javascript - How to change scrollbar position?

Question

Is it possible to change the scrollbar position when the user reaches a certain point scrolling down a web page? For example once you reached half way down down the page the scrollbar would move automatically back to the top.

Answer 1

You can calculate the percentage of the current position of the scrollbar using the onscroll event, and if it reaches the 50 % the scroll position can be set to the top of the page with the scrollTo function:

window.onload = function () { 
  window.onscroll = function () { 
    var doc = document.body, 
    scrollPosition = doc.scrollTop,
    pageSize = (doc.scrollHeight - doc.clientHeight),
    percentageScrolled = Math.floor((scrollPosition / pageSize) * 100); 

     if (percentageScrolled >= 50){ // if the percentage is >= 50, scroll to top
       window.scrollTo(0,0); 
     } 
   }; 
};

You can check my example here.