related Concepts

• How JavaScript Runs
• JavaScript Engines
• Writing Your First JavaScript