How to add to an array in useEffect React JS

Question

I am working on a Firebase project in React JS and I need to append the data from the database to an array. When I add to the array like this:

const db = firebase.firestore()
const docData = []
useEffect(() => {
  db.collection("articles").get().then((querySnapshot) => {
    querySnapshot.forEach((doc) => {
        docData.push(doc.data())

    });
  });
  console.log(docData)
}, [])

This properly displays the data in the console, however when I return the data outside of the useEffect like this:

return (
    <div className="App">
      {docData}
    </div>
  );

This does not return any array. I am new to React JS and any help would be appreciated.

Drew Reese · Accepted Answer · 2021-07-17 20:56:40Z

You will need to place docData into a state variable so you can update and render it. Declare an array to push into in the promise chain callback and then update the component state.

const [docData, setDocData] = React.useState([]);

useEffect(() => {
  db.collection("articles")
    .get()
    .then((querySnapshot) => {
      const docData = []; // locally scoped to callback
      querySnapshot.forEach((doc) => {
        docData.push(doc.data());
      });
      setDocData(docData); // update state
      console.log(docData);
    });
}, []);

...

return (
  <div className="App">
    {docData}
  </div>
);

An alternative that some may consider more clean would be to map the snapshot data to the array.

useEffect(() => {
  db.collection("articles")
    .get()
    .then((querySnapshot) => {
      const docData = querySnapshot.map((doc) => doc.data());
      setDocData(docData);
      console.log(docData);
    });
}, []);

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