title: How to check if a variable is set in JavaScript?
body: <p>I have this object/array thing:</p>
  
  <pre><code>var states = {};
  
  states["CA"] = new State("CA", "California");
  states["AR"] = new State("AR", "Arizona");
  ....
  </code></pre>
  
  <p>How can I check if <code>states["AL"]</code> has been set or not? Will the following work (in all browsers)?</p>
  
  <pre><code>if (states["AL"] == undefined)
     alert("Invalid state");
  </code></pre>