call c# function in JavaScript Function

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C#

Javascript

hiii all
i need to call c# function from javascript function i tried on it and i found a bit problem my javascript function was;

function test()
{
'<%fillSession(); %>
}

and give that function to

<br />
<input id="btn_Chat" type="button" value="Start Chat" onclick="test()"  /><br />

and my C# function is normal function.

The problem here is that this function is loaded in the page load and by default an exception appears. i need to load it in the click of the button.

Any help, please???

Thanks