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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Mounting router.use(requireAuth) below a route that should have been protected

// Wrong: /profile registered BEFORE the auth middleware
router.get('/profile', getProfile); // unprotected!
router.use(requireAuth);

// Correct: middleware first, protected routes after
router.use(requireAuth);
router.get('/profile', getProfile);

The Solution //

router.use() middleware only applies to routes registered AFTER it in the same file — Express evaluates route registration order, not the eventual file structure. A protected route accidentally defined above the router.use(requireAuth) line executes with zero authentication, silently exposing it.

The Error //

An async controller throwing an error crashes the process instead of returning a JSON error

// Wrong: a thrown/rejected error here is never caught by Express
router.get('/:id', async (req, res) => {
  const user = await db.findById(req.params.id); // throws if DB is down
  res.json(user);
});

// Correct
router.get('/:id', async (req, res, next) => {
  try {
    const user = await db.findById(req.params.id);
    res.json(user);
  } catch (err) {
    next(err);
  }
});

The Solution //

Express route handlers don't automatically catch rejected Promises from async functions — an unhandled rejection inside an async controller bypasses Express's default error handling entirely and can crash the process. Wrap async controller logic in try/catch and call next(error), or use a wrapper (like express-async-handler) that forwards rejections to next() automatically.