cache-first getter for firebase references

cacheFirstGet.js

function cacheFirstGet(ref, handlerFunc, timeout = 10000) {
  let options = {};
  if (ref instanceof firebase.firestore.DocumentReference) {
    options.includeMetadataChanges = true;
  } else if (ref instanceof firebase.firestore.Query) {
    options.includeQueryMetadataChanges = true;
  } else {
    throw new Error("Input must implement DocumentReference or Query");
  }

  let cacheIsLoaded = false;
  let unsubscribe = ref.onSnapshot(options, function(snapshot) {
    if (!snapshot.metadata.fromCache) {
      // when we recieve a response from the network we can stop listening
      unsubscribe();
      handlerFunc(snapshot);
      return;
    }
    if (!cacheIsLoaded) {
      cacheIsLoaded = true;
      handlerFunc(snapshot);
    }
  });

  // handles unsubscribing when the network never responds
  window.setTimeout(unsubscribe, timeout);
};

Thanks. This is a nice wrapper.