Clear Varnish from your browser console. Varnish must be configured to accept PURGE HTTP requests, you must be on the ACL if it exists, etc.

gistfile1.js

var xmlHttp=new XMLHttpRequest();
xmlHttp.open('PURGE', top.location.href, true);

xmlHttp.onreadystatechange = function () {
  if (xmlHttp.readyState === 4) {
      if (xmlHttp.status === 200) {
        console.log("Great");
      }
    else {
      console.log("Too bad");
    }
  }
};
xmlHttp.send(null);