第一种

var str = "aaasssksldnfkjr"
var o = {}
for (var i = 0; i < str.length; i++) {
    var name = str[i]
    if (o[name]) {
        o[name]++
    } else {
        o[name] = 1
    }
}
var max = 0;
var maxs = ""
for (var name in o) {
    if (max < o[name]) {
        max = o[name];
        maxs = name
    }
}
console.log(max, maxs)

第二种 正则方法

var str = "aaasssksldnfkjr"
var arr = str.split("")
var Arr = arr.sort()
var s = Arr.join("")
console.log(s)
var max = 0;
var maxs=""
s.replace(/(\w)\1+/g,function(s,s2){
    if(s.length>max){
        max=s.length;
        maxs=s2
    }
})
console.log(max, maxs) 

第三种

var str = "aaasssksldnfkjr"
var arr = str.split("")
var Arr = arr.sort();
var max=0;
var maxs="";
var n=1
for(var i=0;i<Arr.length;i++){
    if(Arr[i]==Arr[i+1]){
        n++
    }else{
        if(n>max){
            max=n;
            maxs=Arr[i]
        }
        n=1
    }
}
console.log(max, maxs)