Strings crossing the WebAssembly boundary must be copied through linear memory by hand

Core WebAssembly has no string type and no direct access to JavaScript objects. Every string passed in or out is encoded to UTF-8 bytes, written into the module's linear memory, and referenced by pointer and length, with the caller responsible for allocating and freeing that buffer. This is why a per-call string round trip costs more than the computation in many naive ports, and why chatty interfaces perform worse than one call moving a batch.

Binding generators such as wasm-bindgen and the Emscripten binding layer hide the copying but do not remove it, so a hot loop calling an exported function with a string per iteration still allocates and copies each time. Design the interface around bulk transfers: pass a pointer to a preallocated buffer, or serialize once into memory and let the module iterate internally. Reference types and garbage collection proposals reduce this cost for host objects without eliminating the string copy in existing toolchains.