recursiveUpdate.ts

async function updateUsers(lastKey: string = '', iteration: number = 0, records: number = 0) {
  const snapshots: DataSnapshot = await admin.database().ref('/users')
    .orderByKey()
    .startAt(lastKey)
    .limitToFirst(10)
    .once('value');

  const batchUpdate = {};
  let nextKey = null;
  let counter = 0;

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

    if (userId !== lastKey) {
      counter++;
      batchUpdate[`/users/${userId}/lastProcessed`] = admin.database.ServerValue.TIMESTAMP;

      nextKey = userId;
    }

    return false;
  });

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

  if (nextKey) {
    await updateUsers(nextKey, iteration + 1, counter + records);
  } else {
    console.log(`Finished: ${iteration} iterations, ${counter + records} records updated.`);
  }
}

updateUsers();