https://leetcode.com/problems/add-binary/

addBinary.js

/*
Sort-of brute force... I know there's a cleverer way to
do this with XORs and bit tricks but if the answer doesn't
fit into 53-bit int I'd be hosed so I'm sticking to what
I know will work =P

O(max(a.length, b.length))
*/

var addBinary = function(a, b) {
    
    const result = [];
    const len = Math.max(a.length, b.length);
    
    let i = 1;
    let carry = 0;

    while(len - i >= 0){
        
        const x = a[a.length-i] === '1' ? 1 : 0;
        const y = b[b.length-i] === '1' ? 1 : 0;
        
        const sum = x + y + carry;
        
        if(sum === 0){
            result.push(0);
            carry = 0;
        }
        else if(sum === 1){
            result.push(1);
            carry = 0;
        }
        else if(sum === 2){
            result.push(0);
            carry = 1;
        }
        else if(sum === 3){
            result.push(1);
            carry = 1;
        }
        
        i++;
    }
    
    if(carry) result.push(1);
    return result.reverse().join('');
};