Serving static files in Express

WHAT THIS TESTS This checks whether you know Express ships a built-in solution for serving assets rather than requiring manual routes, and whether you handle the path robustly.

A GOOD ANSWER COVERS Express provides the express.static middleware for serving files from a folder. You register it with app.use(express.static('public')), and Express then serves any file in that directory at a URL path relative to the directory root: public/css/style.css becomes available at /css/style.css, with no extra routing code. Because a relative path is resolved against the process current working directory, which can vary by how you launch the app, the robust form uses an absolute path: app.use(express.static(path.join(__dirname, 'public'))). You can mount it under a virtual prefix, such as app.use('/static', express.static('public')), so files appear under /static. The middleware also sets caching headers and handles content types automatically.

COMMON WRONG ANSWERS Writing an individual app.get route for each asset, which is unmaintainable. Using a bare relative path and being surprised when files are not found because the working directory differs. Confusing the filesystem path argument with the URL prefix.

LIKELY FOLLOW-UPS How do you mount static files under a URL prefix. Why use __dirname here. How would you set cache-control or maxAge on static assets.

ONE CONCRETE EXAMPLE With app.use(express.static(path.join(__dirname, 'public'))) and a file at public/js/app.js, a browser request to /js/app.js returns that script with the correct Content-Type, no per-file route required. Adding app.use('/assets', express.static(path.join(__dirname, 'public'))) would instead expose it at /assets/js/app.js.