判断一个字符串中出现次数最多的字符，统计这个次数

var str = "abcdefgaaaaaaddda";
var obj = {};
var arr = [];
var letter;
for(var i = 0,len = str.length;i<len;i++){
letter = str[i];
if(!obj[letter]){
obj[letter] = 1;
}else{
obj[letter]++;
}
}

var max_key,max_num=0;
for(key in obj){
if(max_num <obj[key]){
max_num = obj[key];
max_key = key;
}
}
document.write("字母："+max_key+"　次数："+max_num);