mrzgt

function scrollTo (id) {
        var y = $('#'+id).position().top;
            y = y+30;
        scrollTo(y, 800);
        //тут клик по табу $('a[href=\'#specification\']').trigger('click');
    };

Math.easeInOutQuad = function (t, b, c, d) {
        t /= d/2;
        if (t < 1) {
            return c/2*t*t + b
        }
        t--;
        return -c/2 * (t*(t-2) - 1) + b;
    };
var requestAnimFrame = (function(){
        return  window.requestAnimationFrame || window.webkitRequestAnimationFrame || window.mozRequestAnimationFrame || function( callback ){ window.setTimeout(callback, 1000 / 60); };
    })();

    function offsetPosition(element) {
        var offsetLeft = 0, offsetTop = 0;
        do {
            offsetLeft += element.offsetLeft;
            offsetTop  += element.offsetTop
        } while (element = element.offsetParent);
        return [offsetLeft, offsetTop]
    }
function scrollTo(to, duration, callback) {
        if(!to) return;
        // если передаю ид элемента а не число
        if(typeof to === 'string') {
            to = offsetPosition(document.querySelector(to))[1]
        }

        // because it's so fucking difficult to detect the scrolling element, just move them all
        function move(amount) {
            document.documentElement.scrollTop = amount;
            document.body.parentNode.scrollTop = amount;
            document.body.scrollTop = amount;
        }
        function position() {
            return document.documentElement.scrollTop || document.body.parentNode.scrollTop || document.body.scrollTop;
        }
        var start = position(),
            change = to - start,
            currentTime = 0,
            increment = 20;
        duration = (typeof(duration) === 'undefined') ? 500 : duration;
        var animateScroll = function() {
            // increment the time
            currentTime += increment;
            // find the value with the quadratic in-out easing function
            var val = Math.easeInOutQuad(currentTime, start, change, duration);
            // move the document.body
            move(val);
            // do the animation unless its over
            if (currentTime < duration) {
                requestAnimFrame(animateScroll);
            } else {
                if (callback && typeof(callback) === 'function') {
                    // the animation is done so lets callback
                    callback();
                }
            }
        };
        animateScroll();
    }

class='btn-instock' onclick=\"instock.add('<?php echo $product_id; ?>');\"

$(".mfp-bottom-bar").prepend("<div class='hidden-xs'><?php if (!$special) { ?><span class='priceproduct-new'><?php echo $price; ?></span><?php } else { ?><span class='priceproduct-new'><?php echo $special; ?></span>&nbsp;&nbsp;<span class='priceproduct-old'><?php echo $price; ?></span><?php } ?> <?php if ($quantity <= 0 && $status ) { ?><button type='button' class='btn-instock' onclick=\"instock.add('<?php echo $product_id; ?>');\" data-product-id='<?php echo $product_id; ?>'><?php echo $button_instock; ?></button><?php } else { ?><button type='button' id='button-cart' data-loading-text='' class='btn-cart'> <?php echo $button_cart; ?></button><?php } ?></div>");

onclick="window.open('site.ru?addCart=<?php echo $product['product_id']; ?>')"

function $_GET(key) {
    var p = window.location.search;
    p = p.match(new RegExp(key + '=([^&=]+)'));
    return p ? p[1] : false;
}

window.onload = function() {
    cart.add($_GET('addCart'));
}

$sql = "SELECT * FROM " . DB_PREFIX . "review2 WHERE 1=1";

SELECT  t1.email
FROM  " . DB_PREFIX . "stock AS t1
INNER JOIN product ON (product_id =  '" . (int) $product_id."' AND status = 0)

data: {
input: $('input[name=\'aridiusundersell_contact\'],input[name=\'aridiusundersell_firstname\'],input[name=\'aridiusundersell_email\']'),
product: 'product_id=' + product_id
}

'add': function(product_id) { // в этой строке объявляется функция, которая ожидает в качестве аргумента product_id

/* *** */

onclick="instock.add('<?php echo $product['product_id']; ?>');" // при клике вызывается объявленная ранее функция и ей передается product_id

<!-- Кнопке добавлется data-атрибут "data-product-id", куда записывается product_id: -->
<button type="button" data-product-id="<?= $product['product_id'];?>" data-toggle="tooltip" title="<?php echo $button_wishlist; ?>" onclick="instock.add('<?php echo $product['product_id']; ?>');"><?php echo $buttonundersell_title; ?></button>

$(document).on("click touchstart", "#undersell-order-submit", function (product_id) {
    var product_id = $(this).closest('form').find('button[data-product-id]').data('product-id'); // Получение product_id из data-атрибута.

    $.ajax({
        /* *** */