odin-javascript-exercises/05_sumAll/solution/sumAll-solution.js

const sumAll = function (min, max) {
  if (!Number.isInteger(min) || !Number.isInteger(max)) return "ERROR";
  if (min < 0 || max < 0) return "ERROR";
  if (min > max) [min, max] = [max, min];

  let sum = 0;
  for (let i = min; i <= max; i++) {
    sum += i;
  }
  return sum;
};

module.exports = sumAll;