How to re-define a method in a "javascript class"

Question

Please run the following code snippet ¹ and see what is happening in JS console:

My questions are regarding the last line in the snippet:

1. Why is F.prototype.method; changed?
2. How should I redefine Fcustom.prototype.method in order to not change F.prototype.method?

Note: I am using jQuery and underscore to extend the function.

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• ¹ Test code snippet:

  var F = function () {};
  F.prototype.method = function () {
      // some code
  }
  
  F.prototype.method; // it shows "some code"
  
  
  Fcustom = $.extend(true, F, {});
  
  _.extend(Fcustom.prototype, {
  method: function () {
      // other code
      }
  });
  
  Fcustom.prototype.method; // it shows "other code"
  
  F.prototype.method; // it shows "other code" instead of "some code" Why?
  

Answer 1

var obj = { myMethod : function() { 
              //some code
          }
};

var newObj = $.extend(true, {}, obj);

newObj.myMethod = function (){       
   //new method
};

newObj.myMethod();  //should call the new method

While,

obj.myMethod(); //still calls the old "//some code"

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DEMO: