Using deferred promise to test websockets

deferredPromise.ts

const createDeferred = () => {
  let d = {};
  const promise = new Promise((resolve, reject) => {
    d = { resolve, reject };
  });
  return { ...d, promise } as {
    resolve: () => void;
    reject: (err: unknown) => void;
    promise: Promise<void>;
  };
};

await makeServer();

  const d = createDeferred()

  let client = new WebSocket(url);
  client.onclose = (event) => {
    expect(event.code).toBe(1002);
    expect(event.reason).toBe('Protocol Error');
    expect(event.wasClean).toBeTruthy();
    d.resolve()
  };

  await d.promise

another options:

await new Promise(resolve => {
  client.onclose = (event) => {
    expect(event.code).toBe(1002);
    expect(event.reason).toBe('Protocol Error');
    expect(event.wasClean).toBeTruthy();
    resolve()
  }
})

const onClose = (client: WebSocket, handle: WebSocket['onclose']) => {
  return new Promise(resolve => {
    client.onclose = function handleClose(...args) {
      handle.call(this, ...args)
      resolve()
    }
  })
}

await onClose(client, (event) => {
  expect(event.code).toBe(1002);
  expect(event.reason).toBe('Protocol Error');
  expect(event.wasClean).toBeTruthy();
})

type FunctionPropertyNames<T> = {
    [K in keyof T]: NonNullable<T[K]> extends Function ? K : never;
  }[keyof T]

const onEvent = <EventName extends FunctionPropertyNames<WebSocket>>(client: WebSocket, eventName: EventName, handle: NonNullable<WebSocket[EventName]>) => {
  return new Promise(resolve => {
    client[eventName] = function handleEvent(...args: unknown[]) {
      const result = handle.apply(this, args)

      resolve()

      return result
    }
  })
}

await onEvent(client, 'onclose', (event) => {
  expect(event.code).toBe(1002);
  expect(event.reason).toBe('Protocol Error');
  expect(event.wasClean).toBeTruthy();
})

// better infer eventName using template literal types:
// type CastEventName<T extends string> = T extends `on${infer _}` ? T : never
// type FunctionEventNames<T> = CastEventName<FunctionPropertyNames<T> & string>
// const onEvent = <EventName extends FunctionEventNames<WebSocket>>