for asp.net button we have onclick event that will be write on the code behind file by using jquery we can easily stop event handler byusing event.preventDefault

By using javascript

we write 

<script type="text/javascript">
function Javasciptfunc()
{

if(ourcondition)
{

return true;
}
else
{
return false;

}

}
</script>


<asp:Button id="btn1" runat="server" onclick="myeventhandler" onclientclick="return Javasciptfunc()"/>

if you want write more condition write

<script type="text/javascript">
function Javasciptfunc()
{

if(ourcondition)
{

//dont return anything
}
else
{
return false;

}
if(another condition)
{

return true;

}
else
{

return false;
}

}
</script>


<asp:Button id="btn1" runat="server" onclick="myeventhandler" onclientclick="return Javasciptfunc()"/>