passing data between asp.net (c#) and javascript

you can do that through hidden fields
Assign  value to Hidden Fileld which you can access on client side with Javascript like this
var s=document.getElementById('<%=hdnTest.ClientID%>')

so I need to populate a hidden form field with the c# variable and have javascript read it?

You have to pass the ClientID into a Javascript variable, then use that to get the form field by id.  The "HiddenFormField" is the ASP.net ID of the field.  The field must be set to runat server.  I haven't tested this.  If you need more information open a new question:

<script type="text/javascript">
var jsFieldId = '<%=HiddenFormField.ClientID%>';
var jsField = document.getElementById(jsFieldId);
var jsFieldValue = jsField.value;
alert(jsFieldValue);
</script>

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