How to get the 'controlToValidate' property on ClientValidationFunction?

Not verified, just a hint:

var v = document.getElementById('<%=CustomValidator1.FindControl(CustomValidator1.ControlToValidate).ClientID>%');

of course you could simply do it like:

var v = document.getElementById('<%=TextBox1.ClientID%>');

if you know exactly what you're validating. The first method is good when the control to be validated is set dynamically and you don't know beforehand which one it will be.

Also FindControl() might return null so you'd need to test for that too in order to avoid an exception.

Hope this helps.

Actually sender.controltovalidate gives the ClientID of the control. So this seems like a solution.

function ValidationFunction1(sender, args){
    var v = document.getElementById(sender.controltovalidate);
}

I tried and it worked for me. Please notify if it works.