Show custom alert using Jquery

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ASP.NET

jQuery

Hi all I am having a form with 2 check-boxes and a button what I need to do is if the user clicks on a button with our checking any check-box available, I would like to show custom alert saying select one.. If one is selected I don't want to show the alert...

for this I used the following

http://www.abeautifulsite.net/blog/2008/12/jquery-alert-dialogs/[^]

with some changes as follows

<script type="text/javascript">

        $(document).ready(function () {
            var atLeastOneIsChecked = $('#checkArray :checkbox:checked').length > 0;

            if (atLeastOneIsChecked == 0) {
                $("#alert_button").click(function () {
                    jAlert('This is a custom alert box', 'Alert Dialog');
                });
            }
        });

    </script>

<form id="form1" runat="server">
    <fieldset id="checkArray">
    <input type="checkbox" name="chk[]" value="Apples" />
    <input type="checkbox" name="chk[]" value="Bananas" />
</fieldset>
        <p>
            <input id="alert_button" type="button" value="Show Alert" />
        </p>
    </form>

But I am getting alert on both the case can any one help me..