Why does Express 4 not catch async errors?
Express 4 was designed before async/await. It does not catch promise rejections from async handlers. The handler returns a rejected promise that Express ignores, and the client hangs.
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More FAQs in Async Error Handling in Express: Best Practices
Wrap async handlers with asyncHandler: Promise.resolve(fn(req, res, next)).catch(next). The wrapper catches promise rejections and passes them to the error handler. Then you can throw inside async handlers.
Yes. Express 5 catches promise rejections from async handlers and passes them to the error handler. The asyncHandler wrapper is still useful for older versions.
A safety net. process.on('unhandledRejection') and process.on('uncaughtException') catch what your middleware misses. They should not be your main strategy, but they prevent silent crashes. uncaughtException should usually exit the process.
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