Assignments to the 'data' variable from inside React Hook useEffect will be lost after each render

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There is a complete warning message: Assignments to the 'data' variable from inside React Hook useEffect will be lost after each render. To preserve the value over time, store it in a useRef Hook and keep the mutable value in the '.current' property. Otherwise, you can move this variable directly inside useEffect.

I am trying to use data state which is in the useState Hook in Material Table to display the data. That's why i change const to let, But its keep giving me a warning and i dont know how to use useRef Hook with useState Hook.

const filterrr = () => {
    
    
            if (age.length !== 0) {
              return true;
            } 
            else {
              return false;
            }
        
          }
    
          let [data, setData] = useState(Data);
    
    
          useEffect(() => {
    
    
            console.log(Data);
            data = Data.filter(filterrr);
            setData(data);
          }, [age])

so i am trying to use useState outside the useEffect so other functions can use data state.

material-table react-hooks reactjs
1 Answers

React useState() variables are immutable i.e. you cannot directly modify data just by declaring it with the let keyword. To clear this warning, modify your code as follows:

const filterrr = useCallback(() => {
  if (age.length !== 0) {
    return true;
  } else {
    return false;
  }
}, [age]);

const [data, setData] = useState(Data);

useEffect(() => {
  console.log("oldData", data);
  const newData = Data.filter(filterrr);
  console.log("newData", newData);
  setData(newData);
}, [filterrr]);

I have used useCallback() to wrap the filterrr() method so that we can add it to useEffect() dependencies array, this is a good practice in general.

If you don't need filterrr() method elsewhere in your code, simply move it inside useEffect() without the useCallback():

useEffect(() => {
  const filterrr = () => {
    if (age.length !== 0) {
      return true;
    } else {
      return false;
    }
  };
  console.log("oldData", data);
  const newData = Data.filter(filterrr);
  console.log("newData", newData);
  setData(newData);
}, [age]);
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