Store updates JavaScript for Shopify Buy Button - https://www.shopify.com/partners/blog/four-smart-reasons-to-embed-a-shopify-store-in-your-client-websites

four-smart-reasons-to-embed-shopify-store-updates.js

var variantSelectors = generateSelectors(product);
$('.variant-selectors').html(variantSelectors);


function generateSelectors(product) {
  var elements = product.options.map(function(option) {
    return '<select name="' + option.name + '">' + option.values.map(function(value) {
      return '<option value="' + value + '">' + value + '</option>';
    }) + '</select>';
  });

  return elements;
}