RSpec style let() in Jasmine/Mocha

let.js

/**
 * Get RSpec-style let() in your Mocha/Jasmine specs.
 */
var let = function (callback) {
  var value, called = false;
  var memoizer = function() {
    if (called) {
      return value;
    } else {
      called = true;
    }

    return value = callback();
  };

  afterEach(function() {
    value  = undefined;
    called = false;
  });

  return memoizer;
};

// example...

describe('Array', function(){
  var array = let(function(){ return[]; });

  describe('.length', function(){
    describe('when empty', function(){
      it('is zero', function(){
        assert.equal(array().length, 0);
      });
    });

    describe('with two items', function(){
      beforeEach(function(){
        for (var i = 0; i < 2; ++i) { array()[i] = 'anything'; }
      });

      it('is two', function(){
        assert.equal(array().length, 2);
      });
    });
  });

  describe('#push()', function(){
    it('appends items to the array', function(){
      array().push('bob');
      assert.equal(array()[0], 'bob'); // note that the state has been reset
    });
  });
});