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POImplement universal, lightweight, and unobtrusive tagging of arbitrary objects?
 

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  1. POImplement universal, lightweight, and unobtrusive tagging of arbitrary objects?
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    <p><strong>NB:</strong>&nbsp;<em>The material in the subsection titled</em>&nbsp;<strong>Background</strong>&nbsp;<em>is not essential. The full description of the question is fully contained in the preceding paragraphs.</em></p> <p>I'd like to implement a universal, lightweight, and "unobtrusive" way to "tag" arbitrary objects.</p> <p>More specifically, I want to define the equivalent of the (abstract) functions <code>tag</code>, <code>isTagged</code>, and <code>getTagged</code>, such that:</p> <ol> <li><code>isTagged(t)</code> is <code>true</code> if and only if <code>t</code> was the value returned by <code>tag(o)</code>, for some object <code>o</code>;</li> <li><code>getTagged(tag(o))</code> is identical to <code>o</code>, for every object <code>o</code>;</li> <li>if <code>t = tag(o)</code>, then <code>tag(t)</code> should be identical to <code>t</code>;</li> <li>with the exception of the behaviors described in (1), (2), and (3) above, and strict identity tests involving <code>===</code>, <code>tag(o)</code> and <code>o</code> should behave the same way.</li> </ol> <p>[EDIT: One further requirement is that the implementation should not modify the <code>Object</code> class, nor any other standard class, in any way.]</p> <p>For example:</p> <pre><code>&gt;&gt;&gt; isTagged(o = "foo") false &gt;&gt;&gt; isTagged(t = tag(o)) true &gt;&gt;&gt; getTagged(t) === o true &gt;&gt;&gt; tag(t) === t true &gt;&gt;&gt; t.length 3 &gt;&gt;&gt; t.toUpperCase() "FOO" </code></pre> <p>Below I give my best shot at solving this problem. It is (almost) universal, but, as it will soon be clear, it is <strong>anything but</strong> lightweight!!! (Also, it falls rather short of fully satisfying requirement 4 above, so it is not as "unobtrusive" as I'd like. Moreover, I have serious doubts as to its "semantic correctness".)</p> <p>This solution consists of wrapping the object <code>o</code> to be tagged with a "proxy object" <code>p</code>, and copying <strong>all</strong> the properties of <code>o</code> (whether "owned" or "inherited") to <code>p</code>.</p> <p>My question is:</p> <blockquote> <p>is it possible to achieve the specifications given above without having to copy all the properties of the tagged object?</p> </blockquote> <hr> <h2>Background</h2> <p>Here's the implementation alluded to above. It relies on the utility function <code>getProperties</code>, whose definition (FWIW) is given at the very end.</p> <pre><code>function Proxy (o) { this.__obj = o } function isTagged(t) { return t instanceof Proxy; } function getTagged(t) { return t.__obj; } var tag = (function () { function _proxy_property(o, pr) { return (typeof pr === "function") ? function () { return pr.apply(o, arguments) } : pr; } return function (o) { if (isTagged(o)) return o; if (typeof o.__obj !== "undefined") { throw TypeError('object cannot be proxied ' + '(already has an "__obj" property)'); } var proxy = new Proxy(o); var props = getProperties(o); // definition of getProperties given below for (var i = 0; i &lt; props.length; ++i) { proxy[props[i]] = _proxy_property(o, o[props[i]]); } return proxy; } })(); </code></pre> <p>This approach, ham-fisted though it is, at least seems to work:</p> <pre><code>// requirement 1 &gt;&gt;&gt; isTagged(o = "foo") false &gt;&gt;&gt; isTagged(p = tag(o)) true // requirement 2 &gt;&gt;&gt; getTagged(p) === o true // requirement 3 &gt;&gt;&gt; tag(p) === p true // requirement 4 &gt;&gt;&gt; p.length 3 &gt;&gt;&gt; p.toUpperCase() "FOO" </code></pre> <p>...well, <em>almost</em>; requirement (4) is not always satisfied:</p> <pre><code>&gt;&gt;&gt; o == "foo" true &gt;&gt;&gt; p == "foo" false &gt;&gt;&gt; o == o true &gt;&gt;&gt; p == o false </code></pre> <hr> <p><strong>FWIW</strong>, here's the definition of the function <code>getProperties</code>, which is used by the <code>tag</code> function. Criticisms welcome. (<strong>WARNING:</strong> <em><strong>I'm a completely clueless JS noob who doesn't know what he's doing!</em></strong> Use this function at your own risk!)</p> <pre><code>function getProperties(o) { var seen = {}; function _properties(obj) { var ret = []; if (obj === null) { return ret; } try { var ps = Object.getOwnPropertyNames(obj); } catch (e if e instanceof TypeError &amp;&amp; e.message === "obj is not an object") { return _properties(obj.constructor); } for (var i = 0; i &lt; ps.length; ++i) { if (typeof seen[ps[i]] === "undefined") { ret.push(ps[i]); seen[ps[i]] = true; } } return ret.concat(_properties(Object.getPrototypeOf(obj))); } return _properties(o); } </code></pre>
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