static _shuffle(a) {
    for (let i = a.length - 1; i > 0; i--) {
      const j = Math.floor(Math.random() * (i + 1));
      [a[i], a[j]] = [a[j], a[i]];
    }
    return a;
  }
Shuffles array in place. ES6 version
@param {Array} a items An array containing the items.