Node.js Express Interview Questions Middleware, Routing, and Error Handling

Node.js Express Interview Questions

Express is the most popular Node.js web framework. Here are common interview questions.

Q1: What Is Middleware in Express?

Answer: Middleware is a function that runs during the request-response cycle. It can modify the request, send a response, or call the next middleware.

// Middleware function
const logger = (req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next(); // Pass control to the next middleware
};

app.use(logger); // Apply to all routes

Q2: What Is the Difference Between app.use and app.get?

Answer:

// app.use   Runs for ALL HTTP methods on the given path
app.use("/api", authMiddleware);

// app.get   Runs only for GET requests on the given path
app.get("/api/users", getUsersHandler);

Q3: What Are the Types of Middleware?

Answer:

1. Application-level app.use(logger) Runs for all routes
2. Router-level router.use(auth) Runs for specific router
3. Built-in express.json(), express.static()
4. Error-handling (err, req, res, next) => {} 4 params
5. Third-party cors(), helmet(), morgan()

Q4: What Is the Express Request-Response Lifecycle?

Answer:

1. Incoming HTTP request
2. Express parses the request
3. Middleware chain runs in order:
   • express.json() (body parsing)
   • cors() (CORS headers)
   • authMiddleware (verify JWT)
   • validationMiddleware (validate input)
4. Route handler executes
5. Response is sent
6. Error middleware catches any thrown errors

Q5: How Does Error Handling Work in Express?

Answer:

// Error-handling middleware (must have 4 parameters)
app.use((err, req, res, next) => {
  console.error(err.stack);
  const status = err.statusCode || 500;
  res.status(status).json({ 
    message: err.message || "Internal server error",
    ...(process.env.NODE_ENV === "development" && { stack: err.stack })
  });
});

// Throw errors in route handlers
app.get("/users/:id", asyncHandler(async (req, res) => {
  const user = await User.findById(req.params.id);
  if (!user) throw new AppError("User not found", 404);
  res.json({ data: user });
}));

Q6: What Is the asyncHandler Pattern?

Answer: A wrapper that catches async errors and passes them to Express's error handler:

const asyncHandler = (fn) => (req, res, next) => {
  Promise.resolve(fn(req, res, next)).catch(next);
};

// Usage   no try-catch needed
router.get("/users", asyncHandler(async (req, res) => {
  const users = await User.find();
  res.json({ data: users });
}));

Q7: What Is Express Router?

Answer: Express Router modularizes routes:

// routes/auth.js
const router = express.Router();
router.post("/signup", signupHandler);
router.post("/login", loginHandler);
module.exports = router;

// app.js
const authRouter = require("./routes/auth");
app.use("/api", authRouter); // Mount at /api

The Takeaway

Express interview questions cover: middleware (functions in the request-response cycle with next()), app.use vs app.get (all methods vs specific method), middleware types (application, router, built-in, error-handling, third-party), request-response lifecycle, error handling (4-parameter middleware, asyncHandler pattern), and Express Router (modular route organization). Use asyncHandler to avoid try-catch in every route.