Ech, is that right? I don't understand `wordSizeShift`.

I have always done bitset tests as follows:

  this.set = function(bit, value) {
    const index = Math.floor(bit / BitsPerElement);
    const offset = bit - (index * BitsPerElement);

    var element = this.buffer[index];
    if (value)
      element |=  (1 << offset);
    else
      element &= ~(1 << offset);

    this.buffer[index] = element;
  }

  this.get = function(bit) {
    const index = Math.floor(bit / BitsPerElement);
    const offset = bit - (index * BitsPerElement);

    const element = this.buffer[index];
    return (element & (1 << offset));
  }