SOURCE

var arr = [1,5,3,6,6,4,5,3,1];
// 
var aa =  function(arr) {
    arr.sort();
    var num;
    for(i = 0; i < arr.length; i++) {
        if(arr[i] == arr[i+1]) {
            i++;
            continue;
        }else {
            num = arr[i];
            break;
        }
    }
    return num;
}
console.log("aa", aa(arr));

var bb = function(arr) {
    return (',' + arr.sort()).replace(/(,\d)\1|,/g, '');
}
console.log("bb", bb(arr))


let cc = function(arr) {
    return arr.reduce((prev, cur) => {
        console.log(prev, cur)
        return prev ^ cur;
    }); 
}

console.log("cc", cc(arr));

var dd = function(arr) {
    for(i = 0; i < arr.length; i++) {
        if(arr.indexOf(arr[i]) == arr.lastIndexOf(arr[i])) {
            return arr[i];
        }
    }
}
console.log("dd", dd(arr));