branch.ts

store.sync = (key, ref) => {
  // Document
  if (ref instanceof DocumentReference) {
    return onSnapshot(ref, (ds) => {
      if (ds.exists()) {
        store.$patch({ [key]: ds.data() })
      }
    })
  }

  // Collection or Query
  return onSnapshot(ref, (qs) => {
    const datum = qs.docs.map(d => d.data())
    store.$patch((state) => {
      state[key] = datum
    })
  })
}