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.

<div>
            <Child ref="childElement" />
          </div>
        );
      },
    });
    expect(console.error.calls.count()).toBe(0);
    ReactDOM.render(<Parent />, container);
    expect(console.error.calls.count()).toBe(1);
    expect(console.error.calls.argsFor(0)[0]).toContain(
      'Child: `ref` is not a prop. Trying to access it will result ' +
      'in `undefined` being returned. If you need to access the same ' +
      'value within the child component, you should pass it as a different ' +
      'prop. (https://fb.me/react-special-props)'
    );
  });
  it('allows a string to be passed as the type', function() {
    var element = React.createFactory('div')();
    expect(element.type).toBe('div');
    expect(element.key).toBe(null);
    expect(element.ref).toBe(null);
    var expectation = {};
    Object.freeze(expectation);
    expect(element.props).toEqual(expectation);
  });
  it('returns an immutable element', function() {
    var element = React.createFactory(ComponentClass)();
    expect(() => element.type = 'div').toThrow();
  });
  it('does not reuse the original config object', function() {