this is undefined in a member function

Question

export class AccessController extends BaseController {
  appContext!: AppContext;
  constructor(appContext: AppContext) {
    super(appContext);
    this.appContext = appContext;
  }
  public async generateAccess(data, _) {
    return this.appContext.accessService.generate()
  }
}

server.ts

export const accessController = new accessController(appContext);
export const generateAccess =
  functions
      .region(Constant.REGION.toString())
      .https
      .onCall(accessController.generateAccess);

Any idea why this is undefined in the member function and how do I solve this?