Hello
I am trying to understand how to use base utility helper method in lighting component.

Base.cmp
--------
<aura:component abstract="true">
	{!v.body}
</aura:component>


BaseHelper.js
--------------
({
    callServer : function(component,method,callback,params) {
        var action = component.get(method);
        if (params) {
            action.setParams(params);
        }
       
        action.setCallback(this,function(response) {
            var state = response.getState();
            if (state === "SUCCESS") {
                // pass returned value to callback function
                callback.call(this,response.getReturnValue());  
            } else if (state === "ERROR") {
                // generic error handler
                var errors = response.getError();
                if (errors) {
                    console.log("Errors", errors);
                    if (errors[0] && errors[0].message) {
                        throw new Error("Error" + errors[0].message);
                    }
                } else {
                    throw new Error("Unknown Error");
                }
            }
        });
       
        $A.enqueueAction(action);
    }
})


ApexController.axpc
-------------------
public with sharing class ApexController {
    @AuraEnabled
    public static List<Contact> getContacts() {
        List<Contact> contacts = [select Id, Name, MailingStreet, Phone, Email FROM Contact];
        return contacts;
    }
}


CallApexClassCmpBase.cmp
------------------------

<aura:component controller="ApexController">
	
    <lightning:card title="Bring Contact data">
       
          <aura:set attribute="actions">
       
              <lightning:button label="Contact Data" onclick="{!c.Condata}" />
          </aura:set> 
   
    </lightning:card>
    
</aura:component>


In the controller.js of the above component

({
 Condata : function(component,event,helper) {
 helper.callServer(
            component,
            "c.getContacts",
            function(response) { 
               console.log('Contacts data'+JSON.stringify(response));
            }
        );
      
    }
})



<aura:application extends="force:slds">

    <c:CallApexClassCmpBase/>
     
 </aura:application>

when I preview this I am getting an error as below:

This page has an error. You might just need to refresh it. Action failed: etaPrime:CallApexclassCmpBase$controller$Condata [helper.callServer is not a function] Failing descriptor: {etaPrime:CallApexclassCmpBase$controller$Condata}

I am not able to figure out what is causing the issue,

Please help.

smita