Javascript:如何检测浏览器窗口是否滚动到底部?

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.I need to detect if a user is scrolled to the b