Awesome, thanks for the answer. That makes sense to just leave indexing the stack up to the user. Same with letting the user do some legwork to find the address of the function.

Have you considered the (sort of edge) case when arguments are passed in registers? For example (in the VC compiler, as far as I remember) the "this" pointer is generally passed in the register ECX instead of being pushed onto the stack. In those cases it might be useful to have access to the register contents inside the JS callback.

Along a similar line, the EAX register is generally used for return values. It might be useful to provide access to that as well (through another callback?), although that would probably require changing how your trampoline works so that it replaces the return address in the stack... (i miss working with low level things sometimes).

Keep up the awesome work :D