If I map through an array using parseInt it returns unexpected and incorrect results:
console.log(["1234-04-23", "1234", "04", "23"].map(parseInt))
// => [1234, NaN, 0, 2]but redundantly adding an arrow function wrapper works as expected
console.log(["1234-04-23", "1234", "04", "23"].map(e => parseInt(e)))
// => [1234, 1234, 4, 23]These two statements should behave identically, so why does the first one break?
Because map adds a second and third parameters: the index and the array itself. See:
["1234-04-23", "1234", "04", "23"].map(console.log);
// => 1234-04-23 0 ["1234-04-23", "1234", "04", "23"]
// => 1234 1 ["1234-04-23", "1234", "04", "23"]
// => 04 2 ["1234-04-23", "1234", "04", "23"]
// => 23 3 ["1234-04-23", "1234", "04", "23"]
As you can see in the specification, parseInt also accepts a second parameter as base, so you are actually doing this:
console.log(["1234-04-23", "1234", "04", "23"].map((e, e2) => parseInt(e, e2)));
// => [1234, NaN, 0, 2]
The NaN is because:
parseIntstops at the first invalid character and returns whatever it has at that point. If there are no valid characters to parse, it returnsNaN