自己写的javascript map 函数, 用于统计字符出现次数

function Map(s) {
this.keys = new Array();
this.data = new Object();
this.put = function (key, value) {
if (this.data[key] == null) {
this.keys.push(key);
this.data[key] = value;
} else {
this.data[key] =this.data[key]+1;
}
return true;
};
this.get = function (key) {
return this.data[key]
}
for(var i=0; i<s.length;i++) {
this.put(s[i], 1);
}
return this.data;
}