Propagating async errors to Express error handlers

WHAT THIS TESTS Whether you understand the difference between synchronous and asynchronous error capture in Express and can route async failures to centralized handling.

A GOOD ANSWER COVERS Express 4 automatically catches errors thrown synchronously inside a handler, but a rejected promise or a throw inside an async function escapes that mechanism, so the request hangs or crashes unless you forward the error explicitly. The direct way is try/catch around the await and calling next(err) in the catch. To avoid repeating that everywhere, wrap each async handler in a higher-order asyncHandler that calls the function and attaches .catch(next), so any rejection is forwarded to the error middleware. Many teams use express-async-errors or the asyncHandler pattern. Express 5 improves this by awaiting returned promises and forwarding rejections to next automatically, so a plain throw in an async handler reaches the error handler.

COMMON WRONG ANSWERS Assuming Express 4 catches async throws automatically, calling next() without passing the error, or sending a response inside catch instead of delegating to the centralized handler.

LIKELY FOLLOW-UPS The difference between Express 4 and 5 behavior, how asyncHandler works internally, why next(err) is required, and avoiding double responses.

ONE CONCRETE EXAMPLE const asyncHandler = fn => (req, res, next) => Promise.resolve(fn(req, res, next)).catch(next); then app.get('/users/:id', asyncHandler(async (req, res) => { const user = await User.findById(req.params.id); if (!user) throw new NotFoundError(); res.json(user); })). The wrapper turns any rejection or throw into next(err), so the centralized error middleware formats the response.