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 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.
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.
In the order you list them. router.post('/posts', auth, validate, createPost) runs auth, then validate, then createPost. If auth sends a response, validate and createPost never run.
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