Help with code please

In the following code, Why wont null stay as Null with a capital N when I change it and why wont nacm hide itself when I delete the Mobile field. The url if you want to see in action is https://creator.zoho.com/mrcomputer/mrcomputer/#Form:Job

if (input.Mobile != input.Mobile.getAlphaNumeric())
{
    input.Mobile = input.Mobile.getAlphaNumeric();
}
if ((input.Mobile).length() = 11)
{
    show Type_of_text;
    show RFC_text;
    show Send_RFC_email;
    show Customer_contact;
    show Time_of_contact;
    hide No_Mobile_Entered;
    hide nacm;
}
else
{
    hide nacm;
    hide Type_of_text;
    hide RFC_text;
    hide Send_RFC_email;
    hide Customer_contact;
    hide Time_of_contact;
    show No_Mobile_Entered;
    if (input.Mobile != null)
    {
        show nacm;
    }
}

Thanks

Nigel