I believe #5744 introduced a behavioral difference between development and production versions of React. We released it as a part of 15.0, and this difference still exists.

The production behavior hasn’t changed. However the development behavior diverged after this change.

``` js
var el = React.createElement('div', { key: undefined })
document.body.innerHTML = (typeof el.key) + ' ' + el.key
```

What do you expect to appear?
### 0.14.x
- Development (https://jsfiddle.net/sjLcmnq6/1/): `object null`
- Production (https://jsfiddle.net/qro1wck0/1/): `object null`
## 15.x
- Development (https://jsfiddle.net/69z2wepo/43538/):  **`string undefined`**
- Production (https://jsfiddle.net/rakg6uwy/1/): `object null`

The 15.x dev behavior is different because [dev code path only checks the existence of a property but not whether it’s undefined](https://github.com/alitaheri/react/blob/c3980a6d08257d39a08bed8b116d53a56344166b/src/isomorphic/classic/element/ReactElement.js#L130-L133) whereas the [prod code path checks for undefined explicitly](https://github.com/alitaheri/react/blob/c3980a6d08257d39a08bed8b116d53a56344166b/src/isomorphic/classic/element/ReactElement.js#L135-L136).

`React.cloneElement()` is not affected because it doesn’t include those warnings.

---

This doesn’t seem like a major problem (nobody noticed it!) in the real world, but I think we should fix this.
I have a related fix in the works so I’ll add some more tests and include it.

expect(instance.refs.child).not.toBeUndefined();
    });
    it('can capture Child instantiation calls', function() {
      expect(Child.mock.calls[0][0]).toEqual({ foo: 'foo value', children: 'children value' });
    });
  });
  describe('when using parent that uses .createElement()', function() {
    var factory, instance;
    beforeEach(function() {
      var Parent = React.createClass({
        render: function() {
          return React.DOM.div({}, React.createElement(Child, { ref: 'child', foo: 'foo value' }, 'children value'));
        },
      });
      factory = React.createFactory(Parent);
      instance = ReactTestUtils.renderIntoDocument(factory());
    });
    it('should scry children but cannot', function() {
      var children = ReactTestUtils.scryRenderedComponentsWithType(instance, Child);
      expect(children.length).toBe(1);
    });
    it('does not maintain refs', function() {
      expect(instance.refs.child).not.toBeUndefined();
    });
    it('can capture Child instantiation calls', function() {
      expect(Child.mock.calls[0][0]).toEqual({ foo: 'foo value', children: 'children value' });
    });
  });
});