 
  
  function sumStrings(s1, s2) {
  if (s1.length < s2.length) [s1, s2] = [s2, s1];
  s1 = [...s1].reduceRight((a, c, i) => {
    c = +c + ~~s2[i - (s1.length - s2.length)] + a.r;
    a.s = c % 10 + a.s;
    a.r = c / 10 | 0;
    return a;
  }, {r: 0, s: ''});
  return ((s1.r || '') + s1.s).replace(/^0+(?=\d)/, '');
} 
  
  Number.MAX_SAFE_INTEGER
9007199254740991