Is Node.js Single-Threaded?

➡️ Yes AND No.

• JS execution: Single-threaded (via V8 Engine + Call Stack)
• Background tasks like fs, crypto, dns: handled via libuv Thread Pool

libuv Thread Pool — What & Why?

Libuv is C-based library in Node.js that manages:

• 🌀 Event Loop
• 🧵 Thread Pool (Default size = 4 threads)
• 🗃️ I/O Operations like:
  • fs (file system)
  • crypto (e.g., password hashing)
  • dns.lookup
  • Custom async operations

# You can customize thread pool size
process.env.UV_THREADPOOL_SIZE = 8;

Diagram Breakdown (from your image):

1. V8 JS Engine
   • Executes JS (Single-threaded)
   • Manages call stack + memory heap
2. libuv handles:
   • Event loop
   • Callback queue
   • Thread pool (multi-threaded)
3. Thread Pool
   • Handles expensive I/O without blocking JS
   • Each task goes to 1 of the 4 threads
   • Once done → pushes callback to queue → JS thread picks it

🧪 Real Example: crypto.pbkdf2 using Thread Pool

const crypto = require("crypto");

console.time("hash");

for (let i = 0; i < 5; i++) {
  crypto.pbkdf2("password", "salt", 100000, 64, "sha512", () => {
    console.timeEnd("hash");
  });
}