ES6 adds isNaN and isFinite static methods to Number that don't convert their argument to a number first, so Klathmon's version works. But I don't make any claim about whether it's correct or not.

https://babeljs.io/repl/#?babili=false&evaluate=true&lineWra...

Compare step 1 of http://www.ecma-international.org/ecma-262/6.0/#sec-isnan-nu... with http://www.ecma-international.org/ecma-262/6.0/#sec-number.i...