Calculate the sum of merged cells

var columns = [
   ...skip...
   {
     name: "Age",
     fieldName: "Age",
     width: 50,
     mergeRule: "value",
     header: {
       text: "Age",
       styleName: "orange-column"
     },
     numberFormat: "#,##0",
     footer: {
       valueCallback: function(grid, column, footerIndex, columnFooter, value){
         var sum = 0;
         var i = 0;
         var iCnt = grid.getItemCount();
    
         while(i < iCnt) {
             var items = grid.getMergedItems(i, column.name);
             sum += grid.getValue(items[0], column.name);
             i += items.length;
         }
    
         return sum;
       },
       numberFormat: "#,##0",
       styleName: "center-column"
     }
   },
   ...skip...
];