what does glue code do?

[JimmyVV]

The repo is great, but there some code inside I don't understand.

There is a global variable -- Module. However, the glue code is like a black box, how do i know what .wasm exposed. Yes, I could get some exposed fn using WebAssembly.Module.exports/imports, but in this glue code, I should need to do this by Module.ccall.

so, how can get information from glue code? like:

wam['grayScale'] = function (pixelData) {
              const len = pixelData.length
              const mem = _malloc(len); // get the memory of wasm
              HEAPU8.set(pixelData, mem); // transfer the pixelData
              _grayScale(mem, len); // trigger the grayScale fun
              const filtered = HEAPU8.subarray(mem, mem + len); // get the resulted data
              _free(mem); // release memory
              return filtered; // return buffer
            };

where could I find some explanation of the above code, like HEAPU8, or _free.

I feel a little confused.