When you build a web app, the browser shows the user interface. But behind the scenes, a server does the real work: it stores data, checks rules, and talks to other services. For a long time, that server was usually written in PHP, Python, or Java. With Node.js, you can now write both the frontend and the backend in one language: JavaScript. In this chapter, we’ll see how Node.js lets you do that, and how the Express framework helps you build fast, secure web servers with clean code.