What is Node.js and how does it work?

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Accepted Answer

Node.js is a JavaScript runtime built on Google's V8 engine that lets you run JavaScript outside the browser — primarily on the server. It was created by Ryan Dahl in 2009 to handle high-concurrency I/O workloads efficiently. V8 compiles and executes JavaScript into machine code. libuv provides the event loop and handles async I/O via OS-level APIs (epoll, kqueue, IOCP). Node uses a single-threaded, non-blocking I/O model based on the Reactor Pattern — instead of spawning a thread per request, it registers I/O operations with the OS and reacts to events when they're ready. CPU-heavy or blocking operations (filesystem, DNS, crypto) are offloaded to the libuv thread pool , keeping the main thread free. Interviewers want to hear more than 'it runs JavaScript on the server.' Mention V8, the event-driven non-blocking I/O model, libuv, and what types of applications Node.js excels at. Bonus points for mentioning what it is NOT good at (CPU-bound tasks).