State is not updating in useEffect why?

Question

I am trying to make a crypto price tracker website. I am fetching the data after every 10 seconds its updating the coinList variable prevData variable. I want to access the prevData variable inside the useEffect also but inside it the variable is not updating, its value remains []. But if I use prevData outside useEffect the data is changing and I can use it.

I just want the value of prevData inside useEffect which is updating.

function App() {
    const [coinsList, setCoinsList] = useState([]);
    const [prevData, setPrevData] = useState([]);
    let [width, setWidth] = useState(window.innerWidth)
    useEffect(() => {
        async function fetchData() {
            const response = await fetch('api key').then(res => res.json())
setCoinsList(res);
setPrevData(res);
        }
        fetchData()
        async function fetchData2() {
            const res = await fetch('api key').then(res => res.json())
            for (let i = 0; i < prevData.length; i++) {
                if (res[i].current_price > prevData[i].current_price) {
                    let coin = document.getElementById(`${prevData[i].id}`)
                    coin.classList.add('green')
                    console.log(coin)
                    // setTimeout(() => {
                    //     coin.classList.remove('green')
                    // }, 1000)
                } else if (res[i].current_price < prevData[i].current_price) {
                    let coin = document.getElementById(`${prevData[i].id}`)
                    coin.classList.add('red')
                    console.log(coin)

                    // setTimeout(() => {
                    //     coin.classList.remove('red')
                    // }, 1000)
                } else {
                    let coin = document.getElementById(`${prevData[i].id}`)
                    console.log(coin)
                    coin.classList.add('blue')
                    // setTimeout(() => {
                    //     coin.classList.remove('blue')
                    // }, 1000)
                }
            }
            setPrevData(res);
            setCoinsList(res);
        }
        setInterval(() => {
            fetchData2()
            console.log(prevData) // it is not updating
        }, 10000)
    }, [])
    useEffect(() => {
        window.addEventListener('resize', () => {
            setWidth(window.innerWidth)
        })
    }, [window.innerWidth])
    console.log(prevData) // it is updating

user_7882470 · Answer

You have closure on prevData value in setInterval scope, you can use a ref to fix it:

const prevDataRef = useRef();

// Keep the ref updates
useEffect(() => {
  prevDataRef.current = prevData;
}, [prevData]);

// no closure
useEffect(() => {
  setInterval(() => {
    fetchData2(); // use prevDataRef.current
    console.log(prevDataRef.current); // updated
  }, 10000);
}, []);

user_8690857 · Answer

In fetchData2 you've a stale enclosure of the prevData state. It is closed over in the interval callback scope.

One way to remedy is to use a React ref and an additional useEffect hook to cache a copy of the prevData state that can be accessed in the fetchData2 callback.

function App() {
  const [coinsList, setCoinsList] = useState([]);
  const [prevData, setPrevData] = useState([]);

  const prevDataRef = React.useRef(prevData);

  useEffect(() => {
    prevDataRef.current = prevData;
  }, [prevData]);

  ...

  useEffect(() => {
    ...
    async function fetchData2() {
      const prevData = prevDataRef.current; // <-- grab current value

      const res = await fetch('api key').then(res => res.json());

      for (let i = 0; i < prevData.length; i++) {
        if (res[i].current_price > prevData[i].current_price) {
          let coin = document.getElementById(`${prevData[i].id}`);
          coin.classList.add('green');
        } else if (res[i].current_price < prevData[i].current_price) {
          let coin = document.getElementById(`${prevData[i].id}`);
          coin.classList.add('red');
        } else {
          let coin = document.getElementById(`${prevData[i].id}`);
          coin.classList.add('blue');
        }
      }
      setPrevData(res);
      setCoinsList(res);
    }

    setInterval(() => {
      fetchData2();
    }, 10000)
}, []);

State is not updating in useEffect why?

2 Answers