Javascript Promise: Return 'In Progress' Response Until Promise Resolves?

Question

This isn't really an Apollo question, it's a Javascript promises question, but uses an example from Apollo, because that's the only time I recall seeing it.

Apollo has a React hook that looks like this:

const { loading, error, data } = useQuery(GET_DOGS);

I understand how it returns error -- if the promise resolver throws an error, you get an error back.

I understand how it returns data -- when the promise resolver completes, it returns the data.

But how does it return loading and then later return data? I've coded quite a few node.js promise resolvers and haven't yet seen a pattern that could return loading while the operation is in process, and then later return the data.

What Javascript pattern makes this possible?

Answer 1

They'd use a state variable that starts true and is switched to false when they're done, vaguely like this:

function useQuery(/*...*/) {
    const [loading, setLoading] = useState(true);
    const [error, setError] = useState(null);
    const [data, setData] = useState(null);

    useEffect(() => {
        let cancelled = false;
        goGetTheStuff()
        .then(data => {
            if (!cancelled) {
                setData(data);
                setLoading(false);
            }
        })
        .catch(error => {
            if (!cancelled) {
                setError(error);
                setLoading(false);
            }
        });
        return () => {
            cancelled = true;
        };
    }, []);

    return {loading, error, data};
}

Live Example:

const {useState, useEffect} = React;

function goGetTheStuff() {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            if (Math.random() < 0.7) {
                // Emulate success
                resolve({data: "here"});
            } else {
                // Emulate failure
                reject(new Error("Couldn't get the data"));
            }
        }, 800);
    });
}

function useQuery(/*...*/) {
    const [loading, setLoading] = useState(true);
    const [error, setError] = useState(null);
    const [data, setData] = useState(null);

    useEffect(() => {
        let cancelled = false;
        goGetTheStuff()
        .then(data => {
            if (!cancelled) {
                setData(data);
                setLoading(false);
            }
        })
        .catch(error => {
            if (!cancelled) {
                setError(error);
                setLoading(false);
            }
        });
        return () => {
            cancelled = true;
        };
    }, []);

    return {loading, error, data};
}

function Example() {
    const {loading, error, data} = useQuery();
    return (
        <div>
            <div>loading: {JSON.stringify(loading)}</div>
            <div>data: {data && JSON.stringify(data)}</div>
            <div>error: {error && error.message}</div>
        </div>
    );
}

ReactDOM.render(<Example/>, document.getElementById("root"));

<div>70% of the time when you run this, the async operation succeeds; 30% of the time, it fails. Run repeatedly if you want to see both scenarios.</div>
<hr>
<div id="root"></div>

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.13.0/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.13.0/umd/react-dom.production.min.js"></script>