When does an input field value equals '', null or undefined
<p>In which cases do an input field value equals <code>''</code>, <code>null</code> or <code>undefined</code>.</p>

<p>I'm trying to check empty fields</p>

<pre><code>if($('#cur').val() == 'undefined' || $('#cur').val() == '' || $('#cur').val() == 'null'){
    alert('Blank');
}
</code></pre>
