Is Node.js single-threaded? What does that actually mean?

The single-threaded claim refers to the JavaScript execution context. Only one piece of JavaScript runs at a time, so there are no data races between callbacks on the main thread. The process still has multiple OS threads: the event loop, the libuv thread pool, garbage collection helpers and any workers you create.

import { readFile } from "node:fs/promises";
import { pbkdf2 } from "node:crypto";

async function load() {
  const config = await readFile("config.json", "utf8");
  console.log(config.length);
}

pbkdf2("password", "salt", 310000, 32, "sha256", (error, key) => {
  if (error) throw error;
  console.log(key.length);
});

load();

What runs where

• JavaScript callbacks and module evaluation run on the main thread
• The libuv pool defaults to four threads for file system, DNS and crypto operations
• UV_THREADPOOL_SIZE changes the pool size but does not help CPU-bound JavaScript
• worker_threads run JavaScript on separate threads with message passing
• child processes run separate Node instances with their own memory

The model is efficient when work is I/O-bound because waiting does not consume a JavaScript thread. It struggles when a single request does heavy computation, because that computation blocks every other request on the same process.