Using a JS object provides a faster algorithm because it doesn't require the sort operation:

var ones = {}, pairs = 0;
for (var i = 0; i < n; i++) {
    if (ones.hasOwnProperty(ar[i])) {
        pairs++;
        delete ones[ar[i]];
    } else {
        ones[ar[i]] = 0;
    }
}
return pairs

I have also figured out a reduce way of doing it, but it doesn't reduce the time complexity:

var ones = {};
return ar.reduce((pairs, i) => {
    if (ones[i]) {
        delete ones[i];
        return pairs + 1;
    } else {
        ones[i] = true;
        return pairs;
    }
}, 0);