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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Spawning a brand new Worker Thread (or Child Process) on every single incoming HTTP request

// Wrong: creates and tears down a new thread for every request
app.get('/resize', (req, res) => {
  const worker = new Worker('./resize.js', { workerData: req.body });
  worker.on('message', (result) => res.json(result));
});

// Correct: reuse a warm pool of threads
const pool = new Piscina({ filename: './resize.js' });
app.get('/resize', async (req, res) => {
  res.json(await pool.run(req.body));
});

The Solution //

Booting a Worker Thread has real overhead (creating a new V8 isolate, allocating memory) — doing it per-request under real traffic means most of your CPU savings are eaten by constant thread creation/teardown, and you can exhaust system resources under load. Use a thread pool library like piscina that keeps a fixed number of warm threads and reuses them across requests.

The Error //

Reaching for child_process when worker_threads would be lighter and simpler for pure JavaScript computation

// Heavier than necessary for pure JS math
const { spawn } = require('child_process');
spawn('node', ['heavy-math.js']);

// Lighter and purpose-built for JS computation
const { Worker } = require('worker_threads');
new Worker('./heavy-math.js');

The Solution //

child_process.spawn() launches an entirely separate OS process with its own V8 instance, which is significantly heavier to boot and requires JSON-serializing every message across the IPC boundary. If the heavy work is plain JavaScript computation (not a call to an external program like Python or ffmpeg), worker_threads is lighter, boots faster, and is purpose-built for this case.