return a value from javascript from vb.net web page

I have a javascript that returns a value of true or false.  In my code behind in need to run the javascript and get a return value.  but when i try and use
if ClientScript.RegisterClientScriptBlock(Me.GetType(), "ClientScript", "checkCreditCard ('" & ddCardType.SelectedValue & "' , '" & txtCardNo.Text & "') ", True) then

I get message that this doesnt return a value.  Any ideas?

thank you
mgmhicksAsked:
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leakim971Connect With a Mentor PluritechnicianCommented:
in your dropdown/selectbox use : ddCardType

<asp:HiddenField runat="server" ID="HiddenField1" />
<asp:DropDownList runat="server" ID="ddCardType" onchange="document.getElementByid('<%= HiddenField1.ClientID %>').value = this.value;">

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On the server side you will be able to read the value of the asp:HiddenField
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mgmhicksAuthor Commented:
leakim971 getting this error

Microsoft JScript runtime error: Object doesn't support this property or method on the 2nd line you have me.  It highlights starting at "document ...
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mgmhicksAuthor Commented:
I took a different approach.  I used the onchange event to call the function, then get the elements for ddcardtype and txtcardno, test the card and return value true or false back to hiddenfield.  So it was a combo of both suggestions that got me there.
thanks
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