How to detect if browser window is scrolled to bottom?

I need to detect if a user is scrolled to the bottom of a page. If they are at the bottom of the page, when I add new content to the bottom, I will automatically scroll them to the new bottom. If they are not at the bottom, they are reading previous content higher on the page, so I don't want to auto-scroll them since they want to stay where they are.

How can I detect if a user is scrolled to the bottom of the page or if they have scrolled higher on the page?

Solution of the problem

window.onscroll = function(ev) {
 if ((window.innerHeight + window.scrollY) >= document.body.offsetHeight) {
 // you're at the bottom of the page
 }
};

See demo