Nitro is Apple's marketing name for JavaScriptCore, so UIWebView and Safari both make use of Nitro. The difference you're thinking of is whether a JIT compiler

Nitro is Apple's marketing name for JavaScriptCore, so UIWebView and Safari both make use of Nitro. The difference you're thinking of is whether a JIT compiler is used. That difference has nothing to do with the age of the version of WebKit used in UIWebView vs Safari.