Reorganize String

let reorganizeString = function(S) {
    let chars = {}, max = 0;
    for(let i = 0; i < S.length;i++) {
        if (chars[S[i]]) {
            chars[S[i]] = chars[S[i]] + 1;
            max = Math.max(chars[S[i]], max);
        }
        else {
            chars[S[i]] = 1;
        }
    }

    console.log(max, Math.floor((S.length + 1) / 2));
    if (max > (S.length + 1) / 2) {
        return "";
    }

    let result = [], index = 0;
    const keys = Object.keys(chars).sort((a, b) => chars[b] - chars[a]); //?
    while (keys.length) { //?
        const key = keys.shift(); //?
        result[index] = key;

        if (chars[key] - 1 > 0) {
            chars[key] = chars[key] - 1;
            keys.unshift(key); //?
        }

        index += 2; //?
        if (index >= S.length) {
            index = 1;
        }
    }

    return result.join('');
};