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.

expect(ReactDOM.findDOMNode(outer).className).toBe('moo');
  });
  it('throws when adding a prop (in dev) after element creation', function() {
    var container = document.createElement('div');
    var Outer = React.createClass({
      getDefaultProps: () => ({sound: 'meow'}),
      render: function() {
        var el = <div>{this.props.sound}</div>;
        expect(function() {
          el.props.className = 'quack';
        }).toThrow();
        expect(el.props.className).toBe(undefined);
        return el;
      },
    });
    var outer = ReactDOM.render(<Outer />, container);
    expect(ReactDOM.findDOMNode(outer).textContent).toBe('meow');
    expect(ReactDOM.findDOMNode(outer).className).toBe('');
  });
  it('does not warn for NaN props', function() {
    spyOn(console, 'error');
    var Test = React.createClass({
      render: function() {
        return <div />;
      },
    });
    var test = ReactTestUtils.renderIntoDocument(<Test value={+undefined} />);
    expect(test.props.value).toBeNaN();
    expect(console.error.calls.count()).toBe(0);
  });