Middleware and next

Middleware

A middleware is a function that runs before (or between) the handlers, with the same signature plus a third argument: next.

function logger(req, res, next) {
  console.log(req.method, req.url);
  next(); // passes control to the next middleware/handler
}

• If it calls next(), the request continues to the next step.
• If it responds (res.json(...), res.status(...)) and does not call next(), it stops the chain right there. This is the pattern for authentication or validation:

function requireAuth(req, res, next) {
  if (!req.headers.token) {
    res.status(401).json({ error: "Unauthorized" });
    return; // stops: does not call next()
  }
  next(); // there is a token: continue
}

The typical status codes here:

• 401 Unauthorized (authentication is missing).
• 400 Bad Request (malformed or missing data).