Fastboot `renderingContext` macro: https://twitter.com/ryantotweets/status/1011745553447866368

application.js

// app/routes/application.js

export default Route.extend({
  currentUser: service(),
  beforeModel: renderingContext({
    server() {},
    client() {
      return this.get('currentSession.fetch').perform().catch(console.error);
    }
  })
});

rendering-context.js

// app/utils/rendering-context.js

export default function renderingContext(context) {
  return (typeof Fastboot === "undefined") ? context.client : client.server; 
}

session.js

// app/services/session.js

export default Service.extend({
  setupCSRF: renderingContext({
    server() {},
    client(session) {
      let prefilter = (options, originalOptions, jqXHR) => {
        let token = session.get('csrfToken');
        
        if (token && !options.crossDomain) {
          jqXHR.setRequestHeader('X-CSRF-Token', token); 
        }
      };
      
      $.ajaxPrefilter(prefilter);
    }
  })
});

site-header.js

// app/components/site-header.js

export default Component.extend({
  currentUser: service(),
  showAvatar: renderingContext({
    server: false,
    client: readOnly('currentUser.hasAvatar')
  })
});