An instance of JSContext represents a JavaScript execution environment. All
JavaScript execution takes place within a context.
JSContext is also used to manage the life-cycle of objects within the
JavaScript virtual machine. Every instance of JSValue is associated with a
JSContext via a strong reference. The JSValue will keep the JSContext it
references alive so long as the JSValue remains alive. When all of the JSValues
that reference a particular JSContext have been deallocated the JSContext
will be deallocated unless it has been previously retained.
原文：http://www.cnblogs.com/feng9exe/p/6096068.html