/**
   * Gets the access token for the given code.
   * @param code The authorization code.
   * @param callback Optional callback fn.
   */
  getToken(code: string): Promise<GetTokenResponse>;
  getToken(options: GetTokenOptions): Promise<GetTokenResponse>;
  getToken(code: string, callback: GetTokenCallback): void;
  getToken(options: GetTokenOptions, callback: GetTokenCallback): void;
  getToken(
    codeOrOptions: string | GetTokenOptions,
    callback?: GetTokenCallback,
  ): Promise<GetTokenResponse> | void {
    const options =
      typeof codeOrOptions === 'string' ? {code: codeOrOptions} : codeOrOptions;
    if (callback) {
      this.getTokenAsync(options).then(
        r => callback(null, r.tokens, r.res),
        e => callback(e, null, e.response),
      );
    } else {
      return this.getTokenAsync(options);
    }
  }

  private async getTokenAsync(
    options: GetTokenOptions,
  ): Promise<GetTokenResponse> {
    const url = this.endpoints.oauth2TokenUrl.toString();
    const headers = new Headers();
    const values: GetTokenQuery = {
      client_id: options.client_id || this._clientId,
      code_verifier: options.codeVerifier,
      code: options.code,
      grant_type: 'authorization_code',
      redirect_uri: options.redirect_uri || this.redirectUri,
    };