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POHow to properly derive object with private vars using javascript (prototypal) inheritance
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  1. POHow to properly derive object with private vars using javascript (prototypal) inheritance
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    copied!<p>I am new to JavaScript's (prototypal) inheritance and I'm trying to learn more about it. I am using a simple observer pattern as example, in which I want observable objects to be derived from the 'subject' object. This is what I WANT to do:</p> <pre><code>function subject() { var callbacks = {} this.register = function(name, callback) { callbacks[name] = callback; } this.unregister = function(name) { delete callbacks[name]; } var trigger = function() { var a = arguments; var t = this; $.each(callbacks, function(name, callback) { callback.apply(t, a); }); } } list.prototype = new subject() function list() { var items = {} this.add = function(name, item) { items[name] = item; trigger('add', name); } this.remove = function(name) { delete items[name]; trigger('remove', name); } } </code></pre> <p>Now when using the code above like below, I run into my first problem:</p> <pre><code>var l = new list() l.register('observer1', function() { console.log(this, arguments) }); l.add('item1', 'value1'); // &lt;-- ReferenceError: trigger is not defined, trigger('add', name); </code></pre> <p>To continue testing I made the trigger function 'public' using this.trigger instead. Running my example again I run into the next problem:</p> <pre><code>var l = new list() l.register('observer1', function() { console.log(this, arguments) }); l.add('item1', 'value1'); // &lt;-- output: subject, ["add", "item1"] </code></pre> <p>The <code>this</code> object is <code>subject</code>, I want it to be <code>list</code>. My third problem occurs when creating another list:</p> <pre><code>var l2 = new list(); //Don;t register any observers l2.add('item1', 'value1'); // &lt;-- output: subject, ["add", "item1"] </code></pre> <p>The <code>callbacks</code> list is shared between the 2 lists.</p> <p>I've tried similar things with Object.create(new subject()) as well and run into similar problems.</p> <p>My 3 questions in this are:</p> <ol> <li>Can I have private methods that can be used in derived objects <strong>(and should I even care about having them private or public)</strong>?</li> <li>How can I have the <code>this</code> object I want <strong>(without needing to use function.call in the derived object, if possible)</strong>?</li> <li>How can I keep the <code>callbacks</code> list in the base object without it being shared?</li> </ol>
 

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