Hi,
Because "hikashop.js" is a "core file" of HikaShop ; it will be updated during each update/install.
If you want to override the function, you can copy the javascript function in another file and include that file after the hikashop.js file in your website.
It will re-define the function and the HikaShop update won't modify your specific file.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.