Node.js runs JavaScript on a single thread. How can it serve thousands of connections at once? What kinds of workloads suit Node, and which do not?
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Model answer. Your JavaScript runs on one main thread, but I/O is non-blocking. When a request needs the database, a file or another API, Node hands that work to the operating system (or libuv's small thread pool for things like file access, DNS and crypto), registers a callback, and immediately moves on to the next request. When the I/O completes, the event loop runs the callback. One thread can therefore juggle many connections that are mostly waiting.
Good fit (I/O-bound):
- REST and GraphQL APIs, backends-for-frontends
- real-time apps: chat, notifications, WebSockets
- streaming and proxies
- tooling (bundlers, CLIs)
Poor fit (CPU-bound):
- video encoding, image processing, heavy number crunching, large report generation
A CPU-heavy task blocks the event loop: every other request waits until it finishes. Work around it with worker_threads, by moving the job to a queue processed by separate workers, or by using a language better suited to the task.
Original UniCircle question. Last reviewed 22 Oct 2026.