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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Blocking the Event Loop with a synchronous heavy computation or *Sync fs call

// Wrong: freezes the whole server for every user
const data = fs.readFileSync('huge-file.json', 'utf8');

// Correct: OS handles the read, main thread stays free
fs.readFile('huge-file.json', 'utf8', (err, data) => {
  if (err) throw err;
  console.log(data.length);
});

The Solution //

Anything that runs synchronously on the main thread — a large JSON.parse, a naive recursive Fibonacci, fs.readFileSync on a huge file — freezes every other request until it finishes, since Node has only one thread executing JS. Replace synchronous heavy work with its async counterpart (fs.readFile) or move genuinely CPU-bound logic to a worker_threads Worker so the main thread stays free to serve other requests.

The Error //

Assuming setTimeout(fn, 0) runs before a Promise's .then()

setTimeout(() => console.log('macrotask'), 0);
Promise.resolve().then(() => console.log('microtask'));
// Logs: 'microtask' then 'macrotask' — every time

The Solution //

The Event Loop always fully drains the microtask queue (Promise callbacks, queueMicrotask, process.nextTick) before it processes the next macrotask (setTimeout, setInterval, I/O). This means a Promise.resolve().then() scheduled after a setTimeout(fn, 0) still logs first, which surprises developers who assume queues execute strictly in registration order.