solution for kata #40 at http://es6katas.org/

es6kata-40.js

// 40: iterator - one example usage. Build an iterable and use it with some built-in ES6 constructs.
// To do: make all tests pass, leave the assert lines unchanged!

// Consumable users: 
// - `consumableUser` contains a consumable user, 
// - `anyLeft` tells if there is any user left that can be consumed.  
class ConsumableUsers {
  constructor() {
    this.users = ['Alice', 'Bob'];
  }
  get nextUser() {
    if (this.users.length > 0) {
      return `user: ${this.users.shift()}`;
    }
    return void 0;
  }
  get anyLeft() {
    return this.users.length > 0;
  }
}

describe('Iterator usages', () => {

  let usersIterable;
  beforeEach(function(){
    const consumableUsers = new ConsumableUsers();
    const anyLeft = consumableUsers.anyLeft;
    function iteratorFunction() {
      return {
        next: function() {
          return {value: consumableUsers.nextUser, done: !anyLeft}
        }
      }
    }
    
    usersIterable = {
      [Symbol.iterator]: iteratorFunction
    };
  });
  
  describe('create an iterator/iterable', function() {
    it('the `usersIterable` should be iterable', function() {
      const isIterable = Symbol.iterator in usersIterable;
      assert.equal(isIterable, true);
    });
    
    it('the iterator of `usersIterable` should return an object', function() {
      const iterator = usersIterable[Symbol.iterator]();
      assert.equal(typeof iterator, 'object');
    });
    
    it('the iterator of `usersIterable` should have a next function', function() {
      const iterator = usersIterable[Symbol.iterator]();
      assert.equal(typeof iterator.next, 'function');
    });
  });
  
  describe('fill the iterable with content using `ConsumableUsers`', function() {
    
    describe('using the iterator', function() {
      let iterator;
      beforeEach(function(){
        iterator = usersIterable[Symbol.iterator]();
      });
      it('should return `Alice` as first user', function() {
        const firstItem = iterator.next();
        assert.deepEqual(firstItem, {value: "user: Alice", done: false});
      });
      xit('should return `Bob` as second user', function() {
        iterator.next(); // drop the first item
        const secondItem = iterator.next();
        assert.deepEqual(secondItem, {value: "user: Bob", done: false});
      });
      it('should return `done:true`, which means there are no more items', function() {
        iterator.next();
        iterator.next();
        const beyondLast = iterator.next();
        assert.deepEqual(beyondLast, {value: void 0, done: true});
      })
    });
    
    
    describe('using built-in constructs', function() {
      it('use `Array.from()` to convert an iterable to an array', function() {
        const users = Array.from(usersIterable);
        assert.deepEqual(users, ['user: Alice', 'user: Bob']);
      });
      it('use for-of to loop over an iterable', function() {
        const users = [];
        for (let user of usersIterable) users.push(user);
        assert.deepEqual(users, ['user: Alice', 'user: Bob']);
      });
      it('use the spread-operator to convert/add iterable to an array', function() {
        const users = ['noname', ...usersIterable];
        assert.deepEqual(users, ['noname', 'user: Alice', 'user: Bob']);
      });
      it('destructure an iterable like an array', function() {
        const [firstUser, secondUser] = usersIterable;
        assert.equal(firstUser, 'user: Alice');
        assert.equal(secondUser, 'user: Bob');
      })
    });
  });
});