Callbacks & Callback Hell

function middleware(req, res, next) {
  authenticate(req, (err, user) => {
    if (err) return next(err);
    req.user = user;
    next();
  });
}

Error-first callbacks
Continuation-passing style
This demonstrates Continuation-passing style (CPS) callbacks, common in middleware: 1) next function is called to continue execution flow, 2) Error handling is managed through the next function, 3) Allows for both synchronous and asynchronous operations, 4) Enables clean composition of multiple middleware functions, 5) Maintains consistent error handling across the middleware chain, 6) Allows for branching execution paths based on conditions.
Event callbacks
Promise callbacks