function isInViewport(selector, addClass, descBot, descTop, mobBot, mobTop) {

    let viewport = document.querySelectorAll(selector);

    function checkPort() {

        viewport.forEach((element, index) => {

            // calculate distanse element from sides of browser
            let positionTop = element.getBoundingClientRect().top,
                positionBot = element.getBoundingClientRect().bottom;

            // console.log(positionTop, positionBot)

            if (window.innerWidth > 767) {

                dispersion(descBot = 300, descTop = 400);

            } else {

                dispersion(mobBot = 200, mobTop = 200);

            }

            // function calc dispersion
            function dispersion(bottom, top) {

                if (bottom < positionBot && positionTop < top) {
                    element.classList.add(addClass);
                } else {
                    element.classList.remove(addClass);
                }

            }


        })


    }

    document.addEventListener('scroll', checkPort);
}

// usage is in you port
isInViewport('.js-viewport', 'about__row--in-view-port');
// or
// isInViewport('.js-viewport', 'about__row--in-view-port', 400, 300, 200, 200);