I am creating a drop-down list in lightning which show 2 lists of events depends on the drop-down list today, PastDay and future value. I am also creating two buttons Next and Previous. which go to the next 2 list of the events and also come back. But im getting an error when im clicking on the Next button. My code and screenshots are below. please tell me how I  solved it.





My Fisrt Component:
<aura:component controller="EventdayClass">
    
    <aura:attribute name="event" type="Event[]"/>
    <aura:attribute name="eventchange" type="String[]"/>
    <aura:handler name="init" value="{!this}" action="{!c.doInit}"/>
    
    
    <aura:attribute name="offset" type="integer"/>
    <aura:attribute name="next" type="boolean"/>
    <aura:attribute name="prev" type="boolean"/>
    
    <lightning:button label="Next" onclick="{!c.next}"/>
    <lightning:button label="Previous" onclick="{!c.prev}"/>
    
     <ui:inputSelect aura:id="levels" change="{!c.onSelectChange}">
        <aura:iteration items="{!v.eventchange}" var="level">
            <ui:inputSelectOption text="{!level}" label="{!level}"/>
        </aura:iteration>
    </ui:inputSelect>
                <aura:iteration items="{!v.event}" var="items">
                    <c:EventDetails eventDetailsList="{!items}"/>
                   </aura:iteration>
    
</aura:component>

Controller class for this component:
({
    
    doInit: function(component,event,helper){
        var eventname=["Today","PastDay","Future"];
        component.set("v.eventchange",eventname);
        
        var action= component.get("c.daycal");
        action.setParams({"DayName":component.find('levels').get("v.value")});
          
        action.setCallback(this,function(response){
            component.set("v.event",response.getReturnValue())
        });
        
        
        $A.enqueueAction(action);


    },
    onSelectChange : function(component, event, helper) {
        var action= component.get("c.daycal");
        action.setParams({"DayName":component.find('levels').get("v.value")});
          
        action.setCallback(this,function(response){
            component.set("v.event",response.getReturnValue())
        });
        //var selected=  component.find('levels').get("v.value");
        
        
        $A.enqueueAction(action);

                     
    },
    
    next:function(component,event,helper){
        var nxt= ture;
        var prv=false;
        var offset=component.get("v.offset");
       // helper.next(component,nxt,prv,offset);
       
    },
    prev:function(component,event,helper){
        var nxt= false;
        var prv=ture;
        var offset=component.get("v.offset");
        helper.next(component,nxt,prv,offset);
    }
    
})

Helper controller class:
({
    next : function(component,nxt,offset) {
        var total= total||0;
         var action= component.get("c.getbtnlist");
        
        action.setParams({
            "next" : next          
        });
         action.setCallback(this,function(response){
             var result = res.getReturnValue();
              cmp.set('v.total',result.totalcount);
             // cmp.set('v.accounts',result.acc);
              cmp.set('v.next',result.hasnext);
             // cmp.set('v.prev',result.hasprev);
             
           // component.set("v.event",response.getReturnValue());
           // component.set('v.next',result.hasnext.hasnext);
        });
       
        
        
        $A.enqueueAction(action);

        
        
    },
    prev : function(component,nxt,total) {
        var total= total ||0;
         var action= component.get("c.getbtnlist");
        
        action.setParams({
            "prev" : prev          
        });
        action.setCallback(this,function(response){
             var result = res.getReturnValue();
              cmp.set('v.total',result.totalcount);
            
             
             cmp.set('v.prev',result.hasprev);
             
           // component.set("v.event",response.getReturnValue());
           // component.set('v.next',result.hasnext.hasnext);
        
         action.setCallback(this,function(response){
            component.set("v.event",response.getReturnValue());
            component.set('v.prev',result.hasprev);
        });
       
        
        
        $A.enqueueAction(action);

        
        
    }
})

Apex class:
public class EventdayClass {
    
    @AuraEnabled
    public static List<Event> daycal(String DayName){
        if(DayName=='Future'){
            return[select id,Subject,StartDateTime,WhoId,WhatId,Who.Name,What.Name from Event where StartDateTime=:System.today() and StartDateTime!=null limit 2];
        
