Node.js is an open-source, cross-platform runtime that runs JavaScript outside the browser, on Google's V8 engine. It adds what a server needs and a browser lacks, such as file system access and network servers. Its model is asynchronous and event-driven. One process serves many connections on a single thread, because I/O calls do not block while they wait.
That makes Node.js a strong fit for I/O-heavy work such as APIs, chat and streaming. Teams also gain one language across the front end and the back end, and the npm ecosystem. It is a poor fit for long CPU-heavy tasks such as video encoding, because a busy thread delays every other request. Worker threads ease that, but such work often belongs in a separate service.
Rewriting in plainer words…
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