Prevent Overriding Javascript Function

Question

I'm using a Modular Pattern in JavaScript. I wonder if we can prevent the public modules to be overridden. For example, in the code below function1, function2, function3 and function4 can be accessed outside but I don't want to override. If these functions are overridden then I want the compiler to generate an error message

"use strict";

var $ = (function(){
return{
      function1 : function(){
          alert("this is Function1");
      },
      function2 : function(){
          alert("this is Function2");
      },
      function3 : function(){
          alert("this is Function3");
      },
      function4 : function(){
          alert("this is Function4");
      }
    };
}());


$.function1(); //will alert - this is Function1
$.function2(); //will alert - this is Function2

/* 
  I don't want to do this, If I do, then I want the compiler to generate an   
  error message
*/
$.function3=function(){
    alert('function 3 is overridden');

};
$.function3(); //will alert - function 3 is overridden

Answer 1

You can use Object.freeze(obj) to set the entire returned object to be immutable. Additionally, note that you can use const instead of var to avoid the object being reassigned.

'use strict';

const $ = (function() {
  return Object.freeze({
    function1: function() {
      alert('this is Function1');
    },
    function2: function() {
      alert('this is Function2');
    },
    function3: function() {
      alert('this is Function3');
    },
    function4: function() {
      alert('this is Function4');
    }
  });
})();


$.function1(); //will alert - this is Function1
$.function2(); //will alert - this is Function2

// This will now error
$.function3 = function() {
  alert('function 3 is overridden');
};
$.function3(); // will not run

Answer 2

Using Object.defineProperty you can declare the property as read-only.

// Make sure an error is thrown when attempting to overwrite it
// Without strict-mode, re-assigning will fail silently
'use strict';

var API = {};
Object.defineProperty(API, 'function1', {
  writable: false,
  value: function() {
    console.log('Called function1');
  }
});

API.function1();
API.function1 = null;