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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.

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

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

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

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