Get random strings from given string In javascript

The below function return a random string with given length

function randomString(length) {
    chars = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
    var result = '';
    for (var i = length; i > 0; --i) result += chars[Math.round(Math.random() * (chars.length - 1))];
    return result;
}
var res = randomString(4);
alert(res);