Allocates a copy of jsdouble on the GC heap.
cx JSContext * The context in which to create the new number. Requires request. In a JS_THREADSAFE build, the caller must be in a request on this JSContext.
d jsdouble The number to copy.
Warning: The argument d must not be a value that could fit in an integer jsval. If it is, JS_NewDouble creates a bad jsdouble that might behave as desired in some cases but not others.
The new jsdouble is subject to garbage collection until you protect it using a local root, an object property, or the JS_AddRoot function.
On success, JS_NewDouble returns a pointer to the new number. Otherwise it returns NULL.
The application must treat the returned pointer as read-only. Modifying a jsdouble allocated with this function can cause undefined behavior.