Add big integer strings, returns a string

gistfile1.js

function addBigInts(a, b) {
  var result = '';
  var carry = 0;
  
  // convert to arrays and flip around
  a = a.split('').reverse();
  b = b.split('').reverse();
  
  // we have to loop through the longer number to add all the values
  var longerNumber = (a.length > b.length) ? a : b;
  
  longerNumber.map(function(val, i ,arr){
    // check to see if the index exists in the array
    aVal = (typeof a[i] != 'undefined') ? a[i] : 0;
    bVal = (typeof b[i] != 'undefined') ? b[i] : 0;
    var sum = Number(aVal) + Number(bVal) + carry;
    if(sum > 9){
      carry = 1;
      sum = sum + '';
      result += sum[1];
    } else {
      carry = 0;
      result += sum + '';
    }
  });
  
  // if we have a leftover carry digit, add 1 to the end
  if(carry > 0){
    result = result + '1';
  }
 
  // turn the results the right way around
  result = result.split('');
  result.reverse();
  result = result.join('');
  
  return result;
}