$(document).ready(function () {


    // scroll section on
    $(window).on('scroll', function () {
        var pageYOffset = window.pageYOffset + screen.height / 2;

        document.querySelectorAll('.section').forEach(function (elem) {
            if (pageYOffset > elem.offsetTop) {
                elem.classList.add('on');
            } else {
                elem.classList.remove('on');
            }
        });

    });



});