code: function pointToTileFraction(lon, lat, z) {
      var sin = Math.sin(lat * d2r),
          z2 = Math.pow(2, z),
          x = z2 * (lon / 360 + 0.5),
          y = z2 * (0.5 - 0.25 * Math.log((1 + sin) / (1 - sin)) / Math.PI);
  
      // Wrap Tile X
      x = x % z2
      if (x < 0) x = x + z2
      return [x, y, z];
  }
docs: Get the precise fractional tile location for a point at a zoom level
  
  @name pointToTileFraction
  @param {number} lon
  @param {number} lat
  @param {number} z
  @returns {Array<number>} tile fraction
  var tile = pointToTileFraction(30.5, 50.5, 15)
  //=tile