Number to Text , Vietnamese and English only. Origin Post https://www.kimkha.com/2010/03/giai-thuat-oi-tu-so-sang-chu-tieng-viet.html and https://stackoverflow.com/questions/5529934/javascript-numbers-to-words

helper.ts

const NUMERALS = [
  { value: 1000000000000000000000, str: 'sextillion' },
  { value: 1000000000000000000, str: 'quintillion' },
  { value: 1000000000000000, str: 'quadrillion' },
  { value: 1000000000000, str: 'trillion' },
  { value: 1000000000, str: 'billion' },
  { value: 1000000, str: 'million' },
  { value: 1000, str: 'thousand' },
  { value: 100, str: 'hundred' },
  { value: 90, str: 'ninety' },
  { value: 80, str: 'eighty' },
  { value: 70, str: 'seventy' },
  { value: 60, str: 'sixty' },
  { value: 50, str: 'fifty' },
  { value: 40, str: 'forty' },
  { value: 30, str: 'thirty' },
  { value: 20, str: 'twenty' },
  { value: 19, str: 'nineteen' },
  { value: 18, str: 'eighteen' },
  { value: 17, str: 'seventeen' },
  { value: 16, str: 'sixteen' },
  { value: 15, str: 'fifteen' },
  { value: 14, str: 'fourteen' },
  { value: 13, str: 'thirteen' },
  { value: 12, str: 'twelve' },
  { value: 11, str: 'eleven' },
  { value: 10, str: 'ten' },
  { value: 9, str: 'nine' },
  { value: 8, str: 'eight' },
  { value: 7, str: 'seven' },
  { value: 6, str: 'six' },
  { value: 5, str: 'five' },
  { value: 4, str: 'four' },
  { value: 3, str: 'three' },
  { value: 2, str: 'two' },
  { value: 1, str: 'one' },
];

function convertEnglish(n: number): string {
  if (n < 0) {
    return `minus ${convertEnglish(-n)}`;
  } else if (n === 0) {
    return 'zero';
  } else {
    let result = '';
    for (const numeral of NUMERALS) {
      if (n >= numeral.value) {
        if (n < 100) {
          result += numeral.str;
          n -= numeral.value;
          if (n > 0) result += '-';
        } else {
          const times = Math.floor(n / numeral.value);
          result += `${convertEnglish(times)} ${numeral.str}`;
          n -= numeral.value * times;
          if (n > 0) result += ' and ';
        }
      }
    }
    return result;
  }
}

const WORD_NUMBERS = [
  'không',
  'một',
  'hai',
  'ba',
  'bốn',
  'năm',
  'sáu',
  'bảy',
  'tám',
  'chín',
];

function readDozen(no: number, isWhole: boolean) {
  let dozen = '';
  const quotient = Math.floor(no / 10);
  const remainder = no % 10;
  if (quotient > 1) {
    dozen = ` ${WORD_NUMBERS[quotient]} mươi`;
    if (remainder === 1) {
      dozen += ' mốt';
    }
  } else if (quotient === 1) {
    dozen = ' mười';
    if (remainder === 1) {
      dozen += ' một';
    }
  } else if (isWhole && remainder > 0) {
    dozen = ' lẻ';
  }
  if (remainder === 5 && quotient > 1) {
    dozen += ' lăm';
  } else if (remainder > 1 || (remainder === 1 && quotient === 0)) {
    dozen += ` ${WORD_NUMBERS[remainder]}`;
  }
  return dozen;
}

function readHundred(no: number, isWhole: boolean) {
  let text = '';
  const tram = Math.floor(no / 100);
  no = no % 100;
  if (isWhole || tram > 0) {
    text = ` ${WORD_NUMBERS[tram]} trăm`;
    text += readDozen(no, true);
  } else {
    text = readDozen(no, false);
  }
  return text;
}

function readMillion(no: number, isWhole: boolean) {
  let text = '';
  const million = Math.floor(no / 1000000);
  no = no % 1000000;
  if (million > 0) {
    text = `${readHundred(million, isWhole)} triệu`;
    isWhole = true;
  }
  const thousand = Math.floor(no / 1000);
  no = no % 1000;
  if (thousand > 0) {
    text += `${readHundred(thousand, isWhole)} nghìn`;
    isWhole = true;
  }
  if (no > 0) {
    text += readHundred(no, isWhole);
  }
  return text;
}

function convertVietnamese(no: number) {
  if (no === 0) return WORD_NUMBERS[0];
  let text = '',
    postFix = '';
  do {
    const billion = no % 1000000000;
    no = Math.floor(no / 1000000000);
    if (no > 0) {
      text = readMillion(billion, true) + postFix + text;
    } else {
      text = readMillion(billion, false) + postFix + text;
    }
    postFix = ' tỷ';
  } while (no > 0);
  return text;
}

export function convertToText(n: number): string {
  const code = localStorage.getItem('i18nextLng');
  if (code === 'vi') {
    return convertVietnamese(n).trim();
  }
  return convertEnglish(n);
}