calling onchange event of a dropdown list when page refresh and pageload

Has a dropdownlist on page with onchange="hideControls(this.value) depending on the user selected values the controls will visible and be hidden

here is function

function hideControls(ddid)
{
// to hide and show onchange
switch
case ' FirstName':
break;
case ' DOB':
break;
default:
break;

}
when i refresh the page and on pageload i am trying to call this function and switc case to default...how to do this..thanks
i didn't wrote all the code here...i wrote just sample, just i need to know how to call default in switch case........how to pass that value on page refresh and page load..i am not getting that