static shuffle(a) {
      var j, x, i;
      for (i = a.length - 1; i > 0; i--) {
          j = Math.floor(Math.random() * (i + 1));
          x = a[i];
          a[i] = a[j];
          a[j] = x;
      }
      return a;
  }
Shuffles array in place.
https://stackoverflow.com/questions/6274339/how-can-i-shuffle-an-array

@param {Array} a items An array containing the items.