四年之痒之求解出现最多字符

/* 出现最多字符 */
(function(str){
    str = str || '';
    var max_occur_char, max_occurs = 0;
    do{
        var len = str.length, ch = str[0], current_char_occurs;
        str = str.replace(new RegExp(ch, "g"), '');
        current_char_occurs = len - str.length;
        if(max_occurs < current_char_occurs){
            max_occurs = current_char_occurs;
            max_occur_char = ch;
        }
    }
    while(str.length > 0);
    console.log("max occur char: " + max_occur_char + ', occur times: ' + max_occurs);
    return max_occur_char;
})("adfjlajksfjqwetasddddddkfmwrgopqdvnladf");