Client JS cares about UI smoothness and bundle size. Server JS cares about the event loop being non-blocking, efficient database queries, and handling many concurrent connections. The performance priorities are very different, even though the language is the same.
Client-side JS runs in the browser with the DOM and a sandbox. Server-side JS runs on the server with Node.js, with file system, network, and database access. They share the language but differ in environment, APIs, and security model.
Yes. The JavaScript language is the same: functions, closures, promises, objects, arrays. The syntax and core features are identical. The difference is the environment and the available APIs, not the language itself.
Server-side JS has the file system (fs), network (http), databases, process control, and full system access. Client-side JS has the DOM, fetch, localStorage, and a sandboxed environment. The available APIs are environment-specific.
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