What is JavaScript?

JavaScript is a programming language that runs in every major web browser, making web pages interactive instead of just static documents. It's what lets a page respond when you click a button, validate a form before you submit it, update content without reloading, or pop up a menu — all the things that make a website feel alive.

The web is usually described as three layers working together: HTML provides the structure and content of a page, CSS controls how it looks (colors, layout, fonts), and JavaScript controls how it behaves. A page can exist with just HTML and CSS, but it would be completely static — nothing would happen when you interacted with it.

// JavaScript can react to what a user does
button.addEventListener('click', function () {
  alert('You clicked me!');
});

JavaScript was originally created to run only inside web browsers. But since 2009, a runtime called Node.js has let JavaScript run outside the browser too — on servers, in command-line tools, and even to build entire applications. That means learning JavaScript opens the door to far more than just web pages.

JavaScript was originally designed to run inside a ____, but thanks to Node.js it can now run outside one too.

Which of these best describes the role of JavaScript on a web page, compared to HTML and CSS?

JavaScript can only run inside a web browser.

Today, JavaScript is one of the most widely used programming languages in the world. If a website does anything beyond just displaying text and images, there's a very good chance JavaScript is running behind the scenes to make it happen.