Function object in JavaScript

I think the Function object is the only instance in the JavaScript. All the other function(){...} or var xx=function(){...} are the objects inherited from this Function object. That is the prototype based language feature. And in prototype based language there is no Class, and One of the advantages of it is the object can changed anytimes.such as add a method or add a property into it. So, I think we can treat object like a key-values collection. You can add items into it anytimes after you created it. (If I was wrong, Please correct me.)

Say you have the code:

        Function.test=function(){
            alert(Function.name);//"Function"  
            alert(Function.myName);//"fun"
        };
        Function.name="fun";
        Function.myName="fun";
        Function.test();

In the above code, all I want to do is add a test method to the Function object. There is no error found in this code. But why Function.name can not be changed?