js 数组排序 重复项相加返回新数组

   function mergeArr(todos, key) {
       var newArr = [];
       todos.forEach(item => {
           var dataItem = item
           if (newArr.length > 0) {
               var filterValue = newArr.filter(v => {
                   return v.index == dataItem.index
               })
               if (filterValue.length > 0) {
                   newArr.forEach(n => {
                       if (n.index == filterValue[0].index) {
                           n[key]= filterValue[0][key]+ dataItem[key]
                       }
                   })
               } else {
                   newArr.push(dataItem)
               }
           } else {
               newArr.push(dataItem)
           }
       })
       return newArr
   }