text-number.js

const units = [
    'zero', 'one', 'two', 'three', 'four',
    'five', 'six', 'seven', 'eight', 'nine',
    'ten', 'eleven', 'twelve', 'thirteen', 'fourteen',
    'fifteen', 'sixteen', 'seventeen', 'eighteen', 'nineteen',
];

const tens = [
    '', 'ten', 'twenty', 'thirty', 'fourty',
    'fifty', 'sixty', 'seventy', 'eighty', 'ninety',
];

const exponents = [
    '', 'thousand', 'million', 'billion', 'trillion',
    'quadrillion', 'quintillion', 'sextillion', 'septillion', 'octillion',
    'nonillion', 'decillion', 'undecillion', 'duodecillion', 'tredecillion',
    'quattuordecillion', 'quindecillion', 'sexdecillion', 'septendecillion', 'octodecillion',
    'novemdecillion', 'vigintillion',
];

function renderNumberGroup(numberGroup, exponent, exponentCount) {
    const unit = numberGroup.at(-1);
    const ten = numberGroup.at(-2) || 0;
    const hundred = numberGroup.at(-3) || 0;
    const unitAndTen = +`${ten}${unit}`;
    return `${hundred ? `${units[hundred]} hundred${unitAndTen ? ' and ' : ''}` : ''}${unitAndTen && unitAndTen < 20
        ? units[unitAndTen]
        : `${ten ? `${tens[ten]} ` : ''}${unit ? units[unit] : ten || exponentCount > 1 ? '' : units[0]}`}${exponent ? ` ${exponents[exponent]},` : ''}`;
}

function buildTextNumber(number) {
    const numbers = String(number).split('');
    const numberGroups = [];
    numbers.reverse();
    numbers.forEach((number) => {
        const lastGroup = numberGroups.at(-1);
        if (!lastGroup || lastGroup.length === 3) {
            numberGroups.push([+number]);
            return;
        }
        lastGroup.push(+number);
    });
    numberGroups.forEach((group) => group.reverse());
    const namedNumberParts = numberGroups.map((numberGroup, index, numberGroups) => {
        return renderNumberGroup(numberGroup, index, numberGroups.length);
    });
    namedNumberParts.reverse();
    return namedNumberParts.join(' ');
}

export default buildTextNumber;