title: Constantly refresh Javascript values of text fields
body: <p>I'm working on an input form, and I have a Javascript function that gets all of the field values when you press a button. I am looking for a way to <em>automatically</em> refresh the Javascript values (so I can, for example, check if a username is too short on a registration page as they type, and also check if the username is available). Would this be possible?</p>
  
  <p>To clarify, I have an HTML input field (for text), and as the user is typing a result, automatically update.</p>
  
  <p>I'm also open to using PHP or jQuery if it's not possible using solely Javascript, but I'd prefer Javascript if it's possible. Also, sorry if this is a rather basic question, but I've searched and searched and can't find anything on it. I know it's possible because I've seen it on websites (in fact, even on this one, as you type a question, it updates the preview at the bottom).</p>