Modify this javascript

This function copiies down a textbox. It clones it however, I prefer to get a blank /or no contents of teh copied textbox..

function addTextbox(theBoxID){
 theBox = Document.GetElementById(theBoxID);
 theParent = theBox.parentNode;
 newBox = theBox.CloneNode(true);
 theParent.appendChild(newBox);
}