What does a JavaScript engine do?
It parses source code into an AST, JIT-compiles it to bytecode and machine code, and executes it on a single-threaded call stack while managing memory on a heap.
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More FAQs in How the JavaScript Engine Works Behind the Scenes
Just-In-Time compilation means the engine compiles code to machine code at runtime. An interpreter runs first for fast startup, and an optimizing compiler recompiles hot code for speed.
Node.js uses V8, the same JavaScript engine that powers Google Chrome. V8 is written in C++ and developed by Google.
When an optimized assumption breaks (like an object's shape changing), the engine discards optimized machine code and falls back to interpreted bytecode. This hurts performance.
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