The way I'd do this is by attaching a timestamp to all of your documents (new Date().toISOString() in javascript) and then checking to find the most recent one using firebase .where() function in firestore: https://firebase.google.com/docs/firestore/query-data/queries.

This is assuming you're using firebase firestore. If you're using firebase realtime DB there should be a way to apply this concept of checking the most recent timestamps, just with different code.

That'd be:

const ref = firebase.database().ref("Humidity");
const query = ref.orderByChild("time").limitToLast(1);
query.once("child_added").then((snapshot) => {
  console.log(snapshot.val().value);
});

I recommend reading:

• the documentation on reading from Firebase
• the documentation on ordering/filtering data.