JavaScriptCore

JavaScriptCore is the JavaScript engine used by the WebKit browser engine. Developed largely by Apple, it is embedded in Safari on macOS, iOS, and other WebKit-based applications. JavaScriptCore parses, optimizes, and executes JavaScript code, implementing the ECMAScript standard and providing bindings to WebKit’s JavaScript interfaces and runtime.

JavaScriptCore originated from the JavaScript engines used in early WebKit versions, evolving from the SquirrelFish family. It became the default JavaScript engine for Safari and WebKit-based projects. Over time, it introduced multiple just-in-time backends, including a Baseline JIT, the Data Flow Graph (DFG) JIT, and later the Faster-Than-Light (FTL) JIT, to improve performance while maintaining correctness and memory safety.

Architecturally, JavaScriptCore consists of a parser, an interpreter, and multiple JIT compilers, together with a garbage collector. The engine uses an incremental or generational garbage collector to reclaim memory. It integrates with WebKit’s runtime and bindings to support the Document Object Model, WebIDL, and Web APIs.

JavaScriptCore is open source as part of the WebKit project and is distributed under its permissive licenses. It ships with Safari and other WebKit-based applications and is also used in some Linux-based WebKitGTK and other ports. The project continues to evolve with WebKit releases, implementing newer ECMAScript features and performance improvements.