The Node.js Event Loop Explained

Understanding the event loop is fundamental to writing performant Node.js applications. Let’s break down each phase and see how they work together.

The Six Phases

1. Timers — executes setTimeout and setInterval callbacks
2. Pending callbacks — executes I/O callbacks deferred to the next loop iteration
3. Idle, prepare — used internally by Node.js
4. Poll — retrieves new I/O events
5. Check — setImmediate() callbacks are invoked here
6. Close callbacks — socket.on('close', ...), etc.

Common Pitfalls

// This blocks the event loop!
app.get('/heavy', (req, res) => {
  const result = heavyComputation(); // synchronous!
  res.json(result);
});

// Better: offload to a worker
app.get('/heavy', async (req, res) => {
  const result = await runInWorker(heavyComputation);
  res.json(result);
});

Key Takeaways

• Never block the event loop with synchronous computation
• Use worker threads for CPU-intensive tasks
• Understand the difference between setImmediate and process.nextTick