javascriptランダム文字列参照([A~Z],[a~z],[0~9])

function ranNum(l) {
    return Math.floor(Math.random() * (l[1] - l[0])) + l[0];
}

function r(len) {
    var s = "",t;
    for (; len > 0; len--) {
        t = ranNum([[65, 91], [97, 123],[0, 10] ][ranNum([0, 3])]);
        s = s + (t > 10 ? String.fromCharCode(t) : t.toString());
    }
    return s;
}
console.log("  ",r(10));