Codility: Odd Occurrences In Array

gistfile1.txt

// https://app.codility.com/demo/results/trainingTVUZAM-A4K/
function solution(A) {
    // write your code in JavaScript (Node.js 8.9.4)
    if (!Array.isArray(A)) return;
    let res = 0;
    for (let i of A) {
        res ^= i;
    }
    return res;
}

console.log(solution([9,3,9,3,9,7,9]));
console.log(solution([10,7,10,7,9,7,7,8,8]));