SOURCE

let str = 'aaaadf';

function max(str){
    let arr = str.split('');
    return arr.reduce((obj,item)=>{
        if(item in obj){
            obj[item]++;
            if(obj[item]>obj["max"]){
                obj["max"] = obj[item];
            }
        }else{
            obj[item]=1;
        }
        return obj;
    },{"max":0}).max
}
console.log(max(str))