Amount in Words (Lakh-Crore system) in TypeScript

index.ts

const a = [
  "",
  "one ",
  "two ",
  "three ",
  "four ",
  "five ",
  "six ",
  "seven ",
  "eight ",
  "nine ",
  "ten ",
  "eleven ",
  "twelve ",
  "thirteen ",
  "fourteen ",
  "fifteen ",
  "sixteen ",
  "seventeen ",
  "eighteen ",
  "nineteen ",
];
const b = [
  "",
  "",
  "twenty",
  "thirty",
  "forty",
  "fifty",
  "sixty",
  "seventy",
  "eighty",
  "ninety",
];

const regex = /^(\d{2})(\d{2})(\d{2})(\d{1})(\d{2})$/;

const getLT20 = (n: string) => a[Number(n)];
const getGT20 = (n: string) => b[n[0]] + " " + a[n[1]];

function numWords(input: string): string {
  const num = Number(input);
  if (isNaN(num)) return "";
  if (num === 0) return "zero";

  const numStr = num.toString();

  const matched = `000000000${numStr}`.slice(-9).match(regex);

  if (!matched) {
    return "";
  }

  if (numStr.length > 9) {
    return (
      numWords(numStr.slice(0, numStr.length - 7)) +
      " crore " +
      numWords(numStr.slice(-7))
    );
  }

  const [, n1, n2, n3, n4, n5] = matched;

  let str = "";
  str += parseInt(n1) !== 0 ? (getLT20(n1) || getGT20(n1)) + "crore " : "";
  str += parseInt(n2) !== 0 ? (getLT20(n2) || getGT20(n2)) + "lakh " : "";
  str += parseInt(n3) !== 0 ? (getLT20(n3) || getGT20(n3)) + "thousand " : "";
  str += parseInt(n4) !== 0 ? getLT20(n4) + "hundred " : "";
  str += parseInt(n5) !== 0 && str !== "" ? "and " : "";
  str += parseInt(n5) !== 0 ? getLT20(n5) || getGT20(n5) : "";

  return str.trim();
}

// console.log(numWords("19231391230123"));