Some code to do stuff

decimal-to-roman-numeral-converter.ts

interface RomanNumeral {
  readonly number: number
  readonly numeral: string
}

const romanNumerals = [
  { number: 1, numeral: "I" },
  { number: 4, numeral: "IV" },
  { number: 5, numeral: "V" },
  { number: 9, numeral: "IX" },
  { number: 10, numeral: "X" },
  { number: 40, numeral: "XL" },
  { number: 50, numeral: "L" },
  { number: 90, numeral: "XC" },
  { number: 100, numeral: "C" },
  { number: 400, numeral: "CD" },
  { number: 500, numeral: "D" },
  { number: 900, numeral: "CM" },
  { number: 1000, numeral: "M" }
]

function findBefore<T>(arr: ReadonlyArray<T>, predicate: (item: T) => boolean): T | null {
  let result: T | null = null

  for (let element of arr) {
    if (predicate(element)) {
      result = element
    }
  }

  return result
}

function decimalToRomanNumeral(n: number, acc: string = "") {
  if (n <= 0) {
    return acc
  }

  let maxNumeral: RomanNumeral = findBefore<RomanNumeral>(romanNumerals, rn => rn.number <= n)

  return decimalToRomanNumeral(n - maxNumeral.number, acc + maxNumeral.numeral)
}