[russia091]

(function() {
 var elem = document.querySelector('.opacTop'),
 body = document.body,
 html = document.documentElement;
 isVisible = true;
 
 window.addEventListener('scroll', function() {
 var scrollHeight = (html.scrollHeight || body.scrollHeight) - this.innerHeight,
 scrollTop = html.scrollTop || body.scrollTop || 0;
 elem.style.opacity = 1 - 7 / scrollHeight * scrollTop;
 elem.style.opacity = value;
 if(isVisible != !!value) {
 elem.style.display = isVisible ? 'none' : '';
 isVisible = !isVisible;
 }
 });
})();

Я верно прописал?

Потому что не срабатывает.