firebase get selected data from firestore code example

Example 1: get one document based on id in firestore

//retrieve one document and save it to userDetails
const [userDetails, setUserDetails] = useState('')
db.collection('users').doc(id).get()
        .then(snapshot => setUserDetails(snapshot.data()))

Example 2: firebase query multiple keys

var keys = [ 
  "-Ke1uhoT3gpHR_VsehIv",
  "-Ke8qAECkZC9ygGW3dEJ",
  "-Ke8qMU7OEfUnuXSlhhl"
];
var promises = keys.map(function(key) {
  return firebase.database().ref("/items/").child(key).once("value");
});
Promise.all(promises).then(function(snapshots) {
  snapshots.forEach(function(snapshot) {
    console.log(snapshot.key+": "+snapshot.val());
  });
});