[SalesForce] Passing parameter from lightning JS controller to apex class

I tried what you are trying to do but was unsuccessful! :(

However I have couple of alternative for you.

Option 1:

Have an aura:attribute as currentValue and set the very first picklist value in it inside the setCallBack function. Use the variable wherever you need. Also, in the CHANGE event of inputSelect, assign the selected value to the currentValue.

In your component:

<aura:attribute name="currentValue" type="String"/>

<ui:inputSelect aura:id="P" change="{!c.setCurrentValue}">
...
</ui:inputSelect>

In your JS controller:

    setCurrentValue: function(component, event, helper){
            var newValue = component.find("P").get("v.value");
            component.set("currentValue", newValue);
        },
    add : function(component, event, helper) { 
        var v = component.get("v.currentValue");
        console.log('p..'+v);

       }

In your helper class:

checkdefaultvalue:function(component, event, helper){
        var action = component.get("c.temp"); 
        action.setCallback(this, function(response){
            var state = response.getState();    
            if (component.isValid() && state === "SUCCESS") {
                var res = response.getReturnValue();  
                component.set("v.it", res);
                component.set("v.currentValue", res[0]); //This will set the 1st value as current one
                console.log("it optiom: "+res);
            }else{console.log("failure...");}    
        });

I have tested this, its working fine.

Coming to Option 2:

inputSelect has an attribute 'updateOn' which takes an event name, firing which will update the selected Value of the picklist. By defult its CHANGE.

Now if you can fire the change event of inputSelect from JScontroller or helper, it should set the selected value of the inputSelect.

Your Component:

<ui:inputSelect aura:id="P" updateOn="change">
...
</ui:inputSelect>

In your JS Controller:

add : function(component, event, helper) { 
        component.find("P").getEvent("change").fire();
        var defaultDropdown = component.find("P").get("v.value");
        console.log('p..'+defaultDropdown);
    }

I haven't tested the 2nd solution but theoretically it should work.

Yes, @AuraEnabled is required for attributes to be serialized. This is mentioned in the documentation:

• Use @AuraEnabled on Apex class static methods to make them accessible as remote controller actions in your Lightning components.

• Use @AuraEnabled on Apex instance methods and properties to make them serializable when an instance of the class is returned as data from a server-side action.