Using firestore data in state (React) - reactjs

Question

How do I go about setting a specific firestore document field to state.When ever i try to run this i get the following warning.
(Im trying to get an array from firestore if that helps)
Possible Unhndled Promis Rejection (id: 0): TypeError: doc.data is not a function. (In 'doc.data()', 'doc.data' is undefined)
class Chart extends React.Component {
constructor(props) {
super(props);
this.state = {
data: []
};
}
componentDidMount() {
db.collection("users")
.where("email", "==", firebase.auth().currentUser.email)
.get()
.then(doc => {
this.setState({ data: doc.data().prevlevel });
});
}
render(){
const data = this.state.data
.....

Answer 1

You're firing a query against a collection. A query can have multiple results, and thus returns a QuerySnapshot and not a document. Even when there is only one document matching your criteria, it'll still be in a QuerySnapshot.
So your callback will have to iterate over the results. If you only expect one result, you could do that with:
db.collection("users")
.where("email", "==", firebase.auth().currentUser.email)
.get()
.then(querySnapshot => {
querySnapshot.forEach(doc => {
this.setState({ data: doc.data().prevlevel });
})
});

Answer 2

You are currently getting a collection, not a document. If you want to get a doc, you'll need to add that to your request, as in:
db.collection("users")
.where("email", "==", firebase.auth().currentUser.email)
.doc('someId') // <-- Added this, but i don't know exactly what id you need
.then(doc => {
this.setState({ data: doc.data().prevlevel });
});
Or if you do want a collection, then you'll need to use the properties found on collections, as in:
.then(snapshot => {
this.setState({ data: snapshot.docs });
})