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How would you implement a request-logging middleware in Express that captures method, path, status code, and response time for every request?

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Answer

A logging middleware records the request's start time when it first runs, then attaches a listener to the response's 'finish' event (fired once the response has been fully sent) to calculate the elapsed duration and log the complete request/response summary -- this pattern, rather than logging synchronously right after calling next(), is necessary because next() returns before the response is actually sent, so the status code and true total duration aren't known until the response actually finishes.
app.use((req, res, next) => {
  const start = Date.now();
  res.on('finish', () => {
    const duration = Date.now() - start;
    console.log(`${req.method} ${req.path} ${res.statusCode} ${duration}ms`);
  });
  next();
});
Real-world example An API's custom logging middleware records every request's method, path, final status code, and total response time by listening for the response's 'finish' event, giving operations visibility into which endpoints are slow or frequently erroring without needing a third-party logging library.

Common follow-ups: Why does logging immediately after calling next() give an incorrect status code and duration compared to listening for 'finish'?;How does this hand-rolled approach compare to using a mature middleware like morgan for the same purpose?

Logging & Monitoring;Performance Optimization & Profiling
All 15 Express & Middleware questions