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.

},
    });
    var element = React.createElement(StaticMethodComponentClass);
    expect(element.type.someStaticMethod()).toBe('someReturnValue');
    expect(console.error.calls.count()).toBe(0);
  });
  
  
  it('identifies valid elements', function() {
    var Component = React.createClass({
      render: function() {
        return React.createElement('div');
      },
    });
    expect(React.isValidElement(React.createElement('div')))
      .toEqual(true);
    expect(React.isValidElement(React.createElement(Component)))
      .toEqual(true);
    expect(React.isValidElement(null)).toEqual(false);
    expect(React.isValidElement(true)).toEqual(false);
    expect(React.isValidElement({})).toEqual(false);
    expect(React.isValidElement('string')).toEqual(false);
    expect(React.isValidElement(React.DOM.div)).toEqual(false);
    expect(React.isValidElement(Component)).toEqual(false);
    expect(React.isValidElement({ type: 'div', props: {} })).toEqual(false);
    var jsonElement = JSON.stringify(React.createElement('div'));
    expect(React.isValidElement(JSON.parse(jsonElement))).toBe(true);
  });
  it('allows the use of PropTypes validators in statics', function() {