What is Node.js and why is it well suited to I/O-heavy servers?

Summary

Node.js is a JavaScript runtime built on Chrome's V8 engine and libuv. It runs your JavaScript on a single main thread with an event loop and non-blocking I/O, so one process can handle many concurrent connections that mostly wait on network, disk or databases.

Longer Answer

Instead of a thread per request, Node registers I/O operations with the OS (epoll/kqueue/IOCP via libuv) and runs callbacks when results are ready. That makes it efficient for APIs, real-time apps (WebSockets), proxies and streaming. CPU-heavy work blocks the event loop, so it needs worker threads, child processes or separate services.

Key Points

• V8 + libuv runtime
• Single-threaded event loop for JS
• Non-blocking I/O handles high concurrency
• CPU-bound work must be offloaded

What interviewers look for

Clear explanation of why single-threaded doesn't mean slow for I/O, and the CPU-bound caveat.

Follow-up questions

• What does libuv's thread pool do?