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.

childArray[i] = arguments[i + 2];
    }
    props.children = childArray;
  }
  return ReactElement(
    element.type,
    key,
    ref,
    self,
    source,
    owner,
    props
  );
};
/**
 * Verifies the object is a ReactElement.
 * See https://facebook.github.io/react/docs/top-level-api.html#react.isvalidelement
 * @param {?object} object
 * @return {boolean} True if `object` is a valid component.
 * @final
 */
ReactElement.isValidElement = function(object) {
  return (
    typeof object === 'object' &&
    object !== null &&
    object.$$typeof === REACT_ELEMENT_TYPE
  );
};
module.exports = ReactElement;