The Wikipedia article for Webkit gives a pretty good summary:
JavaScriptCore
JavaScriptCore is a framework that provides a JavaScript engine for WebKit implementations, and provides this type of scripting in other contexts within OS X.[13][67] JavaScriptCore is originally derived from KDE's JavaScript engine (KJS) library (which is part of the KDE project) and the PCRE regular expression library. Since forking from KJS and PCRE, JavaScriptCore has been improved with many new features and greatly improved performance.[68] On June 2, 2008, the WebKit project announced they rewrote JavaScriptCore as "SquirrelFish", a bytecode interpreter.[29][30] The project evolved into SquirrelFish Extreme (abbreviated SFX, marketed as Nitro), announced on September 18, 2008, which compiles JavaScript into native machine code, eliminating the need for a bytecode interpreter and thus speeding up JavaScript execution.[31]
So I would say that, nowadays, WebKit's JS engine is called Nitro.