Untitled

<!DOCTYPE html>
<html>
<head>
    <title>Demo</title>
</head>
<body>

    Username : <input type="text" id="user"><br><br>
    Password : <input type="password" id="pass"><br><br>
    <button onclick="Sbmt()">Submit</button>

    <script type="text/javascript">
        function Sbmt() {
            var usr = document.getElementById('user').value;
            var pas = document.getElementById('pass').value;

            if(usr == "" || pas == "") {
                alert("Please Enter Username and Password...");
            } else if (pas.length < 6 || pas.length > 12) {
                alert("Password not valid...");
            } else if (usr.charCodeAt(0) > 65 || usr.charCodeAt(0) < 90 || usr.charCodeAt(0) > 97 || usr.charCodeAt(0) < 122 ) {
                alert("Welcome");
            } else {
                alert("Not valid Username..");
            }
        }
    </script>

</body>
</html>