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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Forgetting to call next() (or send a response) inside a middleware function

// Wrong: request hangs forever
const logger = (req, res, next) => {
  console.log(req.url);
  // missing next()!
};

// Correct
const logger = (req, res, next) => {
  console.log(req.url);
  next();
};

The Solution //

If a middleware neither calls next() nor sends a response with res.send()/res.json()/res.end(), Express has no instruction to move forward, and the client's request hangs until it eventually times out. Every code path in a middleware must either terminate the response or call next() — including inside if/else branches and catch blocks.

The Error //

Calling next() after already sending a response ("Cannot set headers after they are sent")

// Wrong: sends a response, then falls through to next()
const requireAuth = (req, res, next) => {
  if (!req.user) {
    res.status(401).json({ error: 'Unauthorized' });
  }
  next(); // still runs even after the response was sent!
};

// Correct: return stops execution here
const requireAuth = (req, res, next) => {
  if (!req.user) {
    return res.status(401).json({ error: 'Unauthorized' });
  }
  next();
};

The Solution //

If a middleware calls res.send() or res.json() and then also calls next() (or calls next() twice), the request continues down the pipeline and a later handler tries to write to a response that's already closed, throwing the ERR_HTTP_HEADERS_SENT error. Always `return` immediately after sending a response so the function exits before reaching any subsequent next() call.