bignum.js

const add = (a, b) => {
    a = a.split('').reverse();
    b = b.split('').reverse();
    let a1, b1, c, d = 0, e, i = 0;
    do {
        a1 = +a[i] || 0;
        b1 = +b[i] || 0;
        c = a1 + b1 + d;
        d = c / 10 | 0;
        e = c % 10;
        a[i] = e;
        i++;
    } while(d);
    return a.reverse().join('');
}

const a = '155';
const b = '58';

console.log(add(a, b));