Apps can use JavaScriptCore, and JavaScriptCore can use JIT, but these can't happen together, because third-party apps aren't permitted to execute writeable memory.
http://stackoverflow.com/questions/22281265/is-javascriptcor...
http://stackoverflow.com/questions/22281265/is-javascriptcor...
http://trac.webkit.org/wiki/JavaScriptCore
"JavaScriptCore is an optimizing virtual machine. JavaScriptCore 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)."