Checking whether a member is in instance or in prototype
<p>When using the <code>in</code> keyword, one can easily check whether a given key exists within an object. However, prototype functions are also <code>in</code> an object. For example,</p>

<pre><code>var arr = {'a': 'foo', 'b': 'bar'}
Object.prototype.dummyfunc = function() {}
'a' in arr // true
'dummyfunc' in arr // true
</code></pre>

<p>How can I distinguish between a member being really in the instance of the object, or being a prototype function of <code>Object</code>?</p>

<p>Thanks.</p>
