SOURCE

var addBinary = function(a, b) {
    if(a.length > b.length){
        let len = a.length - b.length
        for(i = 0; i < len; i++){
            b = b.slice(0,0).concat('0', b)
        }
    }else if(b.length > a.length){
        let len = b.length - a.length
        for(i = 0; i < len; i++){
            a = a.slice(0,0).concat('0', a)
        }
    }
    let list = []
    for(i = 0; i < a.length; i++){
        list.push(0)
    }
    let data = ""
    for(let i = a.length - 1; i >=0; i--){
        let num = parseInt(a[i]) + parseInt(b[i]) + list[i]
        if(num >= 2){
            data = data.slice(0,0).concat(num % 2, data)
            if(i === 0){
                data = data.slice(0,0).concat('1', data)
            }else{
                list[i - 1] = list[i - 1] + 1
            }
        }else{
            data = data.slice(0,0).concat(num, data)
        }
    }
    return data
};
console.log(addBinary('1','1111'))