问虽然存在Firebase - doc，但doc.exists返回false

EN

  firebase.auth().onAuthStateChanged((user) => {
          if (user) {
            // User is signed in.
            var uid = user.uid
            db.collection("users").doc(uid)
              .get()
              .then((doc) => {
                console.log(doc)
                if (doc.exists) {
                  console.log('returning user')
                } else {
                  console.log('new user')
                }
            }).catch(function(error) {
                console.log("Error getting document:", error);
            });
          } else {
            // User is signed out.
            // ...
          }