js统计数组中元素的重复次数（二）

let list = [
　　{name:"1000积分",serialNumber:6},
　　{name:"500积分",serialNumber:7},
　　{name:"50积分",serialNumber:8},
　　{name:"50积分",serialNumber:8},
　　{name:"50积分",serialNumber:8} ];

怎么把list数组转化为

[
　　{title:"1000积分",num:1},

　　{title:"500积分",num:1},

　　{title:"50积分",num:3}

]
//num:serialNumber相同的元素重复次数

let _res = [];
for(let i = 0; i < list.length;) {
    let count = 0;
    for(let j = i; j < list.length; j++) {
        if(list[i].serialNumber == list[j].serialNumber) {
            count++;
        }
    }
    let obj = {
        title:list[i].name,
        num:count
    }
    _res.push(obj)
    i += count;
}

console.log(_res)

//[{"title":"1000积分","num":1},{"title":"500积分","num":1},{"title":"50积分","num":3}]