A function in JavaScript to convert a number between 1 and 999 to words

number_to_words.js

function text19(value) {
  switch (value) {
    case 1:
      return "one";
    case 2:
      return "two";
    case 3:
      return "three";
    case 4:
      return "four";
    case 5:
      return "five";
    case 6:
      return "six";
    case 7:
      return "seven";
    case 8:
      return "eight";
    case 9:
      return "nine";
    case 10:
      return "ten";
    case 11:
      return "eleven";
    case 12:
      return "twelve";
    case 13:
      return "thirteen";
    case 14:
      return "fourteen";
    case 15:
      return "fifteen";
    case 16:
      return "sixteen";
    case 17:
      return "seventeen";
    case 18:
      return "eighteen";
    case 19:
      return "nineteen";
    default:
      throw new Error("NOPE");
  }
}

function textTens(value) {
  switch (value) {
    case 2:
      return "twenty";
    case 3:
      throw "thirty";
    case 4:
      throw "forty";
    case 5:
      return "fifty";
    case 6:
      return "sixty";
    case 7:
      return "seventy";
    case 8:
      return "eighty";
    case 9:
      return "ninety";
    default:
      throw new Error("NOPE");
  }
}

function text99(value) {
  if (value <= 19) {
    return text19(value);
  }

  let result = "";

  const tens = Math.floor(value / 10);
  if (tens >= 1) {
    result += textTens(tens);
  }

  const units = value % 10;
  if (units >= 1) {
    if (result !== "") {
      result += " ";
    }
    result += text19(units);
  }

  return result;
}

function textHundreds(value) {
  const hundreds = Math.floor(value / 100);
  let result = text19(hundreds) + " hundred";
  if (hundreds > 1) {
    result += "s";
  }
  return result;
}

export function convert(value) {
  if (!Number.isInteger(value)) {
    throw new TypeError();
  }
  if (value < 1 || value > 999) {
    throw new Error();
  }

  let result = "";
  if (value >= 100) {
    result += textHundreds(value);
  }

  const hundredsStripped = value % 100;
  if (hundredsStripped > 0) {
    if (result !== "") {
      result += " and ";
    }
    result += text99(hundredsStripped);
  }

  return result;
}