js计算数组中某个元素重复出现的个数（一）

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

首先要先获取同样的serialNumber的重复次数

//先根据所有的title组合成一个数组

let titleList = list.map(item => {
    return item.name
})

//获得["1000积分", "500积分", "50积分", "50积分", "50积分"]

然后有两种方法将titleList 转化

 // 不用reduce时：
function getRepeatNum(){ 
        var obj = {}; 
        for(var i= 0, l = titleList.length; i< l; i++){ 
            var item = arr[i]; 
            obj[item] = (obj[item] +1 ) || 1; 
        } 
        return obj; 
}

//用reduce时：
//reduce:计算数组元素相加后的总和
//reduce方法接受两个参数，第一个是函数，第二个是初始值
function getRepeatNum(){ 
        return titleList.reduce(function(prev,next){ 
            prev[next] = (prev[next] + 1) || 1; 
            return prev; 
        },{}); 
} 

console.log(getRepeatNum())；//获得{1000积分: 1, 500积分: 1, 50积分: 3}

let myObj = this.getRepeatNum2(titleList),
winningList = []
for(let i in myObj){
    let obj = {
        title:i,
        num:myObj[i]
    }
    winningList.push(obj)
}
console.log(winningList)
//[{"title":"1000积分","num":1},{"title":"500积分","num":1},{"title":"50积分","num":3}]