Web Fundamentals

What Is JavaScript and How Does It Work?

By the CodeCompiler Team · 10 min read

If HTML gives a web page structure and CSS gives it style, JavaScript gives it behavior. It is the language that makes a page do things: respond to clicks, validate a form before it's submitted, fetch fresh data from a server without reloading the page, animate elements, and build entire complex applications that run directly inside your browser tab. JavaScript holds a unique position in the software world: it is the only programming language that runs natively, without any additional installation, in every major web browser on every major operating system.

A Scripting Language for the Browser, Originally

JavaScript was created in 1995 specifically to add interactivity to otherwise static HTML pages — we tell that origin story in much greater detail in The History of JavaScript and Its Impact on the Web. In its earliest days, JavaScript was mostly used for small, simple tasks: showing a popup, validating that a form field wasn't left empty, or swapping an image when a user hovered over it with their mouse. It was seen, fairly or not, as a lightweight scripting language, nowhere near as "serious" as languages used to build desktop software or backend systems.

That perception has changed completely. Modern JavaScript, combined with modern browser engines, is used to build entire single-page applications with the complexity of desktop software — think of the web-based versions of tools like Google Docs, Figma, or Slack, all running inside a browser tab, all built substantially with JavaScript.

How JavaScript Actually Executes

When a browser loads a page containing JavaScript — whether inline in a <script> tag or loaded from an external file — its JavaScript engine (V8 in Chrome and Node.js, SpiderMonkey in Firefox, JavaScriptCore in Safari) parses that code and begins executing it. Modern engines use just-in-time (JIT) compilation, which we cover in Compiler vs. Interpreter: JavaScript starts out being interpreted, and any code that runs frequently gets compiled into fast native machine code on the fly.

One of the most distinctive things about JavaScript is that it is single-threaded with an event loop. This means JavaScript can only execute one piece of code at a time — there's no true parallel execution of your JavaScript logic happening simultaneously. Instead, JavaScript relies heavily on an asynchronous, non-blocking model: when your code needs to do something that takes time — fetching data from a server, reading a file, waiting for a timer — it doesn't freeze the entire page waiting for that operation to finish. Instead, it registers a callback (a function to run later) and moves on immediately to the next piece of work. Once the slow operation completes, the event loop schedules that callback to run. This is why you'll frequently see JavaScript code built around callbacks, promises, and the async/await syntax — all different ways of expressing "do this, and then, whenever it's ready, do that."

JavaScript and the DOM

Inside a browser, JavaScript's most common job is manipulating the Document Object Model, or DOM — the browser's live, in-memory representation of the HTML page. JavaScript can read the DOM to find out what's currently on the page, modify it to change what's displayed, add or remove elements entirely, and attach event listeners that run custom code in response to user actions like clicks, key presses, or scrolling. We go into much more depth on this specific relationship in What Is the DOM and Why Does It Matter?

Beyond the Browser

In 2009, a runtime called Node.js brought JavaScript outside the browser entirely, allowing it to run on servers, build command-line tools, and power backend systems — the same language now works on both the frontend and backend of countless applications, which we discuss further in What Do Frontend and Backend Really Mean? Today, JavaScript (and its statically typed superset, TypeScript) is used to build mobile apps (via frameworks like React Native), desktop apps (via frameworks like Electron), and even some embedded and IoT systems — a remarkable expansion for a language originally built in ten days to add simple interactivity to web pages.

Why JavaScript's Flexibility Is Both a Strength and a Challenge

JavaScript is a dynamically typed, highly flexible language: variables aren't bound to a fixed type, functions can be passed around like any other value, and objects can be reshaped at runtime. This flexibility makes JavaScript approachable to beginners and extremely fast to prototype in. It's also part of why large JavaScript codebases can become error-prone over time, which is exactly the problem TypeScript was created to solve, by adding an optional layer of static typing on top of the same language.