js 數組對象的相同值相加合並

let arr = [
{commodity_id:1,quantity:10,amount:20},
{commodity_id:1,quantity:5,amount:10},
{commodity_id:2,quantity:10,amount:50}]

mergeMap(arr) {
// js數組對象相同值相加，不同添加
    var newArr = [];
    arr.forEach(item => {
        var dataItem = item;
        if (newArr.length > 0) {
            var filterValue = newArr.filter(v => {
                return v.commodity_id == dataItem.commodity_id
            })
            if (filterValue.length > 0) {
                newArr.forEach(n => {
                    if (n.commodity_id == filterValue[0].commodity_id) {
                        n.quantity = filterValue[0].quantity + dataItem.quantity;
                        n.amount = filterValue[0].amount + dataItem.amount;
                    }
                })
            } else {
                newArr.push(dataItem)
            }
        } else {
            newArr.push(dataItem)
        }

    });
    return newArr
},

console.log(mergeMap(arr))