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    <p>It's because you write</p> <pre><code>SubClass.prototype = new Class("proto_class"); </code></pre> <p>you are creating a prototype of <strong>one instance</strong> of <code>Class</code>. What you want is to create a sub-class that inherits from its parent's prototype. As David Flanagan shows in his <a href="http://rads.stackoverflow.com/amzn/click/B002AWX8CI" rel="nofollow noreferrer">JavaScript: The Definitive Guide</a> (§&nbsp;9.5), you have to use a helper function to create a new object with a specified prototype:</p> <pre><code>function heir(p) { function f(){} // dummy constructor function f.prototype = p; // specify prototype object we want return new f(); // create and return new object } </code></pre> <p>(Crockford calls this function <code>Object.create</code>, after the so-called <a href="http://ecma262-5.com/ELS5_Section_15.htm#Section_15.2.3.5" rel="nofollow noreferrer">ES5 object constructor property</a>, but please don't do this, as this <a href="https://stackoverflow.com/questions/3075308/what-modernizer-scripts-exist-for-the-new-ecmascript-5-functions/3075818#3075818">can be misleading</a>.)</p> <p>Within the SubClass constructor, you have to <a href="https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Function/call" rel="nofollow noreferrer">call</a> the Class constructor with <code>this</code> set to the current object:</p> <pre><code>function SubClass() { // call the parent's constructor with // `this` set to the current scope Class.call(this, "proto_class"); } </code></pre> <p>And last, but not least, you only reset <code>Class.asdf2</code> once, but not in the constructor function of <code>Class</code> or <code>SubClass</code>. So add <code>this.asdf2 = [];</code> to one of the constructors.</p> <p>The complete code now reads:</p> <pre><code>function heir(p) { function f(){} // dummy constructor function f.prototype = p; // specify prototype object we want return new f(); // create and return new object } function Class(asdf) { if (typeof asdf != 'undefined') this.asdf = asdf; } Class.prototype.asdf = "default_asdf"; Class.prototype.asdf2 = []; Class.prototype.change_asdf = function() { this.asdf = "changed_asdf"; this.asdf2.push("changed_asdf2"); } function SubClass() { // call the parent's constructor with // `this` set to the current scope Class.call(this, "proto_class"); this.asdf2 = []; } SubClass.prototype = heir(Class.prototype); SubClass.prototype.constructor = SubClass; test1 = new SubClass(); alert("test1 asdf: " + test1.asdf + " " + test1.asdf2); test1.change_asdf(); alert("test1 asdf: " + test1.asdf + " " + test1.asdf2); test2 = new SubClass(); alert("test2 asdf: " + test2.asdf + " " + test2.asdf2); </code></pre>
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    1. POJavaScript prototyping: single prototype object or not?
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