Angular Jest单元测试如何mock Promise覆盖signMessage的if分支

现有如下方法实现：

async signMessage(xml, mbo): Promise<any> {
  try {
    const isSignatureAppAlive = await this.pingDigitalSignatureApp().toPromise();

    if (isSignatureAppAlive.alive) {
      try {
        this.signedMsg = await this.getSignedMsg(xml, mbo).toPromise();
        return this.signedMsg;
      } catch (er) {
        return this.handleSignatureAppErrorCodes(er.error.errorCode);
      }
    }
  } catch (e) {
    this.showSignatureInfoModal();
    return this.handleError({ message: 'empty' });
  }
}

getSignedMsg(msg, mbo): Observable<any> {
  this.signDocument.title = '';
  this.signDocument.content = btoa(msg);
  this.signDocument.contentType = 'application/xml';
  this.signDocument.uriOrIdToSign = '';

  this.postParam.certificateAlias = mbo;
  this.postParam.digestAlgorithm = 'SHA1';
  this.postParam.documents = [];
  this.postParam.documents.push(this.signDocument);
  this.postParam.signingMethod = 'xmldsig';
  this.postParam.envelopingMethod = 'enveloped';

  return this.http.post<any>(SERVICE_URL + 'sign', this.postParam, httpOptions).pipe();
}

目前已编写覆盖错误捕获场景的测试用例：

service.signMessage('xml', 'mbo').catch((errCode) => {
  expect(errCode).toBe('empty');
  done();
});

需要补充什么逻辑才能覆盖方法内的if语句分支？是否需要mock Promise？曾尝试编写如下测试用例：

it('signMessage', async (done) => {
  const spy = spyOn(service, 'pingDigitalSignatureApp').and.returnValue(
    new Promise((resolve) => resolve('someVal')),
  );
  service.pingDigitalSignatureApp();
  expect(spy).toHaveBeenCalled();
  done();
});

该用例执行后未达到预期效果，测试逻辑并未进入if语句分支，需要相关解决方案建议。

之前写的测试没进入if分支，核心问题有3个：

• pingDigitalSignatureApp 本身返回Observable类型，直接mock返回Promise，和原代码调用.toPromise()转Promise的逻辑不匹配
• mock的resolve值是字符串'someVal'，原代码判断条件是isSignatureAppAlive.alive，字符串不存在alive属性，会被判定为假值，自然进不去if块
• 测试逻辑里直接调用了service.pingDigitalSignatureApp()，根本没有调用被测目标signMessage方法，逻辑不可能走到if分支

要覆盖if (isSignatureAppAlive.alive === true)分支，按以下规则mock即可：

• 把pingDigitalSignatureApp mock成返回Observable，这个Observable吐出的对象必须带alive: true属性
• 要覆盖if块内签名成功的分支，同步mock getSignedMsg 方法返回成功的Observable
• 要覆盖if块内签名失败的内层catch分支，mock getSignedMsg 返回抛出指定结构错误的Observable

注意：用到的of、throwError是RxJS提供的构造方法，需要从rxjs包导入，不要手动返回原生Promise，避免和原方法的Observable调用链不匹配。

参考测试用例 #

覆盖「签名服务存活+签名成功」分支 #

it('should enter if branch when signature app is alive and sign success', async () => {
  // mock ping方法返回alive为true的Observable
  spyOn(service, 'pingDigitalSignatureApp').and.returnValue(
    of({ alive: true })
  );
  // mock getSignedMsg返回成功的签名结果
  const mockSignedResult = 'mock-signed-msg';
  spyOn(service, 'getSignedMsg').and.returnValue(of(mockSignedResult));
  spyOn(service, 'handleSignatureAppErrorCodes');

  // 调用被测方法
  const result = await service.signMessage('test-xml', 'test-mbo');

  // 断言逻辑符合预期
  expect(service.pingDigitalSignatureApp).toHaveBeenCalled();
  expect(service.getSignedMsg).toHaveBeenCalledWith('test-xml', 'test-mbo');
  expect(result).toBe(mockSignedResult);
  expect(service.signedMsg).toBe(mockSignedResult);
  expect(service.handleSignatureAppErrorCodes).not.toHaveBeenCalled();
});

覆盖「签名服务存活+签名接口报错」分支 #

it('should handle sign error when signature app is alive but sign request failed', async () => {
  // mock ping返回alive:true
  spyOn(service, 'pingDigitalSignatureApp').and.returnValue(of({ alive: true }));
  // mock getSignedMsg返回错误，错误结构匹配原代码取值逻辑er.error.errorCode
  const mockErrorCode = 'ERR_SIGN_FAILED';
  const mockError = { error: { errorCode: mockErrorCode } };
  spyOn(service, 'getSignedMsg').and.returnValue(throwError(() => mockError));
  spyOn(service, 'handleSignatureAppErrorCodes').and.returnValue(mockErrorCode);

  const result = await service.signMessage('test-xml', 'test-mbo');

  expect(service.getSignedMsg).toHaveBeenCalledWith('test-xml', 'test-mbo');
  expect(service.handleSignatureAppErrorCodes).toHaveBeenCalledWith(mockErrorCode);
  expect(result).toBe(mockErrorCode);
});

如果要覆盖「签名服务不存活」的分支（if条件不满足场景），只需要把ping方法的返回值改成of({ alive: false })即可，此时if块不会执行，方法最终会返回undefined，可根据业务逻辑补充对应断言。

内容的提问来源于stack exchange，提问作者fadingbeat