Set's the form field's value. For input boxes, this sets the value attribute. For textareas, it sets the innerText. For radio boxes and select boxes, it removes the checked/selected attribute from all, and adds it to the one matching the value. For checkboxes, if the value is non-null and not empty, it checks the box. If you set a value that doesn't exist, it throws an exception if makeNew is false. Otherwise, it makes a new input with type=hidden to keep the value.