An example of overriding Backbone.sync and Backbone.parse to load data from localStorage in Backbone

localStorage.js

// Overriding Backbone.sync to either load locally or from the server
sync: function(method, model, options) {
  
  // Pull the key and stored object localStorage
  var storageKey = localStorage.getItem('storage');
  var storageData = localStorage.getItem('storage-' + storageKey);

  // Apply the callback with our local data
  if(storageData) {
    options.success(JSON.parse(storageData));
  }
  // No local data so run .getJSON to fetch it from the server
  else {
    $.getJSON(this.url, function(data, textStatus) {
      if(textStatus !== 'success' || !data) {
        options.error();
      }
        options.success(data);
      });
    }
  }

// Overriding Backbone.parse to fill models from local data
parse: function(response) {
    // Loading from localStorage so we intercept to properly
    // serialize the object in our Backbone.collections
  var self = this;
  
  // In my case, the object loaded from storage was an array
  // Backbone usually passes an object to parse
  if(response.length) {
    
    // Set properties on our model and return it
    return {
      id: response.id,
      myCollection: self.collection.add(response.myCollection),
    };
  }
},