How do I pass a server control's actual client Id to a javascript function?

Question

I have the following textbox server control in my web page:

Answer 1

function ZipCode_OnChange(element, txtCity, txtState) {
    var ClientId = element.getAttribute('id'),
        ret = null,
        value = element.value;

    ret = WebService.GetCityAndState(value, OnComplete1, OnError, ClientId);
}