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<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