Overriding backbone sync to use local storage

localStorageSync.js

// overriding sync to use local storage when possible
  sync : function(method, model, options){
    var key, now, timestamp, refresh;
    if(method === 'read' && this.constants.isStoredInLocalStorage) {
      // only override sync if it is a fetch('read') request
      key = this.getKey();
      if(key) {
        now = new Date().getTime();
        timestamp = $storage.get(key + ":timestamp");
        refresh = options.forceRefresh;
        if(refresh || !timestamp || ((now - timestamp) > this.constants.maxRefresh)) {
          // make a network request and store result in local storage
          var success = options.success;
          options.success = function(resp, status, xhr) {
            // check if this is an add request in which case append to local storage data instead of replace
            if(options.add && resp.values) {
              // clone the response
              var newData = JSON.parse(JSON.stringify(resp));
              // append values
              var prevData = $storage.get(key);
              newData.values = prevData.values.concat(resp.values);
              // store new data in local storage
              $storage.set(key, newData);
            } else {
              // store resp in local storage
              $storage.set(key, resp);
            }
            var now = new Date().getTime();
            $storage.set(key + ":timestamp", now);
            success(resp, status, xhr);
          };
          // call normal backbone sync
          Backbone.sync(method, model, options);
        } else {
          // provide data from local storage instead of a network call
          var data = $storage.get(key);
          // simulate a normal async network call
          setTimeout(function(){
            options.success(data, 'success', null);
          }, 0);
        }
      }
    } else {
      Backbone.sync(method, model, options);
    }
  }