Angular 1.5 :~

backend.js

import angular from 'angular';

// fn

const normaliz = ( entities ) => ( {
  entities: entities.reduce( (r, e) => Object.assign( r, { [e._id]: e } ), {} ),
  ids: entities.map( d => d._id )
} );

const storeItem = ( item ) => ( value ) =>
  window.localStorage[item] = JSON.stringify( value );

const restoreItem = ( item ) => JSON.parse( window.localStorage[item] );

const updateItem = ( trans ) => ({
  entities: Object.assign({}, trans.col.entities, {
    [trans.item._id]: trans.item
  }),
  ids: Array.from(new Set([...trans.col.ids, trans.item._id]))
});

const normalizEmpty = normaliz( [] );

const register = ( store, name ) => Object.assign( register.stores, {
  [name]: store
} ) && store;

register.stores = {};


const store = ( store ) => {
  const storer = storeItem( store );
  const restorer = () => restoreItem( store ) || normalizEmpty;
  const reducer = {
    writer: ( result ) =>
      storer( normaliz( result.data ) ) &&
      result,
    reader: ( id ) => restorer().entities[id],
    updater: ( result ) => storer( updateItem( {
      col: restorer(),
      item: result.data
    } ) )
  };

  return register( reducer, store );
}
// reducers

const documentsStore = store( 'documents' );
const usersStore = store( 'users' );

// Service

function Backend( $http, BackendUrl ) {
  'ngInject';

  const backend = this;
  // create rest request method with subset methods
  // this.$get to pick up one from cache
  // this.storesX => get
  const rest = ( store ) => {
    const s = register.stores[store];
    const url = BackendUrl + `/${store}/`;
    const catcher = catchError( store );

    const extras = {
      [store]: () => $http.get( url ).then(
        s.writer,
        catcher( { url, action: 'index' } )
      ),
      [store + '$get']: s.reader,
      [store + '$update']: ( item ) => $http.put( url, {
          data: item
        } ).then(
          s.updater,
          catcher( { url, action: 'update', item } )
        )

    };

    return Object.assign( backend, extras );
  };

  rest( 'documents' );
  rest( 'users' );
}

// Helpers

function catchError( store ) {
  const type = 'backend';

  return  ( data ) => ( e ) => {
    console.log( `Error on fetching ${store}`, e );
    window.lastError = e;
    window.fire( 'error', { e, type, data } );
    return e;
  };
}

// Module

const module = angular.module( 'app.backend', [] )
module.service( 'Backend', Backend );

// exports

export default module;