Using async/await inside a React functional component

I'm just beginning to use React for a project, and am really struggling with incorporating async/await functionality into one of my components.

I have an asynchronous function called fetchKey that goes and gets an access key from an API I am serving via AWS API Gateway:

const fetchKey = async authProps => { try { const headers = { Authorization: authProps.idToken // using Cognito authorizer }; const response = await axios.post( "", API_GATEWAY_POST_PAYLOAD_TEMPLATE, { headers: headers } ); return response.data.access_token; } catch (e) { console.log(`Axios request failed! : ${e}`); return e; }
};

I am using React's Material UI theme, and waned to make use of one of its Dashboard templates. Unfortunately, the Dashboard template uses a functional stateless component:

const Dashboard = props => { const classes = useStyles(); const token = fetchKey(props.auth); console.log(token); return ( ... rest of the functional component's code

The result of my console.log(token) is a Promise, which is expected, but the screenshot in my Google Chrome browser is somewhat contradictory - is it pending, or is it resolved?

Second, if I try instead token.then((data, error)=> console.log(data, error)), I get undefined for both variables. This seems to indicate to me that the function has not yet completed, and therefore has not resolved any values for data or error. Yet, if I try to place a

const Dashboard = async props => { const classes = useStyles(); const token = await fetchKey(props.auth);

React complains mightily:

> react-dom.development.js:57 Uncaught Invariant Violation: Objects are
> not valid as a React child (found: [object Promise]). If you meant to
> render a collection of children, use an array instead.
> in Dashboard (at App.js:89)
> in Route (at App.js:86)
> in Switch (at App.js:80)
> in div (at App.js:78)
> in Router (created by BrowserRouter)
> in BrowserRouter (at App.js:77)
> in div (at App.js:76)
> in ThemeProvider (at App.js:75)

Now, I'll be the first to state I don't have enough experience to understand what is going on with this error message. If this was a traditional React class component, I'd use the this.setState method to set some state, and then go on my merry way. However, I don't have that option in this functional component.

How do I incorporate async/await logic into my functional React component?

Edit: So I will just say I'm an idiot. The actual response object that is returned is not response.data.access_token. It was response.data.Item.access_token. Doh! That's why the result was being returned as undefined, even though the actual promise was resolved.

4 Answers

You will have to make sure two things

• useEffect is similar to componentDidMount and componentDidUpdate, so if you use setState here then you need to restrict the code execution at some point when used as componentDidUpdate as shown below:

function Dashboard() { const [token, setToken] = useState(''); useEffect(() => { // You need to restrict it at some point // This is just dummy code and should be replaced by actual if (!token) { getToken(); } }, []); const getToken = async () => { const headers = { Authorization: authProps.idToken // using Cognito authorizer }; const response = await axios.post( "", API_GATEWAY_POST_PAYLOAD_TEMPLATE, { headers } ); const data = await response.json(); setToken(data.access_token); }; return ( ... rest of the functional component's code );
}

With React Hooks, you can now achieve the same thing as Class component in functional component now.

import { useState, useEffect } from 'react';
const Dashboard = props => { const classes = useStyles(); const [token, setToken] = useState(null); useEffect(() => { async function getToken() { const token = await fetchKey(props.auth); setToken(token); } getToken(); }, []) return ( ... rest of the functional component's code // Remember to handle the first render when token is null

Also take a look at this: Using Async await in react component

Component might unmount or re-render with different props.auth before fetchKey is resolved:

const Dashboard = props => { const classes = useStyles(); const [token, setToken] = useState(); const [error, setError] = useState(); const unmountedRef = useRef(false); useEffect(()=>()=>(unmountedRef.current = true), []); useEffect(() => { const effectStale = false; // Don't forget ; on the line before self-invoking functions (async function() { const response = await fetchKey(props.auth); /* Component has been unmounted. Stop to avoid "Warning: Can't perform a React state update on an unmounted component." */ if(unmountedRef.current) return; /* Component has re-rendered with different someId value Stop to avoid updating state with stale response */ if(effectStale) return; if(response instanceof Error) setError(response) else setToken(response); })(); return ()=>(effectStale = true); }, [props.auth]); if( error ) return <>Error fetching token...{error.toString()}</> if( ! token ) return <>Fetching token...</> return //... rest of the functional component's code

An alternative is using Suspense and ErrorBoundary:

// render Dashboard with <DashboardSuspend>
const Dashboard = props => { const classes = useStyles(); const [token, idToken] = props.tokenRef.current || []; // Fetch token on first render or when props.auth.idToken has changed if(token === void 0 || idToken !== props.auth.idToken){ /* The thrown promise will be caught by <React.Suspense> which will render it's fallback until the promise is resolved, then it will attempt to render the Dashboard again */ throw (async()=>{ const initRef = props.tokenRef.current; const response = await fetchKey(props.auth); /* Stop if tokenRef has been updated by another <Dashboard> render, example with props.auth changed causing a re-render of <DashboardSuspend> and the first request is slower than the second */ if(initRef !== props.tokenRef.current) return; props.tokenRef.current = [response, props.auth.idToken]; })() } if(props.tokenRef.current instanceof Error){ /* await fetchKey() resolved to an Error, throwing it will be caught by <ErrorBoundary> which will render it's fallback */ throw props.tokenRef.current } return //... rest of the functional component's code
}
const DashboardSuspend = props => { /* The tokenRef.current will reset to void 0 each time this component is mounted/re-mounted. To keep the value move useRef higher up in the hierarchy and pass it down with props or useContext. An alternative is using an external storage object such as Redux. */ const tokenRef = useRef(); const errorFallback = (error, handleRetry)=>{ const onRetry = ()=>{ // Clear tokenRef otherwise <Dashboard> will throw same error again tokenRef.current = void 0; handleRetry(); } return <> Error fetching token...{error.toString()} <Button onClick={onRetry}>Retry</Button> </> } const suspenseFallback = <>Fetching token...</> return <ErrorBoundary fallback={errorFallback}> <React.Suspense fallback={suspenseFallback}> <Dashboard {...props} tokenRef={tokenRef} /> </React.Suspense> </ErrorBoundary>
}
// Original ErrorBoundary class:
class ErrorBoundary extends React.Component { constructor(props) { super(props); this.state = { error: null }; } static getDerivedStateFromError(error) { // Update state so the next render will show the fallback UI. return { error }; } componentDidCatch(error, errorInfo) { // You can also log the error to an error reporting service console.log(error, errorInfo); } render() { if (this.state.error) { // You can render any custom fallback UI const handleRetry = () => this.setState({ error: null }); return typeof this.props.fallback === 'function' ? this.props.fallback(this.state.error, handleRetry) : this.props.fallback } return this.props.children; }
}

const token = fetchKey(props.auth);

This returns a promise. To get the data from it, this is one way to do it:

let token = null;
fetchKey(props.auth).then(result => { console.log(result) token = result;
}).catch(e => { console.log(e)
})

Let me know if that works.

I recreated a similar example: