Theo's Typesafe Cult•17mo ago

Why does react rerender this at the component level but not when using a hook?

I've been toying with a type-writer effect in react and cant understand why react's lifecycle is working this way. Can someone explain why react is re-rendering the <WithoutHook/> component such that it doesnt work While the <WithHook/> one does? https://codesandbox.io/s/damp-moon-v24dzf?file=/src/App.jsx Notice 'strict mode' is on and if you disable it, it still breaks when not using a hook.

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damp-moon-v24dzf - CodeSandbox

damp-moon-v24dzf using react, react-dom, react-scripts

8 Replies

Unknown User•17mo ago

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jakeman•17mo ago

Huh, this is really interesting. Im still confused on how changing the index from a ref to a variable inside the effect schedules it to run after setTypingText(). I remember reading somewhere that setState is async. Any concrete examples of this? Or in other words do you need to await setTypingText in this case?

Unknown User•17mo ago

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jakeman•17mo ago

After more testing this works:

export const WithoutHook = () => {
  const [typingText, setTypingText] = useState("");
  const text = "hello";
  const index = useRef(0);
  
  useEffect( () => {
    const interval = setInterval(() => {
      if (index.current < text.length) {
          let newText = text[index.current];
         setTypingText((prevMessage) => prevMessage + newText);
      } else {
        clearInterval(interval);
      }
      index.current++;
    }, 500);
    return () => clearInterval(interval);
  }, []);

  return (
    <div>
      <h1>{typingText}</h1>
    </div>
  );
};

export const WithoutHook = () => {
  const [typingText, setTypingText] = useState("");
  const text = "hello";
  const index = useRef(0);
  
  useEffect( () => {
    const interval = setInterval(() => {
      if (index.current < text.length) {
          let newText = text[index.current];
         setTypingText((prevMessage) => prevMessage + newText);
      } else {
        clearInterval(interval);
      }
      index.current++;
    }, 500);
    return () => clearInterval(interval);
  }, []);

  return (
    <div>
      <h1>{typingText}</h1>
    </div>
  );
};

the line that fixes it is let newText = text[index.current]; Still confused hahha No doubt the most error prone aspect of react (atleast that I run into)

Unknown User•17mo ago

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jakeman•17mo ago

Than how do you clear the interval?

Unknown User•17mo ago

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jakeman•17mo ago

Yeah makes sense. Thanks for the help dude

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Why does react rerender this at the component level but not when using a hook?