题目一

["b", "c","b", "c","a", "b", "c"] 统计每个元素出现的次数,结果是： {a：1，b: 3, c: 3}

const arr = ["b", "c","b", "c","a", "b", "c"] 

const obj = {}

for (let i = 0; i < arr.length; i++) {
      var key = arr[i]
      if (obj[key] === undefined) {
        obj[key] = 1
      } else {
        obj[key]++
      }
    }

console.log(obj) // ==> {a：1， b: 3, c:  3}

 // 上面中间部分的简写
 
   for (let i = 0; i < arr.length; i++) {
      var key = arr[i]
      obj[key] = (obj[key] + 1) || 1
    }

  const arr = ["b", "c", "b", "c", "a", "b", "c"]
  
    let obj = arr.reduce((obj, cur) => {
      if (cur in obj) {
        obj[cur]++
      } else {
        obj[cur] = 1
      }
      return obj
    }, {})
    
    console.log(obj)

//  上面中间部分的简写

let obj = arr.reduce((a, cur) =>
      ((cur in a) ? a[cur]++ : (a[cur] = 1), a), {}
    )

题目2 用reduce写

var arr = [{ label: '男', value: 1 }, { label: '女', value: 0 }]

function f(arr) {
var obj={}
 arr.reduce((arr, cur) => {
        obj[cur.value] = cur.label
      }, {})
      return obj
}
var obj = f(arr)
console.log(obj) //  {'1': '男',  '0': '女'}