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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Running a large synchronous computation directly inside a request handler

// Wrong: blocks ALL concurrent requests for the duration
app.post("/process", (req, res) => {
  const result = heavySyncComputation(req.body); // blocks everyone
  res.json(result);
});

// Correct: offloaded to a worker thread
const worker = new Worker("./heavy-task.js", { workerData: req.body });

The Solution //

Any synchronous code running inside a request handler blocks the single event loop thread for its entire duration, delaying every other concurrent request being handled by that process, not just the one that triggered the computation. Move genuinely CPU-intensive work to a worker_threads Worker, or break it into chunks yielded with setImmediate if it must stay on the main thread.

The Error //

Having no event loop lag monitoring in place at all, only discovering blocking issues from user complaints

// Add this early — before it becomes a user-facing incident
const histogram = monitorEventLoopDelay({ resolution: 20 });
histogram.enable();

The Solution //

Without an explicit lag metric, event loop blocking issues typically surface only as vague, hard-to-diagnose "the app feels slow sometimes" user reports, long after the underlying cause was introduced. Instrument monitorEventLoopDelay() and alert on a sustained high p99 value to catch this proactively.