add solution for Project Euler 020

Project-Euler/Problem020.js

@@ -0,0 +1,21 @@
+/*
+Factorial digit sum
+
+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 in the number 100!
+*/
+
+const findFactorialDigitSum = (num) => {
+  let result = 0
+  const stringifiedNumber = factorize(num).toLocaleString('fullwide', { useGrouping: false })
+  stringifiedNumber.split('').map(num => { result += Number(num) })
+  return result
+}
+
+const factorize = (num) => num === 0 ? 1 : num * factorize(num - 1)
+
+console.log(findFactorialDigitSum(100))