How to get an array of strings from firestore?

Question

I'm trying to see if any users are already enrolled in a class. I'm trying to set the state classesArray into an array of the classes that they are already enrolled in. For some reason, it performs the action inside if(user) even though user is undefined.

const Button = ({currentPage, currentCourse, user}) =>
{

    const [classesArray, setArray] = useState([]);

    useEffect(()=>{
        if (user)
        {
            let tempArray = db.collection('users').doc(user.id).classes;
            console.log(tempArray)
        }
    })

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– Katayoon, Commented May 12, 2020 at 15:32

Guru Hadadi · Accepted Answer · 2020-05-09 05:22:25Z

if won't get executed for undefined or null or false values. Your user prop may be an empty object (which is considered as ok in the if check). So be specific with your if condition eg: if(user.id)

Note: you need to make corrections in your useEffect code. Also your useEffect runs on each and every re-render, so add a dependency to it(user for ex.)

const Button = ({ currentPage, currentCourse, user }) => {
  const [classesArray, setArray] = useState([]);

  useEffect(() => {
    if (user.id) {
      let docRef = db.collection("users").doc(user.id);

      docRef
        .get()
        .then(function(doc) {
          if (doc.exists) {
            console.log("Document data:", doc.data());
          } else {
            // doc.data() will be undefined in this case
            console.log("No such document!");
          }
        })
        .catch(function(error) {
          console.log("Error getting document:", error);
        });
    }
  }, [user.id]);
};

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