Onsubmit Event Using Required Field Validate In JavaScript

<HTML>

<HEAD>
    <TITLE>Validator</TITLE>
    <SCRIPT type="text/JavaScript"> function checkForm(form) { for (var i = 0; i
        < form.elements.length; i++) { if (form.elements[i].value=="" ) { alert( "Fill out ALL fields.") return false; } } return true } </SCRIPT>
</HEAD>

<BODY>
    <FORM onsubmit="return checkForm(this)"> Please enter all requested information:
        <BR> First Name:
        <INPUT TYPE="text" NAME="firstName">
        <BR> Last Name:
        <INPUT TYPE="text" NAME="lastName">
        <BR> Rank:
        <INPUT TYPE="text" NAME="rank">
        <BR> Serial Number:
        <INPUT TYPE="text" NAME="serialNumber">
        <BR>
        <INPUT TYPE="submit" value="SUBMIT"> </FORM>
</BODY>

</HTML>