In javascript, scalar values (number, string, boolean and null) are immutable. It means you can't modify the value of a variable, you can only reassign another value to a variable (there's a slight difference).
The point is, the v8 tries very hard to keep these immutable as well.
Furthermore, zero-copy can't be done with Javascript variables because they live in the v8-managed memory. The closest thing you can do is to use Buffers in node. Buffers are small JS Objects referring to an allocated memory space outside of the V8-managed memory.
If you're gonna do funky stuff with memory, do it with buffers.