HI,
This is done in the file administrator/components/com_hikashop/types/characteristic.php
But that would mean that you would hack a core file of HikaShop and you would loose your changes after each update.
A cleaner solution would be to edit the file "show_block_characteristic" via the menu Display>Views and change the line:
echo $this->characteristic->displayFE($this->element, $this->params) . '</div>';
You could for example write:
echo str_replace('onclick="','onclick="mycustomJSFunction(this);',$this->characteristic->displayFE($this->element, $this->params) . '</div>');
and then you could do like that in order to add your custom JS:
$js="
function mycustomJSFunction(characteristicDropdownElement){
if(characteristicDropdownElement.value=='value1'){
//do your stuff here
}
}
";
$doc = JFactory::getDocument();
$doc->addScriptDeclaration("\n<!--\n".$js."\n//-->\n");
That way, you won't loose your changes during updates.