Something about scrolling Back

For a more complicated user experience for browsing a website, sometimes we will do more job about animating according to the scroll-bar. This document is mainly talk about something that we should take in consideration.

How to listen to the scrolling of scroll-bars?

Before animating, we should know when users scroll the bar firstly:

window.addEventListener('scroll', function () {
    /** the event handler for listening to the scrolling */    
});

With such a handler, we will be triggered once users scroll.

When to the top and the bottom

If you want to know when users scroll the bar to the top or to the bottom, we can calculate like the following:

/** scroll to the top */
var oTop = document.body.scrollTop === 0 ? document.documentElement.scrollTop : document.body.scrollTop;
var oHeight= document.body.scrollHeight === 0 ? document.documentElement.scrollHeight : document.body.scrollHeight;

if (oTop === 0) {
    /** to the top */
}

if (oTop === oHeight - window.innerHeight) {
    /** to the bottom */
}

Browser Support

With testing, the code snippets mentioned above works in IE11+ or other newer browsers.