Arbitrary-Length Integer Addition in Javascript

Note that this isn't the most efficient way to do it, but it is simple and fast to code, and the logic should be easy to understand.

/* Note that this reverses the order - intentional, to preserve place ordering 
   takes in an arbitrary length string (though limited to an int32 length in practise)
 */
var readNumFromString = function(s) {
  return s.split("").map(Number).reverse();
};

/* adds two arrays created by readNumFromString */
var addNumArrays = function(a,b) {
  let ret = [];
  let carry = 0;
  let i = 0;
  /* shorter one first */
  if (a.length > b.length)
    return addNumArrays(b,a);
  /* add up to the length of the shorter */
  for (i = 0; i < a.length; i++)
  {
    let t = a[i] + b[i] + carry;
    ret[i] = t % 10;
    carry = (t > 9 ? 1 : 0);
  }
  /* handle the undefined values properly, since they don't cast */
  while (i < b.length)
  {
    let t = b[i] + carry;
    ret[i++] = t % 10;
    carry = (t > 9 ? 1 : 0);
  }
  /* possible carry digit at the end */
  if (carry == 1)
    ret[i] = 1;
  return ret;
};

/* turns the arrays back into strings */
var printNumToString = function(a) {
  return a.reverse().join("");
};