performanceTest.js

// First generate a long enough array sequence to test the code with.
var generateArray = Array.from(new Array(100000),(val,index)=>index);

var startMyCode = Date.now();

function findOddMyCode(A) {
    var len = A.length;
    var A_sort = A.slice().sort();

    var count = {};

    A_sort.forEach(function(i) {
        count[i] = (count[i] || 0) + 1;
    });

    for (var key in count) {
        if (count.hasOwnProperty(key)) {

            //  var value = count[key];
            if (count[key] % 2 !== 0) {
                return Number(key);
            }
        }
    }
}

findOddMyCode(generateArray);

var endMyCode = Date.now();

//End of my solution to the problem.

var startFastCode = Date.now();

const findOddFastCode = (xs) => (xs).reduce((a, b) => a ^ b);

findOddFastCode(generateArray);

var endFastCode = Date.now();

// Test the execution time difference
console.log((endMyCode - startMyCode) - (endFastCode - startFastCode));