Why is Node.js single-threaded?

Direct answer

JavaScript on the main thread is single-threaded by design — one call stack avoids locks and race conditions in user code; Node achieves concurrency via non-blocking I/O and libuv's thread pool, not one thread per request.

"Single-threaded" refers to your JavaScript, not the entire Node process — libuv uses a small worker thread pool (default 4) for file compression, crypto, and some DNS work behind the scenes.

	Main JS thread	libuv thread pool
Runs	Your callbacks, Express handlers	Selected blocking OS tasks
Count	One for JavaScript	Default 4 (UV_THREADPOOL_SIZE)
You manage	Avoid long sync loops	Rarely — automatic for fs/crypto

Why one JS thread — simpler mental model; no mutexes in typical app code; great for thousands of idle connections.
Cost of one thread — CPU-heavy JSON parse or bcrypt in a route blocks all clients until done (Q74).
Scaling CPUs — cluster module (one process per core) or worker_threads for parallel JS (Q68–69).

blocking-trap.js

// BAD — blocks entire server for ~2 seconds
app.get("/hash", (req, res) => {
  const start = Date.now();
  while (Date.now() - start < 2000) {} // sync CPU loop
  res.send("done");
});

// GOOD — offload or use async crypto
import { hash } from "node:crypto";
hash("sha256", data, () => res.send("hashed")); // uses thread pool