single-number.arrays.js

/**
 * @param {number[]} nums
 * @return {number}
 */

var singleNumber = function(nums) {
    let obj = {};

    for(let i = 0; i < nums.length; i++) {
        if(!obj[nums[i]]) {
            obj[nums[i]] = 1;
        } else {
            delete obj[nums[i]]
        }
    }

    return Object.keys(obj)[0]
};


singleNumber([2,2,1]) // 1
singleNumber([4,1,2,1,2]) // 4
singleNumber([1]) // 1