A generator function that yields roman numerals in the specified diapason

roman-sequence.js

function* romanSequence (from, to) {
  switch (arguments.length) {
    case 0:
      console.warn('[romanNumerals]: the function expects at least one argument')
      return
    case 1: 
      to = from
      from = 1
  }

  const key = [
    "", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM",
    "", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC",
    "", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"
  ]

  for (; from <= to; from++) {
    const digits = String(from).split("")

    let i = 3
    let roman = ""

    while (i--)
      roman = (key[+digits.pop() + (i * 10)] || "") + roman

    yield Array(+digits.join("") + 1).join("M") + roman
  }
}