sum-big-numbers.js

/**
 * Sum two big numbers given as strings.
 * 
 * @param {string} a
 * @param {string} b
 * @return {string}
 */
function sumStrings(a, b) {
  var zrx = /^0+/; // remove leading zeros
  a = a.replace(zrx, '').split('').reverse();
  b = b.replace(zrx, '').split('').reverse();

  var result = [], max = Math.max(a.length, b.length);
  for (var memo = 0, i = 0; i < max; i++) {
    var res = parseInt(a[i] || 0) + parseInt(b[i] || 0) + memo;
    result[i] = res % 10;
    memo = (res - result[i]) / 10;
  }

  if (memo) {
    result.push(memo);
  }

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