Javascript: Inherit method from base class and return the subclass's private variable

I have the following BaseClass defined:

function BaseClass (arg1,arg2,arg3) {
    //constructor code here then - 
    var privateVar = 7500;
    this.getPrivateVar = function() { return privateVar; };

I want to have the following subclass which allows changing privateVar like so:

function SubClass (arg1,arg2,arg3,privateVar) {
    //constructor code here then - 
    var privateVar = privateVar;
SubClass.prototype = new BaseClass();

Now I want SubClass to inherit the getPrivateVar method. However, when I try this, it always returns 7500 which is the value in the BaseClass and not the value of privateVar.

In other words, is it possible to inherit a BaseClass's public methods but have any references in them refer to the SubClass's properties? And how would I do that?

By the sound of things, it's impossible. The idea was to automate a code-checker for my students code (I tutor kids) but I'll just have to find another way. Thanks anyway.


You are mixing Javascript object model with scoped variables which do not interoperate*.

The inherits idiom of doing SubClass.prototype = new BaseClass(); only works when you are using prototypes and constructors naturally:

function BaseClass(arg1, arg2, arg3) {
    this._privateVar = 7500;
BaseClass.prototype.getPrivateVar = function() {
    return this._privateVar;

function SubClass(arg1, arg2, arg3, privateVar) {

SubClass.prototype = new BaseClass();
//Better way to do it is 
//SubClass.prototype = Object.create(BaseClass.prototype);
SubClass.prototype.constructor = SubClass;

Before you argue that anyone can access the property by just writing _, I could argue back that anyone can access any private in Java, PHP or C# by using reflection. Or using instance_eval or send in Ruby and so on. So it's out of your hands anyway.

*None or most of these don't work when you use scoped variables depending on implementation:

  • Enumerability
  • Writability
  • First-class Accessors
  • Sealedness, Frozedness and state of Extension
  • Reflection through getPropertyNames or keys
  • instanceof operator
  • Generic methods
  • Inheritance

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