javascript add big numbers

1function add(str1, str2) {
2
3    let sum = "";  // our result will be stored in a string.
4
5    // we'll need these in the program many times.
6    let str1Length = str1.length;
7    let str2Length = str2.length;
8
9    // if s2 is longer than s1, swap them.
10    if(str2Length > str1Length ){
11        let temp = str2;
12        str2 = str1;
13        str1 = temp;
14    }
15
16    let carry = 0;  // number that is carried to next decimal place, initially zero.
17    let a;
18    let b;
19    let temp;
20    let digitSum;
21    for (let i = 0; i < str1.length; i++) {
22        a = parseInt(str1.charAt(str1.length - 1 - i));      // get ith digit of str1 from right, we store it in a
23        b = parseInt(str2.charAt(str2.length - 1 - i));      // get ith digit of str2 from right, we store it in b
24        b = (b) ? b : 0;                                    // make sure b is a number, (this is useful in case, str2 is shorter than str1
25        temp = (carry + a + b).toString();                  // add a and b along with carry, store it in a temp string.
26        digitSum = temp.charAt(temp.length - 1);            //
27        carry = parseInt(temp.substr(0, temp.length - 1));  // split the string into carry and digitSum ( least significant digit of abSum.
28        carry = (carry) ? carry : 0;                        // if carry is not number, make it zero.
29
30        sum = (i === str1.length - 1) ? temp + sum : digitSum + sum;  // append digitSum to 'sum'. If we reach leftmost digit, append abSum which includes carry too.
31
32    }
33
34    return sum;     // return sum
35
36}