for those who wish to do this as an override--really preferable to a core hack which is hard to keep track of. i did create the file /templates/MYTEMPLATE/html/com_hikashop/hikashop_cart_js_render.php as above and then just included the following:
$js='';
if($this->cartCount()!=1 && !empty($url)){
$js = 'window.parent.location = \''.urldecode($url).'\';';
}
$js = '
function hikashopModifyQuantity(id,obj,add,form){
if(add){
add=\'&add=1\';
}else{
add=\'\';
}
var qty=1;
if(obj){
qty=parseInt(obj.value);
}
if(form){
var varform = eval(\'document.\'+form);
if(varform){
varform.submit();
}
}else{
try{
new Ajax(\''.$baseUrl.'product_id=\'+id+\'&quantity=\'+qty+add+\''.$url_itemid.'&return_url='.urlencode(base64_encode(urldecode($url))).'\', { method: \'get\', onComplete: function(result) { var hikaModule = window.document.getElementById(\'hikashop_cart_module\'); if(hikaModule){ hikaModule.innerHTML = result;} '.$js.'}}).request();
}catch(err){
new Request({url:\''.$baseUrl.'product_id=\'+id+\'&quantity=\'+qty+add+\''.$url_itemid.'&return_url='.urlencode(base64_encode(urldecode($url))).'\', method: \'get\', onComplete: function(result) { var hikaModule = window.document.getElementById(\'hikashop_cart_module\'); if(hikaModule){ hikaModule.innerHTML = result;} '.$js.'}}).send();
}
}
return false;
}
';
}
which is the code with the modification from the cart php page. but just the part of the cart.php page relevant to that function.