Single Number

solution.js

// hashmap()
// 56 ms, faster than 87.12%
// 37.2 MB, less than 50.00%
function hashmap (nums) { 
    let existance = {};
    for (let i = 0; i < nums.length; i++) {
        if (nums[i] in existance === false) 
            existance[nums[i]] = 1;
        else
            delete existance[nums[i]];
    }
    return Object.keys(existance)[0];
};

// reduce ()
// 56mb ~80%
// 35.3mb 92.31%
// Slower probably because of the inner workings of reduce
//   Potentially because it is a property on the Array prorotype, and not a native function
//   Might use slightly less memory since the accumulator is bulit in

function reduce (nums) { 
    return nums.reduce((mask, num) => mask ^ num, 0);
};

// forLoop ()
// 44ms 99.75%
// 36mb 67.31%
// Memory usage higher probably because of the reassignment:
//   mask = mask ^ nums[i]; mask has to be held twice

function forLoop (nums) {
    let mask = 0;
    for (let i = 0; i < nums.length; i++) {
        mask = mask ^ nums[i];
    }
    return mask;
}

// forEach ()
// 56ms 87.12%
// 35.4mb 82.69%
// Slower than for loop
function forEach (nums) { 
    let m = 0;
    nums.forEach(n => m = m ^ n);
    return m;
};