able-exercise.js

const getDigits = str => str.split("").map(digit => parseInt(digit, 10));

const stripZeros = digits =>
  digits.length <= 1 ? digits : digits.replace(/^0*/, "");

const _addBignums = (num1, num2) => {
  const digits1 = getDigits(num1).reverse(); // [3, 2, 2]
  const digits2 = getDigits(num2).reverse(); // [4, 1]

  let carry = 0; // Me dice si debo agregar un carry en esta iteración

  const result = digits1.map((digit1, index) => {
    const digit2 = digits2[index] || 0;
    let resultDigit = digit1 + digit2 + carry;
    carry = 0;

    if (resultDigit >= 10) {
      carry = 1;
      resultDigit -= 10;
    }

    return resultDigit;
  });

  if (carry) {
    result.push(1);
  }

  return result.reverse().join("");
};

const addBignums = (num1, num2) => {
  const num1_ = stripZeros(num1);
  const num2_ = stripZeros(num2);
  return num1_.length >= num2_.length
    ? _addBignums(num1_, num2_)
    : _addBignums(num2_, num1_);
};

function sumStrings(a, b) {
  return addBignums(a, b);
}