discourse/app/views/common/_persona_javascript.html.erb

<script src="https://login.persona.org/include.js"></script>
<script>
  (function() {
    var readyCalled = false;
    navigator.id.watch({
      onlogin: function(assertion) {
        if (readyCalled) {

          Discourse.ajax({
            type: 'POST',
            url: Discourse.getURL('/auth/persona/callback'),
            data: { 'assertion': assertion },
            success: function(data, textStatus, jqXHR) {
              Discourse.authenticationComplete(data);
            },
            dataType: 'json'
          });
        }
      },
      onlogout: function() {
        if (readyCalled) {
          Discourse.logout();
        }
      },
      onready: function() {
        readyCalled = true;
      }
    });
  }());
</script>