How to define Id as variable
<p>Let say </p>

<pre><code> &lt;div id = "test_id" class = "test_class"&gt; &lt;/div&gt;
</code></pre>

<p><strong><em>JS</em></strong> </p>

<pre><code>  alert( $("#test_id").hasClass('test_class') );  // result is true
</code></pre>

<p>I just write another way to get the same expected result but didn't work, Why?</p>

<pre><code> var test = ' " ' + "test_id" + ' " ' ;

      alert( $(test).hasClass('test_class')  );    // not working, why ?????
</code></pre>
