el-table合并相同数据的行

// 表格列、行宽计算
      arraySpanMethod: function (obj) {
        var prop = obj.column.property;
        var row = obj.row;
        var rowIndex =  obj.rowIndex;
        var columnIndex = obj.columnIndex;
        // 合并行  themeName相同合并、合并第3列，所以合判断columnIndex是否等于3
        if (columnIndex === 3) {
          if (rowIndex === 0 || row.themeName != this.prePlanningList[rowIndex - 1].themeName) {
            var rowspan = 0;
            this.prePlanningList.forEach((element) => {
              if (element.themeName === row.themeName) {
                rowspan++;
              }
            });
            return [rowspan, 1];
          } else {
            return [0, 0];
          }
        }
      },