Expand '1-2,4-5' to '1,2,4,5' and print numbers missing in sequence

range.mjs

const { assert, log } = console
const input = '1-2,4-6'
const output = '1,2,4,5,6'
const outputMissing = '3'
const numericSort = (a, b) => a - b

function expandRange (string) {
  return String(string.split(',').reduce(function (r, a) {
    var b = a.split('-').map(Number)
    do {
      r.push(b[0])
      b[0]++
    } while (b[0] <= b[1])
    return r
  }, []))
}

log(expandRange(input))
assert(expandRange(input) === output, `Got "${expandRange(input)}" expected "${output}"`)

const missingVolumesReduce = (missing, val, i, arr) => {
  const previous = i - 1
  const diff = val - arr[previous]
  if (diff < 1) return []

  let j = 1
  while (j < diff) {
    missing.push(arr[previous] + j)
    j++
  }

  return missing
}

function missingVolumes (string) {
  return String(`0,${string}`
    .split(',')
    .map(Number)
    .sort(numericSort)
    .reduce(missingVolumesReduce, []))
}

log(missingVolumes(expandRange(input)))
assert(missingVolumes(expandRange(input)) === outputMissing, `Got "${missingVolumes(expandRange(input))}" expected "${outputMissing}"`)

// 1-2,3,4-5
// 1,2,3,4,5

// assert(true === false, "Hello")