计算数组中重复元素出现的次数

        const arr = ['1', '2', '2', '3', '3', '4', '4', '4', '5', 'b', 'b', 'd', 'd', 'd']
        /** 不使用 reduce */
        function  repeatCount(arr) {
            let map = {}
            for (let item of arr) {
                map[item] = map[item] + 1 || 1
            }
            return map
        }

        /** 使用 reduce */
        function repeatCountByReduce(arr) {
            let map = arr.reduce((prev, curr) => {
                prev[curr] = prev[curr] + 1 || 1
                return prev
            }, {})
            return map
        }

      const obj = repeatCountByReduce(arr) // {1: 1, 2: 2, 3: 2, 4: 3, 5: 1, b: 2, d: 3}
      const keys = Object.keys(obj) // ['1', '2', '3', '4', '5', 'b', 'd']
      const values = Object.values(obj) // [1, 2, 2, 3, 1, 2, 3]