15-multiples-of-3-and-5.js

/* If we list all the natural numbers below 10 that are multiples
   of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.
   Finish the solution so that it returns the sum of all the multiples of
   3 or 5 below the number passed in.
   Note: If the number is a multiple of both 3 and 5, only count it once. */

export default (number) => {
  const arr = [];
  let i = number - 1;
  while (i > 2) {
    if ((i % 3 === 0) || (i % 5 === 0)) {
      arr.push(i);
    }
    i -= 1;
  }
  return arr.reduce((acc, n, index) => (arr.indexOf(n) === index ? acc + n : acc), 0);
};