Adds two binary strings in JavaScript and returns sum string

addBinary.js

var addBinary = function(a, b) {
    let aLen = a.length, bLen = b.length;
    let maxLen = aLen;
    if(bLen > aLen) {
        maxLen = bLen;
    }
    let carryBit = 0;
    let sumStr = "";
    for(let i = 1; i <= maxLen || carryBit > 0; i++) {
        let bitA = 0, bitB = 0;
        let aOffset = aLen - i;
        let bOffset = bLen - i;
        if(aOffset >= 0 && a[aOffset] == '1') {
            bitA = 1;
        }
        if(bOffset >= 0 && b[bOffset] == '1') {
            bitB = 1;
        }
        let sum = carryBit + bitA + bitB;
        if((sum & 1) == 0) {
            sumStr = "0" + sumStr;
        } else {
            sumStr = "1" + sumStr;
        }
        carryBit = sum >> 1;
    }
    return sumStr;
};