useEffect must not return anything besides a function, which is used for clean-up.

1. This mistake is caused by returning something in the useEffect function.

2. Can only return nothing or a clean-up function in useEffect function:

Wrong usage:

 useEffect(()=>getData(),[])

  async function getData() {
    const url = "http://localhost:8080/hello";
    try {
      const response = await fetch(url);
      setData(response)
    } catch (error) {}
  }

1. getData returns a promise because it is declared to be async. This promise will eventually be resolved when asynchronous operations (like fetch) complete.

2. Cannot return a promise in useEffect function. Only return nothing or the clean-up function.

Correct usage: write asynchronous functions in useEffect and call them so that the useEffect function returns nothing.

useEffect(() => {
    async function getData() {
      const url = "http://localhost:8080/hello";
      try {
        const response = await fetch(url);
        setData(response);
      } catch (error) {}
    }
    getData();
  }, []);