Common Express Interview Questions

Express is the most widely used web framework for Node.js, and interviewers use it to test whether you understand middleware composition, routing, and how the framework builds on top of Node's raw http module. This guide covers the questions that recur most often across Express interviews, with precise answers grounded in how Express actually works internally.

Middleware are functions with the signature (req, res, next) that execute sequentially in the order they are registered via app.use() or route methods, each able to modify req/res, end the request-response cycle, or pass control forward by calling next(). Order matters because Express processes middleware as a pipeline: a middleware that isn't reached (because an earlier one didn't call next()) never runs, and middleware registered after a route handler that sends a response will not execute for that request.

Error-handling middleware has exactly four parameters: (err, req, res, next). Express identifies it as an error handler purely by this arity — even if you don't use all four parameters, you must declare all four for Express to treat the function differently from regular middleware. It is invoked when next(err) is called anywhere upstream, or when a synchronous error is thrown inside a route handler.

Route parameters are named segments defined in the route path (e.g., /users/:id) and are accessed via req.params; they are part of the URL path and typically identify a specific resource. Query strings are key-value pairs appended after a '?' in the URL (e.g., /users?sort=name) and are accessed via req.query; they're typically used for optional filtering, sorting, or pagination rather than identifying a resource.

The http module gives you a bare request/response API where you must manually parse URLs, handle routing with conditional logic, parse request bodies, and set headers yourself. Express is a thin layer built on top of http that adds a declarative routing system (app.get/post/etc.), a middleware pipeline, convenience methods on req/res (like res.json, res.status), and integration points for template engines and third-party middleware, all while still using an http.Server under the hood.

Express provides the built-in express.static(root) middleware, which serves files (HTML, CSS, JS, images) directly from a specified directory. For example, app.use(express.static('public')) serves files in the 'public' folder at the root URL path; you can also mount it under a prefix, e.g., app.use('/assets', express.static('public')).

app.use() mounts middleware for all HTTP methods on a given path prefix (matching the path and anything beneath it unless it's an exact match middleware), making it suitable for cross-cutting concerns like logging, body parsing, or auth. app.get()/app.post()/etc. register route handlers tied to a specific HTTP method and an exact (or pattern-matched) path, used for defining actual endpoint logic.

In Express 4, errors thrown inside synchronous route handlers are caught automatically, but errors thrown inside async functions or rejected promises are NOT automatically caught — you must call next(err) explicitly in a .catch(), or wrap handlers in a helper that forwards rejections to next(). Express 5 changes this by automatically catching rejected promises returned from async handlers and forwarding them to the error-handling middleware.

express.Router() creates a modular, mountable set of route handlers that behaves like a mini Express application. It lets you group related routes (e.g., all /users routes) into a separate file, apply middleware scoped only to that group, and then mount the whole router onto the main app with app.use('/users', userRouter), keeping route definitions organized and maintainable.

Because Express doesn't send a 404 automatically for unmatched routes beyond a default fallback, best practice is to add a catch-all middleware after all defined routes (and before the error handler) that creates a 404 response — since Express processes middleware/routes top-to-bottom, any request that didn't match an earlier route falls through to this handler.