let backToTop = function() {

    let scrollToptimer = setInterval(function() {

        let top = document.body.scrollTop || document.documentElement.scrollTop;

        let speed = top / 4;

        if (document.body.scrollTop !== 0) {

            document.body.scrollTop -= speed;

        } else {

            document.documentElement.scrollTop -= speed;

        }

        if (top === 0) {

            clearInterval(scrollToptimer);

        }

    }, 40);

};