Can someone help me understand how async + await + useEffect work in React?

I have a React app built with the Minimal template and I'm trying to follow along with one of their tutorials, in order to create a Redux slice that feeds some data to a custom component. The data itself is collected from Firebase. Below is my code:

firebase.js - helper

export function getDocuments(col) {
    const colRef = collection(db, col);
    const q = query(colRef, where('uid', '==', auth.currentUser.uid));

    getDocs(q).then((snap) => {
        const data = snap.docs.map((d) => ({ id: d.id, ...d.data() }));
        return data;
    });
   // return [1,2,3]
}

product.js - Redux slice

export function getProducts() {
    return async (dispatch) => {
        dispatch(slice.actions.startLoading());
        try {
            const products = await getDocuments('products');
            dispatch(slice.actions.getProductsSuccess(products));
        } catch (error) {
            dispatch(slice.actions.hasError(error));
        }
    };
}

ProductList.js - component

const dispatch = useDispatch();
const { products } = useSelector((state) => state.client);

useEffect(() => {
    dispatch(getProducts());
}, [dispatch]);

useEffect(() => {
    if (products.length) {
        // setTableData(products);
    }
}, [products]);

If I console log data in the helper function (firebase.js), I get the values I expect, once the promise is resolved/fulfilled. However, if I console.log clients in the product.js slice or later in the component, I get undefined. I assume my problem is not being able to understand how async + await + useEffect work together in order to fix this. My assumption is that I am trying to access the value before the promise is resolved and therefore before the helper function returns it. I confirmed that by returning a simple array [1, 2, 3] in my helper function as a test.

I think I am missing something fundamental here (I am not very experienced with React and JS in general and still learning things on the go). Can someone help me understand what am I doing wrong?

Thank you!

2 answers

  • answered 2022-05-06 20:38 gavin

    Hope this helps :)

    Async and Await are no different in React than in plain JavaScript: When the await keyword is applied, it suspends the calling method and yields control back to its caller until the awaited task is complete. await can only be used inside an async method

    useEffect(): By using this Hook, you tell React that your component needs to do something after rendering. This function will run every time the component is re-rendered.

  • answered 2022-05-06 21:03 Adrian Schweizer

    With await you can await the fulfillment or rejection of a promise, but your getDocuments Function does not return a promise. Change the last line of the function to the following:

    return getDocs(q).then((snap) => {
        const data = snap.docs.map((d) => ({ id: d.id, ...d.data() }));
        return data;
    });
    

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