title: How to get HTTP header from Javascript?
body: <p>I have a Tomcat server that only serves <em>static files</em>(html, css, js). When the request comes in it gets intercepted by a proxy server. Proxy server authenticates the user and adds a   <code>userId</code> field to the header and forwards it my Tomcat server. </p>
  
  <p>How can I access <code>userId</code> that has been stored in the header from javascript? </p>
  
  <p>Thank you</p>