firebaser here

The once('value' method has been replace by a get() function, so:

const snapshot = await get(ref);

Also see the Firebase documentation on reading data once.

To unsubscribe from a realtime listener (so not get()) you now can use the return value from the function you called to subscribe.

So if you subscribe with:

const unsubscribe = onValue(ref, (snapshot) => { ... });

You can then later unsubscribe with:

unsubscribe();

This is what the Firestore SDKs already did, and feedback indicated that devs preferred that over off.

Heads up (~24 Jun 2022; firebase v9): While get() is the recommended way to read data once (see Frank's post), there are currently some unresolved issues with get() where a client's execution of multiple queries with different filters on the same ref path somehow causes interference between the queries resulting in the unexpected triggering of events (see links below). I also experienced this and it's quite hard to debug. It is probably caused by get()'s caching mechanism as hinted at by the issues below:

• [realtime database] concurrent queries on the same path don't work #515
• Queries to the same database path get mixed up and return incorrect results depending on order of execution #6038

Workaround: Instead of retrieving results via get(), you can also use onValue() (see value observer) and then disconnect it immediately after the item has been returned:

const val = await new Promise((resolve, reject) => {
  onValue(query, (snap) => resolve(snap.val()), { onlyOnce: true });
});

Disclaimer: Note that onValue() in combination with onlyOnce: true will return values from firebase's local disk cache immediately (if present), instead of checking for an updated value on the server first (as it is the case with get()).