addTwoBigIntegers.js

function addZeroToString(str1, str2){
    while (str1.length > str2.length) {
        str2 = "0" + str2;
    }
    return str2;
}

function addTwoBugNumbers(a, b) {
    if (a.length > b.length) {
        b = addZeroToString(a,b);
    } else {
        a = addZeroToString(b,a);
    }
    a1 = a.split("");
    b1 = b.split("");
    let sum = 0;
    let carry = 0;
    let array = [];
    for (var i = a1.length-1; i >= 0; i--) {
        sum = parseInt(a[i]) + parseInt(b[i]) + parseInt(carry);
        if (sum >= 10) {
            carry = 1;
            sum = sum - 10;
        } else {
            carry = 0;
        }
        array.push(sum);
    }
    console.log(array.reverse().join(""));
    return array.join("");
}