code: function tileUrlFunction(tileCoord) {
    return ('https://{a-d}.tiles.mapbox.com/v4/mapbox.mapbox-streets-v6/' +
        '{z}/{x}/{y}.vector.pbf?access_token=' + key)
      .replace('{z}', String(tileCoord[0] * 2 - 1))
      .replace('{x}', String(tileCoord[1]))
      .replace('{y}', String(tileCoord[2]))
      .replace('{a-d}', 'abcd'.substr(
        ((tileCoord[1] << tileCoord[0]) + tileCoord[2]) % 4, 1));
  }
docs: Calculation of tile urls for zoom levels 1, 3, 5, 7, 9, 11, 13, 15.