Array.map + parseInt

Question

var timeSplit = timeCaption.innerText.trim().split(' ');

will yield an Array of ["10:00", "–", "18:00"]

var startStr = timeSplit[0].split(':');

will yield an Array of ["10", "00"]

var res = startStr.map(parseInt);

will yield an Array of [10, NaN]

however

var res = startStr.map(function (x) {
   return parseInt(x);
});

works correctly and will yield the "expected" Array of [10, 0]

I expect each string to be passed to parseInt which returns the correct interger value (and doing that separately also yields the correct result, just like the working code).

What am I missing here?

edit: I myself voted to close this question. Pretty obvious mistake. Thx guys!

Answer 1

parseInt accepts 2 arguments:

1. string to be parsed
2. radix

.map calls your function with 3 arguments:

1. the value
2. the index
3. array

If you think about it,

parseInt("00", 1)

doesn't really make sense and it returns NaN. parseInt accepts radixes between 2 and 36.

Answer 2

The second argument to parseInt is the radix, or base (like 10, 16, etc, defaults to base 10). .map passes two arguments to the callback, the value at that index, and also the index. So you're calling parseInt(10, 0) which is trying to figure out what 10 is in base 0.

Answer 3

.map() calls the function with 3 arguments: The array element, the array index, and the array. parseInt takes 2 parameters: the string to parse, and the radix in which to parse it.

So startStr.map(parseInt) is calling:

parseInt("10", 0, startStr);
parseInt("00", 1, startStr);

Apparently parseInt() treats radix = 0 as no radix, so it defaults to base 10. But radix = 1 doesn't make any sense at all, so it returns NaN.