Objects are not valid as a React child (found: [object Promise])

I also received the same error message when creating an async functional component. Functional components should not be async.

const HelloApp = async (props) =>  { //<<== removing async here fixed the issue
  return (
    <div>
      <h2>Hello World</h2>
    </div>
  )
}
ReactDOM.render(<HelloApp />, document.querySelector("#app"))

jsfiddle

Using React Hooks:

UPDATE 2020-08-01: Amended with @ajrussellaudio's suggestion.

import React, {useState, useEffect} from "react"

const ShowPosts = () => {
    const [posts, setPosts] = useState([]);
    
    useEffect( () => { 
        async function fetchData() {
            try {
                const res = await axios.get('/posts'); 
                setPosts(res.data);
            } catch (err) {
                console.log(err);
            }
        }
        fetchData();
    }, []);
    return <div>{posts}</div>
}

this.renderPosts() will return a Promise not the actual data, and AFAIK Reactjs will not resolve Promises implicitly in render.

You need to do it like this

componentDidMount() {
  this.renderPosts();
}

renderPosts = async() => {
  try {
    const res = await axios.get('/posts');
    const posts = res.data;

    // this will re render the view with new data
    this.setState({
      Posts: posts
    });
  } catch (err) {
    console.log(err);
  }
}

render() {
  const posts = this.state.Posts?.map((post, i) => (
    <li key={i} className="list-group-item">{post.text}</li>
  ));

  return (
    <div>
      <ul className="list-group list-group-flush">
        {posts}
      </ul>
    </div>
  );
}