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.

getDefaultProps: function() {
        return {prop: 'testKey'};
      },
      render: function() {
        return <span />;
      },
    });
    var instance = React.createElement(Component);
    var clonedInstance = React.cloneElement(instance, {prop: undefined});
    expect(clonedInstance.props.prop).toBe('testKey');
    var clonedInstance2 = React.cloneElement(instance, {prop: null});
    expect(clonedInstance2.props.prop).toBe(null);
    var instance2 = React.createElement(Component, {prop: 'newTestKey'});
    var cloneInstance3 = React.cloneElement(instance2, {prop: undefined});
    expect(cloneInstance3.props.prop).toBe('testKey');
    var cloneInstance4 = React.cloneElement(instance2, {});
    expect(cloneInstance4.props.prop).toBe('newTestKey');
  });
  it('throws when changing a prop (in dev) after element creation', function() {
    var Outer = React.createClass({
      render: function() {
        var el = <div className="moo" />;
        expect(function() {
          el.props.className = 'quack';
        }).toThrow();
        expect(el.props.className).toBe('moo');
        return el;
      },
    });
    var outer = ReactTestUtils.renderIntoDocument(<Outer color="orange" />);