Script to get text in Lithuanian language from a number (Javascript)

numberToText.js

// According to https://gist.github.com/tadas-subonis/6e23811db350a440cd47e2e5dd40e2cb

const numToText = (num) => {
  const units = [
    "",
    "vienas",
    "du",
    "trys",
    "keturi",
    "penki",
    "šeši",
    "septyni",
    "aštuoni",
    "devyni",
  ];

  const subTen = [
    "",
    "vienuolika",
    "dvylika",
    "trylika",
    "keturiolika",
    "penkiolika",
    "šešiolika",
    "septyniolika",
    "aštuoniolika",
    "devyniolika",
  ];

  const tens = [
    "",
    "dešimt",
    "dvidešimt",
    "trisdešimt",
    "keturiasdešimt",
    "penkiasdešimt",
    "šešiasdešimt",
    "septyniasdešimt",
    "aštuoniasdešimt",
    "devyniasdešimt",
  ];

  const numWord = [
    ["milijonas", "milijonai", "milijonų"],
    ["tūkstantis", "tūkstančiai", "tūkstančių"],
  ];

  if (num < 0 || num >= 10 ** 9) {
    /* If number is less than zero or greater or equal to a billion, 
       return a stringified text of the number
    */
    return `${num}`;
  } else {
    const strInt_num = `${parseInt(`${num}`)}`; // Stringify the number
    const len = 9 - strInt_num.length;
    const numOfZeros = Math.sign(len) === 1 ? len : 0;

    // Precede the number by zeros until the length of the string is 9
    const formatted = `${"0".repeat(numOfZeros)}${strInt_num}`;

    // Split the 9 characters long string above into 3 arrays.
    // Each array has a string with a length of 3
    const numTripletArray = formatted.split(/(.{3})/g).filter((N) => N);
    const numSet = [];
    numTripletArray.forEach((triplet, n) => {
      num = parseInt(triplet) || 0;

      let numCase = 0;

      const hundDiv = Math.floor(num / 100);

      if (hundDiv > 0) {
        if (hundDiv > 1) {
          numSet.push(units[hundDiv]);
        }
        numSet.push(hundDiv > 1 ? "šimtai" : "šimtas");
      }

      // Example: If num =  is 197
      const subHundNum = num % 100; // Becomes 97
      const tensValue = Math.floor(subHundNum / 10); // Becomes 9
      const unitValue = subHundNum % 10; // Becomes 7

      if (10 < subHundNum && subHundNum < 20) {
        // If num is between 10 and 20
        numSet.push(subTen[unitValue]);
        numCase = 2;
      } else {
        // Num that is greater than or equal to 20
        if (tensValue > 0) {
          // If there is a tens value
          numSet.push(tens[tensValue]);
        }
        if (unitValue > 0) {
          // If there is a unit value
          numSet.push(units[unitValue]);
          numCase = unitValue > 1 ? 1 : 0;
        } else {
          numCase = 2;
        }
      }
      if (n < numWord.length && triplet !== "000") {
        numSet.push(numWord[n][numCase]);
      }
    });
    let number = numSet.join(" ").trim();
    number = number.replace("  ", " ");
    return number;
  }
};