JavaScript engine library from WebKitGTK JavaScriptCore is the JavaScript engine used in WebKit. It consists of the following building blocks: lexer, parser, start-up interpreter (LLInt), baseline JIT, a low-latency optimizing JIT (DFG), and a high-throughput optimizing JIT (FTL). This build comes from the GTK port of WebKit (API version 4.0). This package contains the shared libraries.