Why Node.js is Single-Threaded

Node.js is often described as a single-threaded runtime, and that’s true — but with some important details. To understand why, let’s look at JavaScript and the event loop mechanism.

Why a Single Thread?

  • JavaScript was originally designed to run in the browser’s single thread, to avoid complexity with the DOM and race conditions.
  • Node.js inherits this principle: one thread for executing JavaScript code.

This means only one piece of JS code executes at a time.

But How Does It Handle Thousands of Requests?

That’s where the event loop and asynchronous I/O model come into play:

  • System operations (files, network, databases) run in the background via libuv and a C++ thread pool.
  • Node.js doesn’t block while waiting — instead, it registers a callback or Promise.
  • Once the operation completes, the event loop schedules the callback for execution.

This allows Node.js to handle thousands of concurrent connections efficiently.

Important Caveats

  • CPU-bound tasks (heavy calculations) block the event loop and slow everything down.
  • For such workloads, developers use worker threads or external services.
  • For I/O-bound applications (APIs, web services, chats), Node.js is an excellent fit.

Conclusion

Node.js is single-threaded for JavaScript code execution, but under the hood it uses asynchronous mechanisms and thread pools for heavy operations.
This design makes it lightweight, fast, and highly efficient for networked applications.