numsFromNumsAndRanges.js

function numsFromNumsAndRanges(numsAndRanges) {
  const nums = new Set();
  const numsAtStartOfRanges = new Set();
  const numsAtEndOfRanges = new Set();

  if (!numsAndRanges.length > 0) {
    return { nums, numsAtStartOfRanges, numsAtEndOfRanges };
  }

  for (const numOrRange of numsAndRanges.split(',')) {
    if (numOrRange.includes('-')) {
      // range

      let [first, last] = numOrRange.split('-');

      numsAtStartOfRanges.add(Number(first));

      numsAtEndOfRanges.add(Number(last));

      let numsInRange = Array.from({ length: Number(last) - Number(first) + 1 }, (_, i) => Number(first) + i);

      for (const num of numsInRange) {
        nums.add(num);
      }

      continue;
    }

    // num

    nums.add(Number(numOrRange));
    numsAtStartOfRanges.add(Number(numOrRange));
    numsAtEndOfRanges.add(Number(numOrRange));
  }

  return { nums, numsAtStartOfRanges, numsAtEndOfRanges };
}