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POJavaScript: X.prototype.x adds x to base class, too
 

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    <p>I'm fluent in C++ so I have a good background in OOP, but I'm new to JavaScript and I'm trying to understand how to properly implement polymorphism in JavaScript. I have read a number of tutorials and StackOverflow questions related to this topic, but none has explained in detail how to correctly <em>inherit</em> from a base class and also <em>add new members</em> to the child class.</p> <pre><code>function Obj1() {} function Obj2() {} function Obj3() {} Obj2.prototype === Obj3.prototype; // false </code></pre> <p><strong>Curiosity #1:</strong> If both <code>Obj2</code> and <code>Obj3</code> (and, for that matter, <code>Obj1</code>) inherit from <code>Object</code>, why are their prototypes not identical? Did <code>Obj2</code> and <code>Obj3</code> each receive its own <em>copy</em> of <code>Object</code>?</p> <pre><code>Obj2.prototype = Object; Obj2.prototype = Object; Obj2.prototype === Obj3.prototype; // true Obj2.prototype = Obj1; Obj3.prototype = Obj1; Obj2.prototype === Obj3.prototype; // true </code></pre> <p><strong>Curiosity #2:</strong> In both cases, <code>Obj2</code> and <code>Obj3</code> now share the same prototype. What has changed?</p> <pre><code>Obj2.prototype.pi = function() { return 3.14159; } Obj2.prototype.pi(); // 3.14159 Obj3.prototype.pi(); // 3.14159 Obj1.pi(); // 3.14159 </code></pre> <p><strong>The Real Question:</strong> Changing the prototype of <code>Obj2</code> also changes the prototype of <code>Obj3</code> and the function <code>Obj1</code> itself. This is all quite shocking to this C++ programmer, even though it's exactly what I expected from the above syntax: after all, since the prototypes of <code>Obj2</code> and <code>Obj3</code> have both been equated with <code>Obj1</code>, a change to one will produce an identical change to the other two. Nevertheless, I want to use <code>Obj1</code> as a base class for both <code>Obj2</code> and <code>Obj3</code> <em>and</em> insert a new function on <code>Obj2</code> that is <em>not</em> shared in any way by <code>Obj1</code> or <code>Obj3</code>. How do I do that?</p> <p><strong>Further curiosities</strong> arise when these objects are instantiated:</p> <pre><code>var x = new Obj2(); x.pi(); // 3.14159 var y = new Obj3(); y.pi(); // 3.14159 var z = new Obj1(); z.pi(); // Error: pi() not defined on z. </code></pre> <p><code>Obj2</code> and <code>Obj3</code> both have a function <code>pi()</code>, which implies that they still share a <em>base class</em> that contains a function <code>pi()</code>. This base class is none other than <code>Obj1</code>&mdash;but an instance of <code>Obj1</code> does <em>not</em> contain <code>pi()</code>, despite the evidence to the contrary!</p>
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    1. COThe first thing to do is to fully accept the fact that inheritance in JavaScript is a completely different affair than inheritance in C++. It's really only superficially related; it's based on a completely different conceptual model.
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    2. COOh and in particular, there's really no such concept as "class" in JavaScript. There are effects you can get that allow you to *pretend* that there are classes, but it's just a convention.
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    3. COI suggest you start by reading [this](http://ejohn.org/blog/simple-javascript-inheritance/)
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