stringProduct.js

function stringProduct(n1, n2) {
	return toString(toInt(n1) * toInt(n2))
}

const CHAR_0 = "0".charCodeAt(0) // 48

function toInt(str) {
	return [...str].reverse().reduce((acc, c, i) => {
		return acc + BigInt(c.charCodeAt(0) - CHAR_0) * (10n**BigInt(i))
	}, 0n)
}

function toString(int) {
	if (int === 0n) {
		return "0";
	}

	let str = ""
	while (int > 0n) {
		str = String.fromCharCode(CHAR_0 + Number(int % 10n)) + str	
		int = int / 10n
	}

	return str
}