Uncaught TypeError: Cannot read properties of undefined (reading 'map') - Firestore API

Question

I am using react-admin to fetch data from a Firestore API in the following format:

{
  "documents": [
    {
      "name": "",
      "fields": {
        "teacher": {
          "stringValue": ""
        },
        "slot": {
          "stringValue": ""
        },
        "student_name": {
          "stringValue": ""
        }
      },
      "createTime": "2022-03-27T09:49:24.044423Z",
      "updateTime": "2022-03-29T13:50:41.264007Z"
    },
    {
      "name": "",
      "fields": {
        "teacher": {
          "stringValue": ""
        },
        "slot": {
          "stringValue": ""
        },
        "student_name": {
          "stringValue": ""
        }
      },
      "createTime": "2022-03-27T09:49:24.044423Z",
      "updateTime": "2022-03-29T13:50:41.264007Z"
    },
    {
      "name": "",
      "fields": {
        "teacher": {
          "stringValue": ""
        },
        "slot": {
          "stringValue": ""
        },
        "student_name": {
          "stringValue": ""
        }
      },
      "createTime": "2022-03-27T09:49:24.044423Z",
      "updateTime": "2022-03-29T13:50:41.264007Z"
    }
  ]
}

However, when I try to extract certain fields from the data like this,

const List = () => {
    const [error, setError] = useState(null);
    const [isLoaded, setIsLoaded] = useState(false);
    const [items, setItems] = useState([]);
  
    // Note: the empty deps array [] means
    // this useEffect will run once
    // similar to componentDidMount()
    useEffect(() => {
      fetch("MY API URL")
        .then(res => res.json())
        .then(
          (result) => {
            setIsLoaded(true);
            setItems(result);
          },
          // Note: it's important to handle errors here
          // instead of a catch() block so that we don't swallow
          // exceptions from actual bugs in components.
          (error) => {
            setIsLoaded(true);
            setError(error);
          }
        )
    }, [])
  
    if (error) {
      return <div>Error: {error.message}</div>;
    } else if (!isLoaded) {
      return <div>Loading...</div>;
    } else {

      return ( 
        <ul>
          {items.map(item => (
            <li key={item.id}>
              {item.name}
            </li>
          ))}
        </ul> 
      );
    }

I am getting the following error : Uncaught TypeError: Cannot read properties of undefined (reading 'map')

I found a similar answers including this one here : TypeError: Cannot read properties of undefined (reading 'map') Table.render

But was still unable to figure it out. Any help would be great, thanks!

Answer 1

You can try Optional Chaining(?.) at your items.

{items?.map(item => (
   <li key={item.id}>
      {item.name}
   </li>
 ))}

This should prevent the error.

Another important thing is that result you get back from your fetch request might probably be undefined and would raise the TypeError.