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.

Thanks.
LVL 1
kchallAsked:
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mrichmonCommented:
if(document.LoginForm.ContactNo.value.substring(0,4) == "ADMN")
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