Validating a username input field - similar to coldfusion LEFT function

I have this:

function validateForm() {
      var valid = true;
      if (document.LoginForm.ContactNo.value.length == 0)      {
            alert("Please Enter a Username.");
            valid = false;

Which checks if no user name has been entered (if true show alert box)

Also this:

      else if (document.LoginForm.ContactNo.value != "ADMN") {
            alert("Please enter an ADMN number.");
            valid = false;
      else {
            return valid;

Which checks if the entered consists exactly of the letters ADMN

However i need a case which checks if the first FOUR letters only of the entered text consists of the letters ADMN not that the whole string is just ADMN.

I believe in Coldfusion the syntax would be something like: Left(string, 4) EQ "ADMN".

I just cannot work out how to do this in Javascript.

Who is Participating?
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

if(document.LoginForm.ContactNo.value.substring(0,4) == "ADMN")

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
Web Development Software

From novice to tech pro — start learning today.

Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.