<script lang='javascript'>
function CheckFunction()
{
if (document.getElementById(’<%=textbox2.ClientID%>’).value == '')
{
alert('Please enter a value');
return;
}
}
</script>
<asp:textbox id='textbox2' runat='Server'></asp:textbox>
<input type=button id='btn1' onclick='javascript:CheckFunction();' value='Click'>
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