sum_of_multiples1.js

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

  //TODO: Validate inputs

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

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

  var sumMultiples = (sum, currentMultiple) => sum + currentMultiple;
  
  factors.forEach(collectFactorsUniqueMultiples);
  
  return multiples.reduce(sumMultiples, 0);
}