facebook/react — pull request #1727 by ruanwz (2014-06-21 05:14:49+00:00)
Title: test undefined use typeof 

When test react app using jest, I have found the undefined can't be tested

https://github.com/facebook/jest/issues/80

So change the test from one[key]=== undefined to typeof one[key]==='undefined' can fix this error, and in another line in this file, it also use typeof to test. in function _processProps:
typeof props[propName] === 'undefined'