寻找一个字符串出现次数最多的字母以及次数

String.prototype.maxTimesC = function(){
this.str = this.toString();
this.maxTimes = 0;
this.preTimes = 0;
this.maxTimesC = [];
while (this.str!="") {
this.preTimes = this.str.split(this.str.substring(0,1)).length-1;
if(this.preTimes>this.maxTimes){
//当前次数大于最大次数
this.maxTimes = this.preTimes;
this.maxTimesC.length = 0;
this.maxTimesC.push(this.str.substring(0,1));
}else if(this.preTimes == this.maxTimes){
//当前次数等于最大次数
this.maxTimesC.push(this.str.substring(0,1));
}else{
//当前次数小于最大次数
}
this.str = this.str.split(this.str.substring(0,1)).join("");
};
if(str === ""){
alert("字符串为空");
}
var result = [this.maxTimesC,this.maxTimes];
return result;
};