LightningHelper.js

getFitbitData : function(component) {               
  var action = component.get("c.getUserDetails");
  action.setCallback(this,function(response){    
    var state = response.getState();
    if(state === 'SUCCESS'){
      var parseJson = JSON.parse(response.getReturnValue());
      component.set("v.User",parseJson.user);
    }else if (state === "ERROR") {
      console.log('Error');
   }
  });      
  $A.enqueueAction(action);
}