What is the asyncHandler pattern?
A wrapper function: const asyncHandler = (fn) => (req, res, next) => Promise.resolve(fn(req, res, next)).catch(next). It catches rejected promises from async route handlers and passes them to Express's error middleware, eliminating the need for try-catch in every handler.
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More FAQs in Node.js Express Interview Questions Middleware, Routing, and Error Handling
Middleware is a function that runs during the request-response cycle. It receives (req, res, next) and can modify the request, send a response, or call next() to pass control. Types: application-level (app.use), router-level, built-in (express.json), error-handling (4 params), and third-party (cors, helmet).
app.use runs for ALL HTTP methods (GET, POST, PUT, DELETE) on the given path. app.get runs only for GET requests. Use app.use for middleware that should run regardless of method, app.get/post/put/delete for specific route handlers.
Error-handling middleware has 4 parameters (err, req, res, next). It's registered after all routes. When an error is thrown or next(err) is called, Express skips remaining middleware and goes to the error handler. Use an asyncHandler wrapper to catch async errors and pass them to next().
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