Ensuring that a Form Field has Content on Submission

This is a short Javascript function to ensure that a form field has content on submission.
For a field called firstName add the following into the head of the page

<SCRIPT LANGUAGE=javascript>
<!--
function doSubmit(){
  if (formedit.firstName.value.length<1) {
    alert("Please provide a first name.")
    formedit.firstName.focus();
    return false;
  }
}
//-->
</SCRIPT>