It's not exactly checking for falsy values. Although all falsy values will lead to a radix of 10 being applied.

parseInt internally uses the ToInt32 abstract operation on the radix parameter. Once it has that value, it explicitly looks to see if the value is 0. If it is, it uses a radix of 10.

https://people.mozilla.org/~jorendorff/es6-draft.html#sec-pa...

Edit: I hope that doesn't come off as pedantic. My point wasn't to disagree as much as it was to just add some further explanation.

Which is odd. I wonder why they checked for falsiness and not the argument being undefined.