js计算数组中某个元素重复出现的个数

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serialNumber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
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}]

原文地址：https://www.cnblogs.com/yixiancheng/p/12048144.html