js-data + js-data-firebase + js-data-localstorage

app.js

var fb = new DSFirebaseAdapter({
  basePath: 'https://my-app.firebase.io'
});

var ls = new DSLocalStorageAdapter();

var store = new JSData.DS({
  // try firebase first, otherwise try offline data
  fallbackAdapters: ['fb', 'ls'],
  // After creating an item, sync it to localStorage
  afterCreate: function (resource, data) {
    return ls.create(resource, data);
  },
  // After updating an item, sync it to localStorage
  afterUpdate: function (resource, data) {
    if (typeof data === 'array') {
      return Promise.all(data.map(function (item) {
        return ls.update(resource, item[resource.idAttribute], item);
      })); 
    } else {
      return ls.update(resource, data[resource.idAttribute], data);
    }
  },
  // After destroying an item, sync it to localStorage
  afterDestroy: function (resource, data) {
    if (typeof data === 'array') {
      return Promise.all(data.map(function (item) {
        return ls.destroy(resource, item[resource.idAttribute], item);
      })); 
    } else {
      return ls.destroy(resource, data[resource.idAttribute], data);
    }
  }
});

store.registerAdapter('ls', ls);
store.registerAdapter('fb', fb, { default: true });

// define resources
var User = store.defineResource('user');
var Post = store.defineResource('post');
var Comment = store.defineResource('comment');

// load all data from localStorage into the data store
Promise.all([
  User.findAll({}, { adapter: 'ls' }),
  Post.findAll({}, { adapter: 'ls' }),
  Comment.findAll({}, { adapter: 'ls' })
]).then(function (data) {
  data[0]; // all users already in localStorage
  data[1]; // all posts already in localStorage
  data[2]; // all comments already in localStorage
})

function isOffline() {
  // return whether the app is "offline"
}

// From here on the firebase adapter will be used by default.
// Unless you specify a different adapter during a method call...

function chooseAdapter() {
  return isOffline() ? 'ls' : 'fb';
}

// so you can do this:
User.find(1, { adapter: chooseAdapter() }).then(function (user) {
  if (isOffline()) {
    // then "user" was loaded from localStorage
  } else {
    // then "user" was loaded from Firebase
  }
});

// or this:
// try firebase, if fails, try localstorage
User.find(1, { strategy: 'fallback' }).then(function (user) {
  
});