Integer to words coding challenge

number-to-words.js

UNIT_WORD = [
	'zero',
	'one',
	'two',
	'three',
	'four',
	'five',
	'six',
	'seven',
	'eight',
	'nine',
];

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

TEENS_WORD = [
	'ten',
	'eleven',
	'twelve',
	'thirteen',
	'fourteen',
	'fifteen',
	'sixteen',
	'seventeen',
	'eighteen',
	'nineteen',
];

MULT_WORD = [
	'',
	'thousand',
	'million',
	'billion',
	'trillion',
];

function words(number) {
	const numberString = number.toString();

	if (!/^([1-9]\d*|0)$/.test(numberString)) {
		throw new Error("Input must be a non-negative integer with no leading zeroes");
	}

	if (numberString.length > MULT_WORD.length * 3) {
		throw new Error("Can't handle numbers this big");
	}

	// Split into array of integer digits
	const digits = numberString.split('').map(s => parseInt(s, 10));

	// Short-circuit for single digit (also the only way to output "zero")
	if (digits.length === 1) {
		return UNIT_WORD[digits[0]];
	}

	// Split into chunks of three, from end to start
	const triples = [];
	while (digits.length >= 3) {
		triples.unshift(digits.splice(-3, 3));
	}
	if (digits.length) {
		// Pad leftovers with zeros
		while (digits.length < 3) {
			digits.unshift(0);
		}
		triples.unshift(digits);
	}

	// Turn it into words
	let output = '';
	for (const [triple, [h, t, u]] of Object.entries(triples)) {
		let words = '';
		let comma = true;
		if (h) {
			words = `${UNIT_WORD[h]} hundred`;
		}
		if (h && (t || u)) {
			words += " and ";
		}
		if (t === 1 && u) {
			words += TEENS_WORD[u];
		} else if (t) {
			words += TENS_WORD[t];
			if (u) {
				words += "-";
			}
		}
		if (t !== 1 && u) {
			words += UNIT_WORD[u];
		}
		if (h || t || u) {
			const mult = triples.length - triple - 1;
			if (mult > 0) {
				words += " " + MULT_WORD[mult];
			} else if (!h) {
				comma = false;
			}
		}
		if (words.length) {
			if (output.length) {
				if (comma) {
					output += ', ';
				} else {
					output += ' and ';
				}
			}
			output += words;
		}
	}

	return output;
}

function test(input) {
	console.log(`Input: ${input}, output: ${words(input)}`);
}

for (let i = 0; i < 20; i++) {
	test(i);
}
for (i of [
	20,
	25,
	99,
	100,
	105,
	120,
	123,
	200,
	333,
	1000,
	1001,
	1010,
	1011,
	1100,
	1101,
	1110,
	1111,
	1234,
	12345,
	123456,
	1234567,
	12345678,
	123456789,
	1234567890,
	12345678901,
	123456789012,
	1234567890123,
	12345678901234,
	123456789012345,
	300000000000000,
	300000000000001,
	999999999999999,
]) {
	test(i);
}