backbone.localStorage.js

// A simple module to replace `Backbone.sync` with *localStorage*-based
// persistence. Models are given GUIDS, and saved into a JSON object. Simple
// as that.

// Generate four random hex digits.
function S4() {
   return (((1+Math.random())*0x10000)|0).toString(16).substring(1);
};

// Generate a pseudo-GUID by concatenating random hexadecimal.
function guid() {
   return (S4()+S4()+"-"+S4()+"-"+S4()+"-"+S4()+"-"+S4()+S4()+S4());
};

// Our Store is represented by a single JS object in *localStorage*. Create it
// with a meaningful name, like the name you'd give a table.
window.Store = function(name) {
  this.name = name;
  var store = localStorage.getItem(this.name);
  this.records = (store && store.split(",")) || [];
};

_.extend(Store.prototype, {

  // Save the current state of the **Store** to *localStorage*.
  save: function() {
    localStorage.setItem(this.name, this.records.join(","));
  },

  // Add a model, giving it a (hopefully)-unique GUID, if it doesn't already
  // have an id of it's own.
  create: function(model) {
    if (!model.id) model.id = model.attributes.id = guid();
    localStorage.setItem(this.name+"-"+model.id, JSON.stringify(model));
    this.records.push(model.id.toString());
    this.save();
    return model;
  },

  // Update a model by replacing its copy in `this.data`.
  update: function(model) {
    localStorage.setItem(this.name+"-"+model.id, JSON.stringify(model));
    if (!_.include(this.records, model.id.toString())) this.records.push(model.id.toString()); this.save();
    return model;
  },

  // Retrieve a model from `this.data` by id.
  find: function(model) {
    return JSON.parse(localStorage.getItem(this.name+"-"+model.id));
  },

  // Return the array of all models currently in storage.
  findAll: function() {
    return _.map(this.records, function(id){return JSON.parse(localStorage.getItem(this.name+"-"+id));}, this);
  },

  // Delete a model from `this.data`, returning it.
  destroy: function(model) {
    localStorage.removeItem(this.name+"-"+model.id);
    this.records = _.reject(this.records, function(record_id){return record_id == model.id.toString();});
    this.save();
    return model;
  }

});

// localSync delegate to the model or collection's
// *localStorage* property, which should be an instance of `Store`.
Backbone.localSync = function(method, model, options, error) {

  // Backwards compatibility with Backbone <= 0.3.3
  if (typeof options == 'function') {
    options = {
      success: options,
      error: error
    };
  }

  var resp;
  var store = model.localStorage || model.collection.localStorage;

  switch (method) {
    case "read":    resp = model.id ? store.find(model) : store.findAll(); break;
    case "create":  resp = store.create(model);                            break;
    case "update":  resp = store.update(model);                            break;
    case "delete":  resp = store.destroy(model);                           break;
  }

  if (resp) {
    options.success(resp);
  } else {
    options.error("Record not found");
  }
};

// Override 'Backbone.sync' to default to localSync, 
// the original 'Backbone.sync' is still available in 'Backbone.ajaxSync'
Backbone.ajaxSync = Backbone.sync;
Backbone.sync = Backbone.localSync;