Is that JSON invalid?
$.getJSON() is an async function. You cannot do
var data = $.getJSON("http://ocart.com/index.php?route=ajaxhelper/test&product_id=" + product_id);return data;Instead, you have to process the results when it comes back:
$.getJSON("http://ocart.com/index.php?route=ajaxhelper/test&product_id=" + product_id) .then(function(data){ //Do what you need to do with data });