怎么根据哈希表的value筛选出对应的key进行输出？

题目是一个数组里只有唯一一个不重复的元素，其他元素都重复两次，输出唯一不重复的元素的值。
用哈希表写了以后不输出，求问！！
以下代码输出undefined：

class HashCode {
            constructor() {
                this.storage = [];  //数组存放index
                this.limit = 10;
                this.count = 0;
            }
            hashFunc(num, max) {
                let hashCode = 0;
                for (i = 0; i < num.length; i++) {
                    hashCode = num[i];
                }
                let index = hashCode / max;
                return hashCode;
            }

            put(key, value) {
                // 根据哈希函数获取index值
                const index1 = this.hashFunc(key, this.limit);
                //  根据index值获取对应的链值
                let bucket = this.storage[index1];
                //  判断bucket是否为空
                if (bucket === undefined) {
                    bucket = [];
                    this.storage[index1] = bucket;
                }
                //  判断是否修改数据
                let overide = false;
                for (let i = 0; i < bucket.length; i++) {
                    let tuple = bucket[i];
                    if (tuple[0] == key) {
                        tuple[1] = value;
                        //  如果bucket中有key属性，则令他对应的值为value
                        overide = true;
                    }
                }
                if (!overide) {
                    bucket.push([key, value]);
                    this.count++;
                }
            }

        }

        var hashIndex = new HashCode();
        let index = 0;

        var singleNumber = function (nums) {
            for (i = 0; i < nums.length; i++) {
                for (j = i; j < nums.length; j++) {
                    if (nums[i] == nums[j]) {
                        index++;
                    }
                    hashIndex = hashIndex.put(nums[i], index);
                    return hashIndex;
                }
            }
            for (k = 0; k < hashIndex.length; k++) {
                console.log(hashIndex.key);
            }
        };

        let num = [1, 2, 2, 3, 3, 4, 4];
        console.log(singleNumber(num));

a=[1,2,3,4,5,4,3,2,1];
result=null;
a.forEach(v=>{if(a.indexOf(v)===a.lastIndexOf(v)){result=v;}});
console.log(result);
//结果是5

关于数组的判重，去重等你可以参考这个文章