What exactly does it mean that a component unmounts in React? Does it disappear after it's unmounted?

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let say I have a functional Home component:

const Home = () => {
  console.log('outside useEffect')
  useEffect(() => {
    console.log('inside useEffect')
    
    // cleanup function
    return () => { console.log('unmounted') }
  }, [])

  return <div>Hello Word</div>
}

The output in the console shows:

outside useEffect
inside useEffect
unmounted
inside useEffect

My questions are:

  1. When does the component unmount, does it happen automatically after it has rendered once or when?
  2. What exactly does happen when the component unmounts? Does the component get deleted/removed? If yes then how am I still able to see the Hello World even if it unmounts.
reactjs javascript
2 Answers

Normally a component gets unmounted when for example it's rendered conditionally and the conditional become false. DummyComponent below gets unmounted every time state become false. And when a component is unmounted, it's removed from the DOM, and therefore you won't see it in the page.

const DummyComponent = () => {
  useEffect(() => {
    console.log("mounted");
    return () => {
      console.log("unmounted");
    };
  }, []);
  return <div>Hello Word</div>;
};

const App = () => {
  const [state, setState] = useState(true);
  return (
    <div>
      <button onClick={() => setState(!state)}>Toggle Dummy Component</button>
      {state ? <DummyComponent /> : <></>}
    </div>
  );
};

What you have right now is introduced by React v18 where each component gets mounted and immediately unmounted and mounted again, when you are in strict development mode. I explained the reason why in this QA on Stack Overflow. And it's so quick that you are not noticing. The production mode is as before, explained in the first section.

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