framework/js/lib/initializers/preload.js

import Session from 'flarum/Session';

/**
 * The `preload` initializer creates the application session and preloads it
 * with data that has been set on the application's `preload` property. It also
 * preloads any data on the application's `preload` property into the store.
 * Finally, it sets the application's `forum` instance to the one that was
 * preloaded.
 *
 * `app.preload.session` should be the same as the response from the /api/token
 * endpoint: it should contain `token` and `userId` keys.
 *
 * @param {App} app
 */
export default function preload(app) {
  app.store.pushPayload({data: app.preload.data});

  app.forum = app.store.getById('forums', 1);

  app.session = new Session(
    app.store.getById('users', app.preload.session.userId),
    app.preload.session.csrfToken
  );
}