Check if the bottom of the page is visible using JavaScript

A very common need in web development is to check if the bottom of the page is visible. This can be useful for lazy loading content, infinite scrolling, or other similar features. While JavaScript does't provide a built-in check method or property for this, all you need to implement it yourself is a little math.
Given an HTML element, you can use Element.scrollHeight to get the total height of the element, and Element.clientHeight to get the height of the visible part of the element. Applying that to the Document.documentElement will give you the height of the entire page and the height of the visible part of the page.
Then, using Window.scrollY, you can get the current scroll position of the window. Finally, by adding the client height and scroll position, you can determine if the bottom of the page is visible.

const bottomVisible = () =>
  document.documentElement.clientHeight + window.scrollY >=
  (document.documentElement.scrollHeight ||
    document.documentElement.clientHeight);

bottomVisible(); // true

Then, using Window.scrollY, you can get the current scroll position of the window. Finally, by adding the client height and scroll position, you can determine if the bottom of the page is visible.

From 30-seconds-of-code: bottom-visible