add class

scroll.js

const scroll = fromEvent(window, 'scroll');
scroll.subscribe(showSection);

function showSection() {
    const sectionObs = section => {
        const interSection = new IntersectionObserver(entrie => {
            entrie[0].intersectionRatio > 0.15 ?
                !section.classList.contains('active') && section.classList.add('active')
                : section.classList.remove('active');
        });

        interSection.observe(section);
    };
    forEach(sectionObs, sectionsForAnimate);
}