var targetNode = document.getElementsByClassName('containerModal')[0];
var observer = new MutationObserver(function() {
    if (targetNode.style.opacity == '0') {
        document.body.style.overflow = "visible";
    }
    if (targetNode.style.opacity == '1') {
        document.body.style.overflow = "hidden";
    }
});
observer.observe(targetNode, {
    attributes: true,
    childList: true
});