The target of the JIT compiler is indeed JavaScript, since WasmGC is still too limited.
This is fundamentally an implementation detail though, when we adopt WasmGC there will be no user visible change beside a potential performance improvement
How do you deal with numbers? Javascript doesn't do 64-bit numbers and naively multiplying 32-bit numbers can loose lower bits. If I remember right GWT used two numbers to represent 64-bit and ignored the 32-bit issue.