Add Binary

Problem statement

Given two binary strings a and b, return their sum as a binary string.

Example 1:

Input: a = "11", b = "1"Output: "100"

Example 2:

Input: a = "1010", b = "1011"Output: "10101"

Constraints:

• 1 <= a.length, b.length <= 104
• a and b consist only of '0' or '1' characters.
• Each string does not contain leading zeros except for the zero itself.

My solution

/**
 * @param {string} a
 * @param {string} b
 * @return {string}
 */
var addBinary = function(a, b) {
    let aLen = a.length - 1;
    let bLen = b.length - 1;
    
    let total = ''
    let reminder = 0;
    
    while (aLen >=0 || bLen >= 0) {
        const aInt = parseInt(a.charAt(aLen)) || 0
        const bInt = parseInt(b.charAt(bLen)) || 0
        const sum = aInt + bInt + reminder;
        
        const digit = sum % 2;
        total = digit + total;
        
        reminder = sum > 1 ? 1 : 0
        
        aLen--
        bLen--
    }
    
    total = `${reminder ? "1" : ""}${total}`
    
    return total
};