numeral.js

// we don't need the zero terms, but indexing is easier this way
const ones = [null, "ūnus", "duo", "trēs", "quattuor", "quīnque", "sex", "septem", "octō", "novem"];
const tenPlusOnes = [null, "ūndecim", "duodecim", "trēdecim", "quattuordecim", "quīndecim", "sēdecim", "septendecim", null, null];
const tens = [null, "decem", "vīgintī", "trīgintā", "quadrāgintā", "quīnquāgintā", "sexāgintā", "septuāgintā", "octōgintā", "nōnāgintā"];
const hundreds = [null, "centum", "ducentī", "trecentī", "quadringentī", "quīngentī", "sescentī", "septingentī", "octingentī", "nōngentī"];

function cardinalNumberLessThanThousand(input) {
	if (input <= 0 || input >= 1000)
		return null;
	var result = "", pending = "";
	let digits = input.toString().split("").map(x=>+x).reverse();
	digits.push(0); digits.push(0); // for carrying
	// position = n for 10^n digit
	for (position = 0; position < digits.length; position++) {
		let digit = digits[position];
		if (digit == 0)
			continue;
		var term = null;
		switch (position) {
			case 0: // ones
				if ((digit == 8 || digit == 9) && digits[1] != 0 && digits[1] != 9) {
					pending = digit == 8 ? "duodē" : "ūndē";
					digits[1]++;
				} else if (digits[1] == 1) {
					term = tenPlusOnes[digit];
					digits[1] = 0;
				} else {
					term = ones[digit];
				}
				break;
			case 1: // tens
				term = tens[digit];
				break;
			case 2: // hundreds
				term = hundreds[digit];
				break;
		}
		if (term != null) {
			result = pending + term + (result == "" ? "" : (" " + result));
			pending = "";
		}
	}
	return result;
}

function cardinalNumber(input) {
	if (input == 0)
		return "nihil";
	if (input < 0 || input >= 1000000)
		return "";
	// Last part (last three digits)
	let remainder = input % 1000;
	var result = cardinalNumberLessThanThousand(remainder);
	if (input < 1000)
		return result;
	if (result == null) {
		result = "";
	} else {
		result = " " + result;
	}
	// First part
	let thousands = Math.floor(input / 1000);
	if (thousands == 1) {
		result = "mīlle" + result;
	} else {
		result = cardinalNumberLessThanThousand(thousands) + " mīlia" + result;
	}
	return result;
}

var result = "";
for (i = 0; i < 1000000; i++) {
	result += i + "," + cardinalNumber(i) + "\n";
}
console.log(result);