The Event Loop Is Single-Threaded — Don't Block It

The problem:

app.get("/process", (req, res) => {
  for (let i = 0; i < 100000; i++) {
    expensiveSync(i);  // Blocks the event loop
  }
  res.send("done");
});

// Another request arrives while the loop runs
// It waits. And waits. And waits.

Your server appears frozen.

The fix: Yield to the event loop with setImmediate.

async function processChunked(items: Item[]) {
  for (const item of items) {
    expensiveSync(item);
    await new Promise(resolve => setImmediate(resolve));  // Yield
  }
}

Other requests now execute between chunks. No freezing.

For true CPU work (crypto, machine learning), use worker_threads to parallelize on multiple cores.

Single thread, many I/O? setImmediate. Multiple cores, CPU-bound? worker_threads.

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