js-5、[1,2,3].map(parseInt)等於多少：1,NaN,NaN

5、["1","2","3"].map(parseInt)等於多少：1,NaN,NaN

這道題的第二個數值為什麼是NaN 啊 我感覺應該是2 啊
(因為 parseInt 需要兩個參數 (val, radix) 但 map 傳了 3 個 (**element, index, array**) 這是什麼意思？

最佳回答：

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