Actually, good js implementations (like v8, for example) will only create one code object shared by all the unpack functions, with separate closures assigned to

Actually, good js implementations (like v8, for example) will only create one code object shared by all the unpack functions, with separate closures assigned to each. And since closures can (usually) be statically determined, they can use an offset lookup for the closured variable in place of a hashtable. So it's not quite as inefficient as it looks :).