tl;dr: parseInt takes an optional second argument, the radix. It defaults to 10 (so the number is parsed in base 10). Since map() passes in each item in the arr

tl;dr: parseInt takes an optional second argument, the radix. It defaults to 10 (so the number is parsed in base 10). Since map() passes in each item in the array as the first argument, and the index as the second, you're parsing each string with the radix of whatever the index is. Not too weird IMO.