Centralized error-handling middleware

What's really being asked

This assesses understanding of Express's error pipeline and how to consolidate error responses instead of duplicating try/catch handling in every route.

The full answer

Error-handling middleware is distinguished solely by having four parameters: err, req, res, and next. Express inspects a middleware function's arity, and a four-argument function is treated as an error handler rather than a normal one. Errors reach it in two ways: when any handler calls next(err) with an argument, or when a synchronous route handler throws, which Express catches automatically. Note that for asynchronous errors in older Express you must pass them to next yourself, since Express 4 does not auto-catch rejected promises. You implement a centralized catch-all by defining one error handler after all your routes and other middleware. Inside it you log the error, optionally inspect a status property to choose a code, and send a single consistent response, for example a JSON error body with an appropriate status, avoiding leaking stack traces to clients in production.

The mistakes people make

Defining the handler with three parameters, so Express never invokes it for errors. Placing it before the routes, so errors thrown later never reach it. Assuming async rejections are caught automatically in Express 4 without next(err). Sending raw stack traces to the client.

What usually comes next

How do you forward async errors to the handler. How does Express 5 change async error handling. How would you attach a status code to a custom error.

A concrete example

After all routes you add app.use((err, req, res, next) => { console.error(err); res.status(err.status || 500).json({ error: err.message }); });. A route doing next(new Error('boom')) or throwing synchronously lands here, producing a uniform 500 JSON response and one place to log every failure.