batchUpdate.ts

const snapshots: DataSnapshot = await firebase.database().ref('/users')
  .once('value');

const batchUpdate = {};

snapshots.forEach(snapshot => {
  const userId = snapshot.key;

  batchUpdate[`/users/${userId}/lastProcessed`] = firebase.database.ServerValue.TIMESTAMP;

  return false;
});

await firebase.database().ref('/')
  .update(batchUpdate);