index.js

// this is for understanding reduce, counter qty when same item

const serverData = [
            {
              code: 1,
              item: "ball",
              salesman: "Braan",
              quantity: 5,
              price: 10.0,
            },
            {
              code: 1,
              item: "shoe",
              salesman: "Alex",
              quantity: 5,
              price: 20.0,
            },
            {
              code: 1,
              item: "ball",
              salesman: "Max",
              quantity: 3,
              price: 10.0,
            },
            {
              code: 1,
              item: "shirt",
              salesman: "Braan",
              quantity: 5,
              price: 15.0,
            },
          ];

          const result = serverData
            .map((item, i, array) => {
              const defaultValue = {
                code: item.code,
                item: item.item,
                quantity: 0,
                price: 0,
              };
              const finalValue = array
                .filter((other) => other.item === item.item) //we filter the same items
                .reduce((accum, currentVal) => {
                  //we reduce them into a single entry
                  accum.quantity += currentVal.quantity;
                  accum.price += currentVal.price;
                  return accum;
                }, defaultValue);

              return finalValue;
            })
            .filter((item, thisIndex, array) => {
              //now our new array has duplicates, lets remove them
              const index = array.findIndex(
                (otherItem, otherIndex) =>
                  otherItem.item === item.item &&
                  otherIndex !== thisIndex &&
                  otherIndex > thisIndex
              );

              return index === -1;
            });

          console.log(result);