How JavaScript Engines Work

JavaScript engines parse JavaScript source code, compile it to bytecode or machine code, and execute it. The major engines — V8, SpiderMonkey, and JavaScriptCore — are all written in C++.

V8 (Google / Node.js / Chrome)

V8 is an open-source JavaScript and WebAssembly engine written in C++. It compiles JavaScript directly to machine code before executing it (JIT compilation). V8 powers Google Chrome, Node.js, and Deno.

SpiderMonkey (Firefox)

SpiderMonkey is Mozilla's JavaScript engine, written in C++ and Rust. It was the first JavaScript engine ever created (1995) and powers Firefox. It uses a tiered JIT compilation approach.

JavaScriptCore (Safari / WebKit)

JavaScriptCore (also called Nitro) powers Safari and all iOS browsers. It is written in C++ and uses a four-tier JIT architecture.

Why C++?

JavaScript engines are written in C++ for performance: direct memory management, fine-grained control over compilation, and low-level platform access. JavaScript itself is a high-level dynamic language — only its engine is in C++.

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