code.js

var Storage = (function() {
  var uniqueId = 0;
  function Storage() {}

  Storage.prototype.add = function(item) {
    var id = this.generateId()
      , data = this._toJSON(item);

    localStorage[id] = data;
  };

  Storage.prototype.generateId = function() {
    return ++uniqueId;
  };

  Storage.prototype.reset = function() {
    uniqueId = 0;
  };

  Storage.prototype._toJSON = function(items) {
    return JSON.stringify(items);
  };

  Storage.prototype._fromJSON = function(json) {
    return JSON.parse(json);
  };

  return Storage;
})();

window.Storage = Storage;

spec.js

describe("Storage", function() {
  var store;

  beforeEach(function() {
    store = new Storage();
    store.reset();
  });

  it("can generate unique ids", function() {
    var ids = [
      store.generateId(),
      store.generateId(),
      store.generateId(),
      store.generateId()
    ];

    expect(ids).toEqual([1,2,3,4]);
  });

  it("can reset the id counter", function() {
    store.generateId();
    var last = store.generateId();

    expect(last).toEqual(2);

    store.reset();
    last = store.generateId();

    expect(last).toEqual(1);
  });

  it("can encode content", function(){
    data = {one: 1, two: 2};

    encoded = store._toJSON(data);
    expect(encoded).toEqual('{"one":1,"two":2}');
  });

  it("can decode content", function(){
    data = '{"one":1,"two":2}';
    encoded = store._fromJSON(data);
    expect(encoded).toEqual({one: 1, two: 2});
  });

  it("can store an item", function() {
    var data = {item: "Test"};

    store.add(data);
    expect(localStorage[1]).toEqual(JSON.stringify(data));
  });
  
  it("can fail", function() {
      expect(true).toBeFalsy();
  });

});