What a JavaScript engine does

• Separate the engine (runs the language) from the host (browser or Node) that supplies extra APIs.

Model answer. JavaScript is a dynamically typed, garbage-collected language with first-class functions. It started in browsers and now also runs on servers (Node.js, Deno, Bun).

A JavaScript engine turns source text into running machine code:

1. Parse the source into an abstract syntax tree (AST).
2. Interpret it quickly as bytecode, so code starts running straight away.
3. Optimise: functions that run often ("hot" code) are compiled just-in-time to fast machine code, based on the types actually seen.
4. Manage memory: allocate objects and free unreachable ones with a garbage collector.

The engine alone knows nothing about pages or files. The host adds those: a browser adds the DOM, fetch and timers; Node.js adds fs, http and its own event loop. That is why document exists in Chrome but not in Node, even though both use V8.