Ttdfjt

<aura:component controller="DisplayconsClass">
 <aura:attribute type="Contact[]" name="consList"/>
 <aura:attribute name="mycolumns" type="List"/>
 <aura:handler name="init" value="!this" action="!c.init"/> 
 <lightning:datatable keyField="id" data="!v.consList" columns="!v.mycolumns" hideCheckboxColumn="true" />

</aura:component>

//controller

(
 init : function(component, event, helper) 

 helper.fetchContacts(component, event, helper);
 
)

Helper:

(
 fetchContacts: function(component, event)

 

 component.set('v.mycolumns', [
 label: 'First Name',
 fieldName: 'firstName',
 type: 'text'
 ,
 
 label: 'Last Name',
 fieldName: 'lastname',
 type: 'text'
 

 ]);


 var action = component.get("c.fetchCons");

 action.setCallback(this, function(response) 

 var state = response.getState();

 if (state === "SUCCESS") 

 var records = response.getReturnValue();

 component.set("v.consList", records);

 
 );

 $A.enqueueAction(action);
 
)

Output:

Apex controller:

public class DisplayconsClass 

 @AuraEnabled
 public static List <contact> fetchCons() 

 return [ SELECT firstName, lastname FROM contact LIMIT 10 ];

 

Field names are cAsE-sEnSiTiVe. It should be FirstName and LastName, not firstName and lastname. You must use the case from their API name, not however you've written it in your Apex code query.

 component.set('v.mycolumns', [
 label: 'First Name',
 fieldName: 'FirstName',
 type: 'text'
 ,
 
 label: 'Last Name',
 fieldName: 'LastName',
 type: 'text'
 

 ]);

1. 
   

   Change the apex controller to the following:

   
   
   

   public class DisplayconsClass

   
   
   

   @AuraEnabled
   public static List fetchCons()
   List listToReturn = new List();
   
   List tempList = SELECT firstName, lastname FROM contact LIMIT 10 ];

   
   
   

    for(Contact temp : tempList)
    CotactWrapper tempWrap = new ContactWrapper();
    tempWrap.firstName = temp.firstName;
    tempWrap.lastname = temp.lastname;
    listToReturn.add(tempWrap);
    
    return ListToReturn;
   
   
   public class ContactWrapper
    @AuraEnabled public String firstName;
    @AuraEnabled public String lastname;
   
   

   
   
   
   


2. 
   

   In your JavaScript controller change to the following code:

   
   
   

   if (state === "SUCCESS") 
    var records = response.getReturnValue();
    var toJson = JSON.stringify(records);
    component.set("v.consList", toJson);
   
   
   

   
   

Tbh I'm not sure whether it's JSON.stringify or JSON.parse so just try both.

3. in the component change the attribute:

<aura:attribute type="Object" name="consList"/>

or Object[]