js program to find factorials of larger numbers

factorial.js

const toInt = (str) => str - "0";

function factorial(num) {
	num = trimLeadingZero(num);
	if (num === "0" || num === "1") return "1";

	return stringMultiply(num, factorial(stringSubtract(num, 1)));
}

function trimLeadingZero(num) {
	let numArray = Array.from(num);
	if (num.length === 1) return num;
	for (let i = 0; i < num.length; i++) {
		if (num[i] === "0") numArray.shift();
		else return numArray.join("");
	}

	return numArray.join("");
}

function stringSubtract(minuend, subtrahend) {
	let carry = subtrahend;
	let numArray = Array.from(minuend);

	for (let i = numArray.length - 1; i >= 0; --i) {
		if (toInt(numArray[i]) < carry) {
			// we need to carry over 1
			numArray[i] = toInt(numArray[i]) + 10 - carry + "";
			carry = 1;
		} else {
			numArray[i] = toInt(numArray[i]) - carry + "";
			carry = 0;
		}
	}

	return numArray.join("");
}

// only to be used when multiplier is single digit
function stringSingleMultiply(multiplicand, multiplier) {
	if (multiplier == 0) return "0";

	let carry = 0;
	let numArray = Array.from(multiplicand);

	for (let i = numArray.length - 1; i >= 0; --i) {
		let num = toInt(numArray[i]) * multiplier;
		carry += num;
		numArray[i] = (carry % 10) + "";
		carry = carry != 0 ? Math.floor(carry / 10) : 0;
	}

	if (carry != 0) numArray.unshift(carry + "");

	return numArray.join("");
}

function stringMultiply(multiplicand, multiplier) {
	let result = "0";
	for (let i = multiplier.length - 1, power = 0; i >= 0; --i, ++power) {
		result = stringAddition(
			result,
			stringSingleMultiply(
				multiplyBase10Expo(multiplicand, power),
				multiplier[i]
			)
		);
	}
	return result;
}

function stringAddition(num, addend) {
	if (addend === "0") return num;

	let finalLength = num.length > addend.length ? num.length : addend.length;
	let finalString = [];
	let carry = 0;

	for (i = 0; i < finalLength; i++) {
		if (i < num.length) {
			carry += toInt(num[num.length - i - 1]);
		}
		if (i < addend.length) {
			carry += toInt(addend[addend.length - i - 1]);
		}

		finalString.unshift((carry % 10) + "");
		carry = Math.floor(carry / 10);
	}

	if (carry != 0) finalString.unshift(carry + "");

	return finalString.join("");
}

// basically does (num * 10 ^ power)
function multiplyBase10Expo(num, power) {
	for (let i = 0; i < power; i++) {
		num += "0";
	}

	return num;
}

console.log(factorial("1000"));