code: function tileToQuadkey(tile) {
      var index = '';
      for (var z = tile[2]; z > 0; z--) {
          var b = 0;
          var mask = 1 << (z - 1);
          if ((tile[0] & mask) !== 0) b++;
          if ((tile[1] & mask) !== 0) b += 2;
          index += b.toString();
      }
      return index;
  }
docs: Get the quadkey for a tile
  
  @name tileToQuadkey
  @param {Array<number>} tile
  @returns {string} quadkey
  @example
  var quadkey = tileToQuadkey([0, 1, 5])
  //=quadkey