为什么要声明相同类类型的静态字段？

到目前为止，我对 TypeScript 并不熟悉，但我还没有遇到过任何语言的这个，我在 ASP.NET Core + React-Redux + TS 样板中看到了这段代码。请解释为什么需要在声明的描述中声明一个相同类型的静态字段？

import { AxiosResponse } from 'axios';
import { BaseService } from './base.service';
import { IAuthUser, ICredentials } from '../store/auth/types';

/**
 * Auth API abstraction layer communication via Axios (typescript singleton pattern)
 */
class AuthService extends BaseService {
  private static _authService: AuthService; // вот эта строка
  private static _controllerName: string = 'Auth';

  private constructor(controllerName: string) {
    super(controllerName);
  }

  public static get Instance(): AuthService {
    return this._authService || (this._authService = new this(this._controllerName));
  }

  public async logoutAsync(): Promise<AxiosResponse> {
    return await this.$http.post('Logout');
  }

  public async loginAsync(credentials: ICredentials): Promise<IAuthUser> {
    const { data } = await this.$http.post<IAuthUser>('Login', credentials);
    return data;
  }
}

export const AuthApi = AuthService.Instance;