title: Explain XMLHttpRequest creation
body: <p>I'm learning XMLHttpRequest from w3schools. I don't understand the following snippet of code. What does <code>window.XMLHttpRequest</code> signify? What makes it true or false? Is this entire if/else structure only there to account for ie6 and ie5, and if so can it all be replaced by one line which reads <code>xmlhttp = new XMLHttpRequest()</code> ?</p>
  
  <pre><code> if (window.XMLHttpRequest) {
       // code for IE7+, Firefox, Chrome, Opera, Safari
       xmlhttp = new XMLHttpRequest();
   } else {
       // code for IE6, IE5
       xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
   }
  </code></pre>