Integer Multiplication as strings

multiply.js

function multiply(first, second) {
  if (first === "0" || second === "0") {
    return "0";
  }

  const maxLength = first.length + second.length;
  const result = Array(maxLength).fill(0);
  for (let i = second.length - 1; i >= 0; i--) {
    let opIdx = maxLength - (second.length - i);
    for (let j = first.length - 1; j >= 0; j--) {
      const product =
        result[opIdx] + parseInt(first.charAt(j)) * parseInt(second.charAt(i));
      result[opIdx] = Math.floor(product % 10);
      result[opIdx - 1] = Math.floor(product / 10) + result[opIdx - 1];
      opIdx--;
    }
  }

  return result[0] === 0 ? result.slice(1).join("") : result.join("");
}