JavaScriptCore is a cross-platform javascript engine on it's own that is used by default in WebKit browsers (Safari, Mobile Safari, Konqueror, etc.).

The new thing on iOS 7 is that this framework is exposed as a public API you can code against. So you don't have to initialize a heavy-weight WebView just to execute Javascript.

Safari gets Nitro, Apple's JIT, but historically it has been unavailable to apps (via UIWebView) for security reasons. I'm unsure if the new Objective-C JavaScriptCore API gets Nitro.