Asynchronously map JSX in React

Question

I have async function that basically fetches a list from the backend, and I would create a html list on the FE with those elements. For that I want to do a map on that list. The problem is that since it's the result of a async function, it seems that I need add some details.

Based on my searches , I should do something similar to this:

const listItems = await Promises.all( podcastList.map(
      (podcast) =>{
      <ListItem value = {podcast} />}
      ));

However, I get the error:

Compiled with problems:X

ERROR in ./src/App.js

Module build failed (from ./node_modules/babel-loader/lib/index.js):
SyntaxError: Unexpected reserved word 'await'. (21:20)

The remaining functions are:

function ListItem(props) {
  return <li>{props.value}</li>;
}

async function podcast_list() {
  let podcast_list;
  const headers = { 'Content-Type': 'application/json' };
  const res = await fetch('/list_podcasts', { method: 'GET',headers});
  podcast_list = await res.json();
  console.log(podcast_list);
  return podcast_list
}
podcastList = podcast_list();

Answer 1

JSX can't be returned from a component asynchronously. Separate your raw data from your JSX-- await the request for the raw data, set state to trigger a rerender once the data arrives, then build the JSX UI.

 const {useEffect, useState} = React; // Mock for demonstration fetch = () => Promise.resolve({ json: () => Promise.resolve([ "some podcast", "some other podcast", ]) }); // const ListItem = ({value}) => ( <li key={value}>{value}</li> ); const Podcasts = () => { const [podcasts, setPodcasts] = useState([]); useEffect(() => { fetch("/list_podcasts").then(res => res.json()).then(data => { setPodcasts(data); }); }, []); return ( <div> {podcasts.length? ( <ul> {podcasts.map(e => <ListItem value={e} />)} </ul> ): <div>Loading...</div> } </div> ); }; ReactDOM.render(<Podcasts />, document.querySelector("#root"));

 <script src="https://cdnjs.cloudflare.com/ajax/libs/babel-standalone/6.26.0/babel.min.js"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/react/17.0.2/umd/react.production.min.js"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/17.0.2/umd/react-dom.production.min.js"></script> <div id="root"></div>

Use camelCase in JavaScript, not snake_case and don't forget to add a unique key attribute to list items.

Answer 2

You don't need to await on maps when rendering JSX elements. Just:

const listItems = podcastList.map((podcast) => <ListItem value={podcast} />)