Node js is single-threaded or multithreaded?

All your application's JavaScript code runs on a single main thread called the Event Loop.

Non-Blocking: Because it is single-threaded, Node.js uses asynchronous, non-blocking I/O to handle thousands of concurrent connections without the overhead of creating a new thread for every request

Libuv Thread Pool: Node.js uses a C++ library called libuv, which maintains a default pool of 4 background threads (configurable via UV_THREADPOOL_SIZE).

Internal Tasks: These background threads handle intensive operations that would otherwise block the main thread, such as File System (I/O) operations, Cryptographic functions (e.g., hashing), Compression (Zlib), DNS lookups.

V8 Engine Threads: The V8 engine itself uses separate threads for tasks like garbage collection.