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What is Express.js, and what problem does it solve compared to using Node.js's raw http module directly? What is middleware in Express, and what is the significance of the order in which middleware is registered? What is the difference between application-level, router-level, and route-specific middleware in Express? What does the express.json() and express.urlencoded() built-in middleware do, and why must it be registered before routes that need to read req.body? What is the purpose of the helmet middleware package for an Express application, and what specific security headers does it set? What is CORS middleware in Express, and how does the cors package simplify configuring Cross-Origin Resource Sharing? What is the difference between res.send(), res.json(), and res.end() in Express, and when should each be used? How do you write custom Express middleware that needs to perform asynchronous work, like checking a value against a database, before calling next()? How would you implement request validation middleware in Express using a schema-validation library like Zod or Joi? How would you implement a request-logging middleware in Express that captures method, path, status code, and response time for every request?
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What is the purpose of the express.static() built-in middleware, and how would you use it to serve a folder of static assets?

Beginner

Answer

express.static() serves files directly from a specified directory (images, CSS, client-side JavaScript, HTML) without needing a custom route handler for each individual file -- it automatically handles setting appropriate Content-Type headers, supports conditional GET requests via ETags for caching, and maps request URLs directly to file paths within the configured directory.
app.use(express.static('public'));

// A request to /images/logo.png automatically serves ./public/images/logo.png
// without needing an explicit route defined for it
Real-world example A single-page application's Express backend serves its built frontend assets (HTML, CSS, JS bundle) directly via express.static('dist'), letting the same Node.js server that handles the API also serve the frontend, without a separate static file server.

Common follow-ups: How would you configure a custom cache duration for static assets served this way?;What's the security consideration of exposing an entire directory via express.static() if it accidentally contains sensitive files?

HTTP & HTTPS Modules;Caching
All 15 Express & Middleware questions