[Eron]

var func=(function(){
    var users = {
        "1": {
            "balance": 300,
            "name": "Юрий"
        },"2": {
            "balance": 399,
            "name": "Юрий"
        },"3": {
            "balance": 3000,
            "name": "Юрий"
        },"4": {
            "balance": 5000,
            "name": "Юрий"
        },"5": {
            "balance": 5001,
            "name": "Юрий"
        },"6": {
            "balance": 5000,
            "name": "Юрий"
        },"7": {
            "balance": 5e9,
            "name": "Юрий"
        }
    },
    sortedList=Object.keys(users).sort((a,b)=>users[a].balance-users[b].balance);
 
    return {
        getUserByPosition:function(position){
            return --position in sortedList?users[sortedList[position]]:null;
        },
        getPositionByUserId:function(userId){
            var result=sortedList.indexOf(userId+'');
 
            return result<0?null:result+1;
        }
    };
})();
 
console.log(func.getPositionByUserId(5));