Sum strings as numbers

stringAdd.js

function stringAdd(a, b) {
  /*
  *  Adds two numbers, in a way you would do with pen and paper
  *  Since the numbers are kept as strings, it can handle really big numbers
  *  Useful when you need to calculate sum of two big integers that are bigger than Number.MAX_SAFE_INTEGER
  */
  a += '' , b += '';
  var res = [], carry = 0, lena = a.length, lenb = b.length;
  if(lena > lenb) {
    b = new Array(lena - lenb + 1).join('0') + b;
  } else {
    a = new Array(lenb - lena + 1).join('0') + a;
  }
  for(var i=Math.max(lena, lenb)-1; i>=0; i--) {
    var add = +(a.charAt(i)) + +(b.charAt(i)) + carry;
    res.push(add%10);
    carry = (add - (add%10)) / 10;
  }
  if(carry) res.push(carry);
  return res.reverse().join('');
}