attachOnVariantSelectListeners 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-attachOnVariantSelectListeners.js

function attachOnVariantSelectListeners(product) {
  $('.variant-selectors').on('change', 'select', function(event) {
    var $element = $(event.target);
    var name = $element.attr('name');
    var value = $element.val();
    product.options.filter(function(option) {
      return option.name === name;
    })[0].selected = value;

    var selectedVariant = product.selectedVariant;
    var selectedVariantImage = product.selectedVariantImage;
    updateVariantImage(selectedVariantImage);
    updateVariantTitle(selectedVariant);
    updateVariantPrice(selectedVariant);
  });
}