js計算數組中某個元素重復出現的個數（一）

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

怎么把list數組轉化為

[
　　{title:"1000積分",num:1},

　　{title:"500積分",num:1},

　　{title:"50積分",num:3}

]
//num:serialNumber相同的元素重復次數

首先要先獲取同樣的serialNumber的重復次數

//先根據所有的title組合成一個數組

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

//獲得["1000積分", "500積分", "50積分", "50積分", "50積分"]

然后有兩種方法將titleList 轉化

 // 不用reduce時：
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時： //reduce:計算數組元素相加后的總和
//reduce方法接受兩個參數，第一個是函數，第二個是初始值 function getRepeatNum(){ return titleList.reduce(function(prev,next){ prev[next] = (prev[next] + 1) || 1; return prev; },{}); } console.log(getRepeatNum())；//獲得{1000積分: 1, 500積分: 1, 50積分: 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積分","num":1},{"title":"500積分","num":1},{"title":"50積分","num":3}]