Emscripten - how to pass an array of floating point numbers from JavaScript to a C callback, to let the latter fill it with data.

library,js

// "use strict";

var LibraryTst = {
  initialize: function (callback) {
    callback = Runtime.getFuncWrapper(callback, 'vi')
    var numBytes = 2 * Float32Array.BYTES_PER_ELEMENT
    var ptr = Module._malloc(numBytes)

    try {
      callback(ptr)

      console.log('Callback filled buffer like so:')
      var i = 0
      for (i = 0; i < 2; ++i) {
        console.log(HEAPF32[(ptr/Float32Array.BYTES_PER_ELEMENT)+i])
      }
    }
    finally {
      Module._free(ptr)
    }
  },
}

mergeInto(LibraryManager.library, LibraryTst)

tst.cpp

#include <cstdio>

extern "C" {
    void initialize(void (*callback)(float*));
}

void callback(float* output)
{
    for (int i = 0; i < 2; ++i)
    {
        output[i] = i+1;
    }
    printf("Callback was invoked\n");
}

int main()
{
    initialize(&callback);
    return 0;
}