Custom client redirection after social authentication using passport, koa and koa-router

redirection.js

/**
  * This is how I configure koa-router
  * with passport to redirect the user
  * to exactly where they were in the
  * application after a successful
  * social authentification.
  */

// store client-provided redirection url
// in session cookie to retrieve it later.
// the user must pass a callback parameter
// to his request, containing the uri-
// encoded location they come from.
function* setCallbackUrlInSession(next) {
  if (this.request.query && this.request.query.callback) {
    this.session.callback = this.request.query.callback;
  }
  yield next;
}

// when the user comes back with their token
// after successful login, authenticate and
// then redirect them to the location they
// were at originately.
function getCallbackHandler(socialNetwork) {
  return function* (next) {
    var ctx = this;
    const { callback } = this.session;
    const appended = callback ? decodeURIComponent(callback) : '';
    yield* passport.authenticate(socialNetwork, function*(err, user, info) {
      if (err) throw err
        if (user === false) {
          ctx.status = 401
          ctx.body = { success: false }
        } else {
          yield ctx.login(user)
          ctx.body = { success: true }
        }
        ctx.response.redirect(successRedirect + `${appended}`);
    }).call(this, next);
  }
}

// koa-router configuration
router.get('/auth/facebook', setCallbackUrlInSession, passport.authenticate('facebook'));
router.get('/auth/facebook/callback', getCallbackHandler('facebook'));

Hi, I am learning generators and I'm intrigued about how the yield* passport.authenticate(socialNetwork, function*(err, user, info) { works. Is that a wrapped passport that takes a generator or does yield* fn(arg, genfn*) provide the proper context to run the genfn to completion?

Also, why don't you store the uri-decoded callback in the session?