        }
        else if(DayName=='PastDay'){
            return[select id,Subject,StartDateTime,WhoId,WhatId,Who.Name,What.Name from Event where StartDateTime<:System.today() and StartDateTime!=null limit 2];
        }
        else if(DayName=='Today'){
            return[select id,Subject,StartDateTime,WhoId,WhatId,Who.Name,What.Name from Event where StartDateTime>:System.today() and StartDateTime!=null limit 2];
        }
        else{
            return[select id,Subject,StartDateTime,WhoId,WhatId,Who.Name,What.Name from Event where StartDateTime=:System.today() and StartDateTime!=null limit 2];
        }
        
    }
    
    
    //
    //
    //
    
   @AuraEnabled
     public list<event> evn;
      
     @AuraEnabled
     public integer totalcount;
      
     
     @AuraEnabled
     public boolean hasprev;
      
     @AuraEnabled
     public boolean hasnext;
    
    private static integer pagesize=2;
    private static integer total;
   @AuraEnabled
    public static EventdayClass getbtnlist(Boolean next,Boolean prev,decimal off){
        total = (integer)off;
        System.debug('total'+total);
        List<Event> EventList= new List<Event>();
        integer listlenght=[select count() from event where StartDateTime!=null];
        if(next==true && prev==false && (total+pagesize<=listlenght)){
            total=total+pagesize;
            System.debug('total next'+total);
            EventList=[select id,Subject,StartDateTime,WhoId,WhatId,Who.Name,What.Name from Event where StartDateTime!=null limit:pagesize OFFSET:total];
            System.debug('EventList Next'+EventList);
            
        }
        else if(prev==true && total>0){
            total=total-pagesize;
            System.debug('total prev'+total);
            EventList=[select id,Subject,StartDateTime,WhoId,WhatId,Who.Name,What.Name from Event where StartDateTime!=null limit:pagesize OFFSET:total];
            System.debug('EventList prev'+EventList);
        }
        
        EventdayClass ec= new EventdayClass();
        ec.evn=EventList;
        System.debug('ec.evn'+ec.evn);
        ec.totalcount=total;
        System.debug('ec.totalcount'+ec.totalcount);
        ec.hasprev = hasprev(total);   
        System.debug('ec.hasprev'+ec.hasprev);
        ec.hasnext = hasnxt(total,listlenght,pagesize);
        System.debug('ec.hasnext'+ec.hasnext);
        return ec;
       
        
    }
    private static boolean hasprev(integer off){
            if(off>0)
                return false;
            return true; 
        }
        private static boolean hasnxt(integer off,integer li,integer ps){
        if(off+ps<li)
            return false;
        return true;
        }
        
          
    
      
  }
child component that i call in parent componenet:

<aura:component >
    
    <aura:attribute name="eventDetailsList" type="Event"/>
    
    
    
    
   <aura:iteration items="{!v.eventDetailsList}" var="items">
                    <c:Evenlistbtnnn eventbutton="{!items}"/>
   </aura:iteration>
            
                            
        
    
    
    
         
          

   
        
  <table>  
      
                {!v.eventDetailsList.Subject}
               
               <a href="{!'#/sObject/'+v.eventDetailsList.Who.Id+'/view'}">{!v.eventDetailsList.Who.Name}</a>
                <a href="{!'#/sObject/'+v.eventDetailsList.What.Id+'/view'}">{!v.eventDetailsList.What.Name}</a>
                {!v.eventDetailsList.StartDateTime}

             
    </table>
    
    
  
    <!--Next Button-->
    <!--a href="{!'#/sObject/'+v.eventDetailsList.Id+'/view'}">{!v.eventDetailsList.Subject}</a>
    <a href="{!'#/sObject/'+v.eventDetailsList.Who.Id+'/view'}">{!v.eventDetailsList.Who.Name}</a>
    <a href="{!'#/sObject/'+v.eventDetailsList.What.Id+'/view'}">{!v.eventDetailsList.What.Name}</a>
    <a href="{!'#/sObject/'+v.eventDetailsList.RecurrenceStartDateTime.Id+'/view'}">{!v.eventDetailsList.RecurrenceStartDateTime}</a-->
    
    <aura:iteration items="{!v.eventDetailsList}" var="items">
                    <c:Evenlistbtn eventbutton="{!items}"/>
    </aura:iteration>
    
    
    
</aura:component>