Why is the error handler always last in Express?
Express runs middlewares in order. An error handler in the middle would miss errors from routes declared after it. The error handler must be the last middleware registered.
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More FAQs in Express Middleware Order and Execution: How It Works
Express runs middlewares in the order you add them. Wrong order causes silent bugs: body parser after routes means req.body is undefined; logger after routes means 404s are not logged; error handler not last means some errors are not caught.
Body parser, helmet, CORS, logger, then routes (with per-router and per-route middlewares), then error handler last. Body parser before routes so req.body is defined; helmet early for security headers; error handler last so all errors are caught.
Express skips everything between that middleware and the error handler. The error handler then runs and sends a response. This is how errors short-circuit the rest of the chain.
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