23.form_validation.html

<html>

<head>
    <script type="text/javascript">
        function validator()
        {
            var name = document.my_form.username.value  // gives me the value entered in username field
            if (document.my_form.thebox.checked)
            {
                alert("It's checked !!!");
                alert('My username is: ' + name);
            }

            else
                alert("Noooo! It's not checked !!!")
        }
    </script>
</head>

<body>
    <form name=my_form>
        <input type="checkbox" name="thebox" />
        Username: <input type="text" name="username" />
        <input type="button" value="Press me !" onClick="validator()" />
        <!-- validator method in script will be called when we click on button -->
    </form>
</body>

</html>