ipv6 address validate using Javascript

// check whether the str is a right IPv6 address
function checkIPv6(str) {
    var idx = str.indexOf("::");
    // there is no "::" in the ip address
    if (idx == -1) {
        var items = str.split(":");
        if (items.length != 8) {
            return false;
        } else {
            for (i in items) {
                if (!isHex(items[i])) {
                    return false;
                }
            }
            return true;
        }
    } else {
        // at least, there are two "::" in the ip address
        if (idx != str.lastIndexOf("::")) {
            return false;
        } else {
            var items = str.split("::");
            var items0 = items[0].split(":");
            var items1 = items[1].split(":");
            if ((items0.length + items1.length) > 7) {
                return false;
            } else {
                for (i in items0) {
                    if (!isHex(items0[i])) {
                        return false;
                    }
                }
                for (i in items1) {
                    if (!isHex(items1[i])) {
                        return false;
                    }
                }
                return true;
            }
        }
    }
}

// check whether every char of the str is a Hex char(0~9,a~f,A~F)
function isHex(str) {
    if(str.length == 0 || str.length > 4) {
        return false;
    }
    str = str.toLowerCase();
    var ch;
    for(var i=0; i< str.length; i++) {
        ch = str.charAt(i);
        if(!(ch >= '0' && ch <= '9') && !(ch >= 'a' && ch <= 'f')) {
            return false;
        }
    }
    return true;
}