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