gistfile1.js

     var selectCallback = function(variant, selector) {
                        if (variant && variant.available == true) {
                            // selected a valid variant
                            $('#productinfo #detail #purchase').removeClass('disabled').removeAttr('disabled'); // remove unavailable class from add-to-cart button, and re-enable button
                            var newHtml = Shopify.formatMoney(variant.price, "");
                            if(variant.compare_at_price > variant.price) {
                                newHtml += '<div class="was-price">was ' + Shopify.formatMoney(variant.compare_at_price, "") + '</div>';
                            }
                            $('#price-field').html(newHtml);  // update price field
                            
                            // added for Back In Stock app
                            if (variant.inventory_quantity == 0) { // some products are out of stock, but still available
                                $('#BIS_trigger').show();   
                            } else {
                                $('#BIS_trigger').hide();   
                            }
                        } else {
                            // variant doesn't exist
                            $('#productinfo #detail #purchase').addClass('disabled').attr('disabled', 'disabled'); // set add-to-cart button to unavailable class and disable button
                            var message = variant ? "This Size is Sold Out" : "Unavailable";
                            $('#price-field').text(message); // update price-field message
                            $('#BIS_trigger').show();  // added for Back In Stock app
                        }
                    };