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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? 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 Express.js, and what problem does it solve compared to using Node.js's raw http module directly?

Beginner

Answer

Express is a minimal, unopinionated web framework built on top of Node's core http module, providing routing (mapping URLs and HTTP methods to handler functions), middleware composition, request/response helper methods (like res.json(), req.params), and template-engine integration -- all of which would otherwise need to be implemented manually and repetitively using the lower-level, more verbose raw http module.
// Raw http module: verbose, manual routing
const http = require('node:http');
http.createServer((req, res) => {
  if (req.url === '/users' && req.method === 'GET') { res.end(JSON.stringify(users)); }
}).listen(3000);

// Express: concise, declarative routing
const express = require('express');
const app = express();
app.get('/users', (req, res) => res.json(users));
app.listen(3000);
Real-world example A team building a REST API chooses Express over the raw http module specifically for its concise routing syntax and rich middleware ecosystem (body parsing, CORS, authentication), avoiding having to hand-roll all of that boilerplate themselves.

Common follow-ups: In what scenario might a team deliberately avoid Express and use the raw http module directly instead?;How does Express compare to newer, similarly-purposed frameworks like Fastify or Koa?

HTTP & HTTPS Modules;RESTful API Design with Express
All 15 Express & Middleware questions