[simonh]

The only reason it wasn't allowed until now was security. With iOS 8 cross process communication improvements, the JITed JavaScript engine can be used by third party apps in a segregated fashion without sacrificing security.

[carloscm]

Is this also true when using JavaScriptCore directly? Or do you only get JIT compilation in the context of a web view?

[kenferry]

Only in a web view. The point is that the JITed code is in an isolated process with few permissions.

[gsnedders]

And vitally it can allocate executable memory, which nothing you write can.