How do I revise "if" syntax in form validation JavaScript?

Question

I added a script to my "Contact" page to validate any submitted email. Just one problem: the visitor is not required to give me their reply email if they don't want to. I want the script to validate if they input an email, but do nothing if they leave that form field blank. This should be easy, but as a result of having gotten into the habit of avoiding Web desgn that relies on JavaScript, I don't have the know-how to fix a script when I decide it's needed!  Link:
http://www.keithpurtell.com/kthings/mail02.htm

Answer 1

Just leave the conditional in but have it do nothing...
    if (fld.value == "") {
         // do nothing
    } else if (!emailFilter.test(tfld)) {         //test for illegal characters
        fld.style.background = 'Yellow';
        error = "Please enter a valid email address.\n";
    } else if (fld.value.match(illegalChars)) {
        fld.style.background = 'Yellow';
        error = "The email address contains illegal characters.\n";
    } else {
        fld.style.background = 'White';