SOURCE

var addStrings = function(nums1, nums2) {
let i=nums1.length-1;
    let j=nums2.length-1;
    let carry = 0;
    let str = '';
    while(i>=0 || j>=0) {
        let a = i>=0? nums1.charCodeAt(i) :48;
        let b = j>=0? nums2.charCodeAt(j) :48;
        let res = a + b + carry - 96;
        if(res >= 10) {
            carry = 1;
            res -= 10;
        } else {
            carry = 0;
        }
        str = res + str;
        i--;
        j--;
    }
    return carry ? '1' + str : str;
};
console.log(addStrings("6913259244","71103343"));