// helper function to multiply numbers
const multiply = (num1, num2) => {
  // @params num1, num2 are array of digits in
  // reversed order
  let result = [];
  outer: for(let i = 0; i < num2.length; i++) {
    let carry = 0;
    inner: for(let j = 0; j < num1.length; j++) {
      let product = num2[i] * num1[j];
      if (carry !== 0 ) {
        product += carry;
      }
      if (result[i + j]) {
        product += result[i + j];
      }
      carry = Math.floor(product / 10);
      result[i + j] = product % 10;
    }
    // end of inner loop
    if (carry !== 0) result.push(carry);
  }
  return result;
}
// helper function to convert numbers to 
// digits in reversed order
const toReverseDigits = (number) => {
  let reversedDigits = [];
  while (number > 0) {
    reversedDigits.push(number % 10);
    number = Math.floor(number / 10);
  }
  return reversedDigits;
}

const factorial = n => {
  if (n === 0) return "1";
  if (n === 1) return "1";
  let total = toReverseDigits(n)
  for (let i = n; i > 1; i--) {
    total = multiply(total, toReverseDigits(i - 1));
  }
  return total.reverse().join("");
}