Is there any way to prevent override/overwrite of functions/variables in singleton instance?

Question

Consider this pseudo code:

(function(window){
   var options = { /*where everything goes */ };

   var instance = (functio         


        

Answer 1

What if is_allowed would be completely local?

(function(window){
   var options = {}, is_allowed;

   var instance = (function(options){
     for (var i in options) {
     if (options.hasOwnProperty(i)) {
        this[i] = options[i];
       }
     }
     return this;
   })(options);

   instance.callbacks = function(cb){
       /* ... */
   };

   function check_allowed(){
     /* check and let this function set [is_allowed] */
   };

  window.instance = check_allowed()
                     ? instance
                     : { callbacks: function(){(alert('not allowed'));}  };

} (this) );

jsBin mockup

BTW: in your code, window.instance would be undefined.