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Problem 20: Factorial digit sum

curriculum/challenges/english/blocks/project-euler-problems-1-to-100/5900f3801000cf542c50fe93.md

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--description--

n! means n × (n − 1) × ... × 3 × 2 × 1

For example, 10! = 10 × 9 × ... × 3 × 2 × 1 = 3628800,
and the sum of the digits in the number 10! is 3 + 6 + 2 + 8 + 8 + 0 + 0 = 27.

Find the sum of the digits n!

--hints--

sumFactorialDigits(10) should return a number.

js
assert.isNumber(sumFactorialDigits(10));

sumFactorialDigits(10) should return 27.

js
assert.strictEqual(sumFactorialDigits(10), 27);

sumFactorialDigits(25) should return 72.

js
assert.strictEqual(sumFactorialDigits(25), 72);

sumFactorialDigits(50) should return 216.

js
assert.strictEqual(sumFactorialDigits(50), 216);

sumFactorialDigits(75) should return 432.

js
assert.strictEqual(sumFactorialDigits(75), 432);

sumFactorialDigits(100) should return 648.

js
assert.strictEqual(sumFactorialDigits(100), 648);

--seed--

--seed-contents--

js
function sumFactorialDigits(n) {

  return n;
}

sumFactorialDigits(100);

--solutions--

js
let factorial = (n) => n <= 1 ? BigInt(n) : BigInt(n) * BigInt(factorial(--n));

let sumDigits = n => n.toString().split('').map(x => parseInt(x)).reduce((a,b) => a + b);

function sumFactorialDigits(n) {
  return sumDigits(factorial(n));
}