gistfile1.js

var selectCallback = function(variant, selector) {
  if (variant && variant.available == true) {
    // selected a valid variant
    jQuery('#add-to-cart').removeClass('disabled').removeAttr('disabled').val('Add to Cart'); // remove unavailable class from  add-to-cart button, and re-enable button
    if(variant.price < variant.compare_at_price){
      jQuery('#price-preview').html(Shopify.formatMoney(variant.price) + " <span>was " + Shopify.formatMoney(variant.compare_at_price) + "</span>");
    } else {
      jQuery('#price-preview').html(Shopify.formatMoney(variant.price));
    }
    jQuery('#BIS_trigger').hide(); // This line added for Back In Stock app
  } else {
    // variant doesn't exist
    var message = variant ? "Sold Out" : "Unavailable";
    jQuery('#add-to-cart').addClass('disabled').attr('disabled', 'disabled').val('Sold Out');      // set add-to-cart button to unavailable class and disable button
    jQuery('#product .variants .price').text(message); // update price-field message
    jQuery('#BIS_trigger').show(); // This line added for Back In Stock app
  }
};