In-place, non-recursive qsort in Javascript

qsort.js

function qsort(a, k, l, r) {
    // a: array to sort, k: key to sort by,
    // l, r: optional array index array range

    // i: stack index, s: stack,
    // p: pivot index, v: pivot value,
    // t: temporary array item,
    // x, y: partion low/high

    var i, s, p, v, t, x, y;

    l = l || 0;
    r = r || a.length - 1;

    i = 2;
    s = [l, r];

    while (i > 0) {
        r = s[--i];
        l = s[--i];

        if (l < r) {
            // partition

            x = l;
            y = r - 1;

            p = l;
            v = a[p];
            a[p] = a[r];

            while (true) {
                while (
                    x <= y &&
                    a[x] != undefined &&
                    a[x][k] < v[k])
                    x++;
                while (
                    x <= y &&
                    a[y] != undefined &&
                    a[y][k] >= v[k])
                    y--;
                if (x > y)
                    break;
                t = a[x];
                a[x] = a[y];
                a[y] = t;
            }

            a[r] = a[x];
            a[x] = v;

            // end

            s[i++] = l;
            s[i++] = x - 1;
            s[i++] = x + 1;
            s[i++] = r;
        }
    }
}

variable names like these are unproductive