You are scraping some external site(s). You can't be sure the HTML all fits exactly the same structure, so you need to be defensive on how you traverse it.
var product = $('#product');
if (!product) return console.log('Cannot find a product element');
var productClass = product.attr('class');
if (!productClass) return console.log('Product element does not have a class defined');
var productNumber = productClass.split(/\s+/)[3].split("_")[1];
console.log(productNumber);
This'll help you debug where things are going wrong, and perhaps indicate that you can't scrape your dataset as easily as you'd hoped.