How to run useEffect part first in React-Hooks?

Question

How to execute the axios part and send the updated states props to Important component.

When I console.log I see that state passed as props with an empty object but after a fraction of seconds again states is updated with a new fetched value that means my return is running first then my usEffect axios part is running,

How can I make sure that axios part should run first then my return part. In first go updated part should be sent not the blank empty part

const initialState = {
    Important: [{}],
    Error: false
}

const reducer = (state, action) => {
    switch (action.type) {
        case "STEPFIRST":
            return {
                ...state,
                Important: action.payload,
            };
        case "STEPSecond":
            return {
                Error: true,
            };
        default:
            return state;
    }
}

const Landing = () => {
    const [states, dispatch] = useReducer(reducer, initialState)
    console.log(states)

    useEffect(() => {
        axios.get("https://example.com/")
            .then(response => {
                dispatch({
                    type: "STEPFIRST",
                    payload: response.data
                });
            })
            .catch(error => {
                dispatch({
                    type: "STEPSecond"
                });
            });
    },[]);

    const [xyz, xyzfn] = useState();
    console.log(xyz)

    return (
        <div>
        <Important states = {states} xyzfn={xyzfn} />
        <Foo xyz={xyz}/> 
        </div>
    );
};

export default Landing;

Jagrati · Accepted Answer · 2020-05-04 04:38:44Z

useEffect will always run after first rendering is done. You can have a loading state in your state and return the component accordingly.

const initialState = {
    Important: [{}],
    Error: false,
    isLoading: true
}

const reducer = (state, action) => {
    switch (action.type) {
        case "STEPFIRST":
            return {
                ...state,
                Important: action.payload,
                isLoading: false
            };
        case "STEPSecond":
            return {
                Error: true,
                isLoading: false
            };

        default:
            return state;
    }
}


const Landing = () => {
    const [states, dispatch] = useReducer(reducer, initialState)

    console.log(states)

    useEffect(() => {
        axios.get("https://example.com/")
            .then(response => {
                dispatch({
                    type: "STEPFIRST",
                    payload: response.data
                });
            })
            .catch(error => {
                dispatch({
                    type: "STEPSecond"
                });
            });
    },[]);

    const [xyz, xyzfn] = useState();
    console.log(xyz)

   if(state.isLoading){
       return <div>Loading....</div>
   }

    return (
        <div>
        <Important states = {states} xyzfn={xyzfn} />
        <Foo xyz={xyz}/> 
        </div>
    );
};

Dennis Vash · Accepted Answer · 2020-05-03 19:01:26Z

0

useEffect callback runs after the render phase.

Also, fetch calls are asynchronous, so you want to use conditional rendering:

const Landing = () => {
  const [states, dispatch] = useReducer(reducer, initialState);

  useEffect(() => {
    axios
      .get("https://example.com/")
      .then((response) => {
        dispatch({
          type: "STEPFIRST",
          payload: response.data,
        });
      })
      .catch((error) => {
        dispatch({
          type: "STEPSecond",
        });
      });
  }, []);

  // Use any comparison function to indicate that `states` changed.
  // like deep comparison function `isEqual` from lodash lib.
  return (
    <div>
      {!lodash.isEqual(states, initialState) && (
        <Important states={states} xyzfn={xyzfn} />
      )}
    </div>
  );
};

edited May 3, 2020 at 19:01

answered May 3, 2020 at 18:47

Dennis Vash

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2 Comments

puru Over a year ago

Hi @Dennis Vash I didn't understand this part !lodash.isEqual(states, initialState) . what exactly is lodash.isEqual?

Dennis Vash Over a year ago

See the comment in the answer, its a function from lodash library, you can google it. You need somehow to tell if states is equal to your initialState.

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