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useEffect(fn, []) is not the new componentDidMount()

  • They’re actually mechanically different, so you might not get what you expect if consider them the same (which we talk about below).
  • Thinking in terms of time, like “call my side effect once on mount” can hinder your learning of hooks.
  • Refactoring from classes to hooks will not mean you simply replace your componentDidMount’s with useEffect(fn, []).