React.createElement(type, { key: undefined }) is handled incorrectly

[gaearon]

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.

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 whereas the prod code path checks for undefined explicitly.

React.cloneElement() is not affected because it doesn’t include those warnings.

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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.