Example of a async singleton class

AsyncSingleton.ts

export class TestServer {
  static _instance: TestServer;
  static app: Thing;
  constructor() {
    return (async () => {
      if (TestServer._instance) {
        return TestServer._instance;
      }
      const app = await Something();
      TestServer.app = app;
      TestServer._instance = this;
      return this;
    })();
  }
}