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.

if (Object.freeze) {
      Object.freeze(element.props);
      Object.freeze(element);
    }
  }
  return element;
};
/**
 * Create and return a new ReactElement of the given type.
 * See https://facebook.github.io/react/docs/top-level-api.html#react.createelement
 */
ReactElement.createElement = function(type, config, children) {
  var propName;
  
  var props = {};
  var key = null;
  var ref = null;
  var self = null;
  var source = null;
  if (config != null) {
    if (__DEV__) {
      warning(
        
        config.__proto__ == null || config.__proto__ === Object.prototype,
        
        'React.createElement(...): Expected props argument to be a plain object. ' +
        'Properties defined in its prototype chain will be ignored.'
      );
      ref = !hasOwnProperty.call(config, 'ref') ||
        Object.getOwnPropertyDescriptor(config, 'ref').get ? null : config.ref;