It's just a small leap from the examples you linked to, let's assume you have isElementInViewport
from your example link, test each page's <div>
for being in the viewport until you get a match, then return that Node's id .
var pages = document.getElementsByClassName('pagina');
function getCurrentPageId() {
var i;
for (i = 0; i < pages.length; ++i)
if (isElementInViewport(pages[i]))
return pages[i].getAttribute('id');
}