MrYupi

$query->registerRuntimeField("store",[
            "data_type" => "\Bitrix\Catalog\StoreProductTable",
            'reference' => [
                'this.ID' => 'ref.PRODUCT_ID'
            ],
        ]);

        $query->registerRuntimeField("DETAIL_AMOUNT",[
            "data_type" => "integer",
            "expression" => array("sum(%s)", "store.AMOUNT")
        ]);

const className = 'class';
const index = 1;

const element = document.querySelector(`.${className}:nth-child(${index + 1})`);

const element = document.querySelectorAll('.' + className)[index];
// или
const element = document.getElementsByClassName(className).item(index);