Super Digit Problem on Hackerrank

superdigit.js

function superDigit(n, k) {
    //This reminds me of a clever math trick, and makes me
    //wonder if there are more. If you recursively sum a number
    //and it the final number like this is divisible by 3,
    //then the number is divisble by 3. Therefore, if n, k times
    //will just become sum(n) * k and sum(n) * k % 3 === 0
    //then the super digit is 3, 6, 9.
    //
    //In fact, if the sum of digits is 9, then the super number IS 9.
    //In fact, there IS a relationship between the sum of the digits
    //and the base number itself that is solveable in O(1) time.
    //Yep. O(1). sum_of_digits(n) = (n - 1) % 9 + 1
    //It's not even complicated.
    //Though it is thanks to https://www.sjsu.edu/faculty/watkins/Digitsum369.htm
    //We also know can see that k sums of the digits of n, can factor
    //a k out front so that it is actually k * (sum of digits of n)
    //More importantly, it seems k * (sum of digits of n) == k * sum_of_digits(n)
    //that is, the sum all the way to a single digit.
    return Number(((((BigInt(n) - 1n) % 9n + 1n) * BigInt(k)) - 1n) % 9n + 1n);
}