code: function(i, j) {
          var id = null;
          if (this.json) {
              var resolution = this.utfgridResolution;
              var row = Math.floor(j / resolution);
              var col = Math.floor(i / resolution);
              var charCode = this.json.grid[row].charCodeAt(col);
              var index = this.indexFromCharCode(charCode);
              var keys = this.json.keys;
              if (!isNaN(index) && (index in keys)) {
                  id = keys[index];
              }
          }
          return id;
      }
docs: Method: getFeatureId
  Get the identifier for the feature associated with a pixel offset.
  
  Parameters:
  i - {Number} X-axis pixel offset (from top left of tile)
  j - {Number} Y-axis pixel offset (from top left of tile)
  
  Returns:
  {Object} The feature identifier corresponding to the given pixel offset.
  Returns null if pixel doesn't correspond to a feature.