这道题的第二个数值为什么是NaN 啊 我感觉应该是2 啊
(因为 parseInt 需要两个参数 (val, radix) 但 map 传了 3 个 (**element, index, array**) 这是什么意思？
callback is invoked with three arguments: the value of the element, the index of the element, and the Array object being traversed."
So if you call a function which actually expects two arguments, the second argument will be the index of the element.
In this case, you ended up calling parseInt with radix 0, 1 and 2 in turn. The first is the same as not supplying the parameter, so it defaulted to base 10. Base 1 is an impossible number base, and 3 is not a valid number in base 2:
parseInt('1', 0); // OK - gives 1
parseInt('2', 1); // FAIL - 1 isn't a legal radix
parseInt('3', 2); // FAIL - 3 isn't legal in base 2