At least when compiled with emscripten, the same memory-access-related optimizations used in native code also apply to cross-compiled JS code since the memory layout is exactly the same. emscripten uses a big linear memory buffer (a single big JS typed array) as heap and dlmalloc for dynamic memory management within this heap. If your code is cache-friendly in the natively compiled version of the code it will also be cache friendly in emscripten compiled code. Cheerp seemt to use a different approach though and seems to generate 'traditional' high level JS objects (which also means it requires garbage collection, which emscripten generated code doesn't).