If you want to check if an email exists in Firebase, here is a similar solution

firebase.app().database().ref("shops").orderByChild("email")
   .equalTo(user.email).once("value", snapshot => {

            const userData = snapshot.val();

            // Check if it is a SHOP.
            if (userData) {
              console.log("Shop logged in!");
              this.setState({
                isAdminLoggedIn: false,
                isUserLoggedIn: false,
                isShopLoggedIn: true,
                isNoneLoggedIn: false
              });

            // Check if it is a USER.
            } else {
              console.log("User logged in");
              this.setState({
                isAdminLoggedIn: false,
                isUserLoggedIn: true,
                isShopLoggedIn: false,
                isNoneLoggedIn: false
              });
            }
        });

The

exists() method is part of the snapshot object returned by the Firebase query. So keep in mind that you won't be able to avoid retrieving the data to verify that it exists .

// Firebase Namespaced SDK (v8 & older)
// import firebase as appropriate

const userQueryByID = firebase.database()
  .ref("users")
  .orderByChild("ID")
  .equalTo("U1EL5623");

// using a snapshot listener
userQueryByID
  .once(
    "value",
    snapshot => {
      if (snapshot.exists()){
        const userData = snapshot.val();
        console.log("exists!", userData);
      }
    }
  );

// OR, using a promise
userQueryByID.get()
  .then(snapshot => {
    if (snapshot.exists()){
      const userData = snapshot.val();
      console.log("exists!", userData);
    }
  });

// Firebase Modular SDK (v9+)
// import each function from "firebase/database"

const rtdb = getDatabase();
const userQueryByID = query(
  ref(rtdb, "users"),
  orderByChild("ID"),
  equalTo("U1EL5623")
);

// using a snapshot listener
onValue(
  userQueryByID,
  snapshot => {
    if (snapshot.exists()){
      const userData = snapshot.val();
      console.log("exists!", userData);
    }
  },
  { onlyOnce: true }
);

// OR, using a promise
get(userQueryByID)
  .then(snapshot => {
    if (snapshot.exists()){
      const userData = snapshot.val();
      console.log("exists!", userData);
    }
  });

Observation results:

If you are in a different scenario and you have the exact reference path where the object might be, there is no need to add orderByChild and equalTo. In this case you can get the path to the object directly, so no search processing is required from firebase. Also, if you know one of the properties that the object must have, you can follow the code snippet below and have it retrieve only that property instead of the entire object. The result will be faster inspections.

For example, if each user had a username in their data, you could use these:

// Firebase Namespaced SDK (v8 & older)
// import firebase as appropriate

const usernameRef = firebase.database()
  .ref(`users/${userId}/username`);

// using a snapshot listener
usernameRef
  .once(
    "value",
    snapshot => {
      if (snapshot.exists()){
        const username = snapshot.val();
        console.log("exists!", username);
      }
    }
  );

// OR, use usernameRef.get() for a promise, as above

// Firebase Modular SDK (v9+)
// import each function from "firebase/database"

const rtdb = getDatabase();
const usernameRef = ref(rtdb, `users/${userId}/username`);

// using a snapshot listener
onValue(
  usernameRef,
  snapshot => {
    if (snapshot.exists()){
      const username = snapshot.val();
      console.log("exists!", username);
    }
  },
  { onlyOnce: true }
);

// OR, use get(usernameRef) for a promise, as above