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    <blockquote> <p>NOTE: This answer was written in <strong>2008</strong>, when javascript was still in <strong>ES3</strong> from <strong>1999</strong>. A lot of new functionality has been added since then, so now better solutions exists. This answer is kept for historical reasons. </p> </blockquote> <p>The benefit of the code below is that you don't need to specify the name of the function twice and it works for anonymous functions too. </p> <pre><code>function x() { if ( (this instanceof arguments.callee) ) { alert("called as constructor"); } else { alert("called as function"); } } </code></pre> <p><strong>Update</strong> As <a href="https://stackoverflow.com/users/15055/claudiu">claudiu</a> have pointed out in a comment below, the above code doesn't work if you assign the constructor to the same object it has created. I have never written code that does that and have newer seen anyone else do that eighter. </p> <p>Claudius example:</p> <pre><code>var Z = new x(); Z.lolol = x; Z.lolol(); </code></pre> <p>By adding a property to the object, it's possible to detect if the object has been initialized. </p> <pre><code>function x() { if ( (this instanceof arguments.callee &amp;&amp; !this.hasOwnProperty("__ClaudiusCornerCase")) ) { this.__ClaudiusCornerCase=1; alert("called as constructor"); } else { alert("called as function"); } } </code></pre> <p>Even the code above will break if you delete the added property. <strong><em>You can however overwrite it with any value you like, including <code>undefined</code>, and it still works.</em></strong> But if you delete it, it will break.</p> <p>There is at this time no native support in ecmascript for detecting if a function was called as a constructor. This is the closest thing I have come up with so far, and it should work unless you delete the property.</p>
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    1. POHow to detect if a function is called as constructor?
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    1. COThis doesn't work. Behold: { var Z = new x(); Z.lolol = x; Z.lolol();}. It thinks the 2nd invocation is it being called as a constructor.
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    2. CO@Claudiu: Does this work for you?
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    3. COThis doesn't seem to be allowed by `"use strict";`.
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      1. USMatt Fenwick
 

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