sum_of_multiples.js

function sumOfMultiples(limit, factors = [3, 5]) {
  var multiple;
  var multiples = [];

  var isValidInt = num => Number.isInteger(num) && num > 0;
  var validFactors = array => Array.isArray(array) && array.every(isValidInt);

  var collectFactorsUniqueMultiples = function(factor) {
    multiple = factor;

    while (multiple < limit) {
      if (!multiples.includes(multiple)) multiples.push(multiple);
      multiple += factor;
    }
  };

  var sumMultiples = (sum, currentMultiple) => sum + currentMultiple;
  
  // Validate inputs. Return null for invalid inputs
  if (!isValidInt(limit) || !validFactors(factors) || factors.length === 0) return null;

  factors.forEach(collectFactorsUniqueMultiples);
  
  return multiples.reduce(sumMultiples, 0);
}