Edge case here, but what if you want to know that a page has really completed loading external to React code? For instance, you have React components, and you can check for DOMContentLoaded, however, it seems rendering is still occurring even when that event fires.

In Rails, for instance, you might need to check in some other view to determine whether or not React 'managed/rendered' input components have really completed loading and then fire some callback that's outside the scope of React.

Is there no or could there not be some custom event that gets fired when React is "really done"?

Disclaimer: I Googled my brains out on this one and kept being met with, try useEffect, however, this is "how do I do this 'external' to React, so to speak.