Conditional middleware execution in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Middleware execution based on request path

Consider this Express middleware setup. What will be the output when a GET request is made to /api/data?

Express

const express = require('express');
const app = express();

function logMiddleware(req, res, next) {
  console.log('Middleware triggered');
  next();
}

app.use((req, res, next) => {
  if (req.path.startsWith('/api')) {
    logMiddleware(req, res, next);
  } else {
    next();
  }
});

app.get('/api/data', (req, res) => {
  res.send('Data response');
});

app.listen(3000);

A Console logs nothing and client receives 404 Not Found.

B Console logs nothing and client receives 'Data response'.

C Console logs 'Middleware triggered' but client hangs without response.

D Console logs 'Middleware triggered' and client receives 'Data response'.

💡 Hint

Think about when the middleware function is called based on the request path.

The middleware checks if the request path starts with '/api'. For '/api/data', it does, so logMiddleware runs, logging the message. Then it calls next() to continue to the route handler, which sends the response.

📝 Syntax

intermediate

Correct conditional middleware syntax

Which option correctly applies middleware only for POST requests to '/submit' in Express?

A app.post('/submit', (req, res, next) => { next(); });

B app.use((req, res, next) => { if (req.path === '/submit' && req.method === 'POST') next(); else next(); });

C app.use('/submit', (req, res, next) => { if (req.method === 'POST') next(); else res.sendStatus(405); });

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💡 Hint

Remember the difference between app.use and app.post for method-specific middleware.

Option A uses app.post which applies middleware only for POST requests to '/submit'. Option A uses app.use but does not handle the response properly. Option A has an assignment (=) instead of comparison (===), causing a bug.

🔧 Debug

advanced

Middleware not executing as expected

Given this code, why does the middleware never log 'Middleware active' when accessing '/dashboard'?

Express

const express = require('express');
const app = express();

app.use('/dashboard', (req, res, next) => {
  console.log('Middleware active');
  next();
});

app.get('/dashboard', (req, res) => {
  res.send('Dashboard page');
});

app.listen(3000);

A Because the middleware is applied after the route handler, so it never runs.

B Because app.use with a path only matches requests starting with that path, so it should work and the problem is elsewhere.

C Because the middleware is applied to '/dashboard' but the request path includes a trailing slash '/dashboard/', so it doesn't match.

D Because the middleware is synchronous and blocks the route handler.

💡 Hint

Check how Express matches paths with and without trailing slashes.

Express matches paths as prefixes for app.use with a path. If the request URL is '/dashboard/' (with trailing slash), it still matches '/dashboard' prefix, so the middleware runs. The problem is likely elsewhere.

❓ state_output

advanced

Middleware modifying request state conditionally

What will be the value of req.user in the route handler after this middleware runs for a request to '/profile' with header Authorization: Bearer token123?

Express

const express = require('express');
const app = express();

app.use((req, res, next) => {
  if (req.headers.authorization?.startsWith('Bearer ')) {
    req.user = { id: 1, name: 'Alice' };
  }
  next();
});

app.get('/profile', (req, res) => {
  res.json(req.user || null);
});

app.listen(3000);

A {"id":1,"name":"Alice"}

B null

C undefined

D Throws a TypeError

💡 Hint

Look at how the middleware sets req.user based on the Authorization header.

The middleware checks if the Authorization header starts with 'Bearer '. Since the request has 'Bearer token123', it sets req.user to the user object. The route handler then returns this object as JSON.

🧠 Conceptual

expert

Understanding conditional middleware order and effect

In Express, if you want to run a logging middleware only for routes starting with '/admin' and ensure it runs before any route handler, which setup is correct?

A Place <code>app.use(loggerMiddleware)</code> after all '/admin' routes are defined and check path inside middleware.

B Define <code>app.get('/admin/*', loggerMiddleware, handler)</code> for each route individually.

C Place <code>app.use('/admin', loggerMiddleware)</code> before defining any '/admin' routes.

D Use <code>app.use((req, res, next) => { if (req.path.startsWith('/admin')) loggerMiddleware(req, res, next); else next(); })</code> after routes.

💡 Hint

Middleware order matters in Express; earlier middleware runs before later routes.

Middleware must be registered before the routes it affects to run first. Using app.use('/admin', loggerMiddleware) before defining '/admin' routes ensures it runs only for those routes and before their handlers.