This is behaving properly, exactly as documented. See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/parseInt
– jamisonSep 27 '17 at 15:17
parseInt accepts a second argument (and integer that specifies the radix/base), and map provides its callback a second argument - the array index.
– jamisonSep 27 '17 at 15:19