. We can only take .getboundingClientRect () for each element, and then cooperate with document.body.scrollTop to judge that the performance will not be too good.

if you need to determine whether certain elements enter the window, you can use the following method:

var isInViewport = function (elem) {
    var bounding = elem.getBoundingClientRect();
    return (
        bounding.top >= 0 &&
        bounding.left >= 0 &&
        bounding.bottom <= (window.innerHeight || document.documentElement.clientHeight) &&
        bounding.right <= (window.innerWidth || document.documentElement.clientWidth)
    );
};
window.addEventListener('scroll', function (event) {
    if (isInViewport(elem)) { //elem
        
    }
}, false);

I don't know if I understand it correctly. See if this stickUp plugin is the effect you want
http://lirancohen.github.io/s.