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The state value may turn undefined in the useEffect() hook if it is not properly initialized or if it is updated after the component has already mounted. This can happen if the state value is fetched asynchronously, and there is a delay in receiving the value.

To avoid this, you can use a conditional statement to check if the state value is undefined before using it in the useEffect() hook.

Example:

import React, { useState, useEffect } from 'react';

function App() {
  const [data, setData] = useState(undefined);

  useEffect(() => {
    if(data === undefined) {
      fetchData();
    }
  }, [data]);

  const fetchData = async () => {
    const response = await fetch('https://api.example.com/data');
    const data = await response.json();
    setData(data);
  }

  return (
    <div>
      {data && <p>{data.name}</p>}
    </div>
  );
}

export default App;

In the example above, the fetchData() function is called only if the data state value is undefined. This ensures that the state value is properly initialized before it is used in the useEffect() hook.