title: What do the different readystates in XMLHttpRequest mean, and how can I use them?
body: <p><code>XMLHttpRequest</code> has 5 <code>readyState</code>s, and I only use 1 of them (the last one, <code>4</code>).</p>
  
  <p>What are the others for, and what practical applications can I use them in?</p>