HttpInterceptor单元测试异常：断言结果不符合预期求助

核心问题 #

你的测试不管断言写toBeTruthy还是toBeFalsy都能通过，本质是断言根本没被执行。因为RxJS的Observable.subscribe是异步操作，jest默认不会等待异步回调完成，测试代码执行到result.subscribe()后就直接结束了，完全没等到error或next回调里的断言/fail执行。

验证方式 #

你可以在断言里加个控制台输出验证：

error: (err) => {
  console.log('断言执行了');
  expect(err).toBeFalsy();
}

运行测试会发现这条日志根本不会输出，证明回调从未被触发。

修复方案 #

下面是几种通用的解决方式，按需选择：

方案1：使用jest的done回调 #

通过done告诉jest等待异步操作完成：

it.each([500, 501])('should handle 500/501 errors', (statusCode, done) => {
  const response = new HttpErrorResponse({ status: statusCode });
  const request = new HttpRequest('GET', '/fakeUrl');
  const mockHandler = { handle: jest.fn(() => throwError(() => response )) };
  const result = errorInterceptor.intercept(request, mockHandler);

  result.subscribe({
    next: () => {
      fail('error should be thrown');
      done.fail('next callback should not be called');
    },
    error: (err) => {
      expect(err).toBeTruthy();
      done(); // 通知jest测试完成
    },
    complete: () => done.fail('complete callback should not be called')
  });
});

方案2：用RxJS的firstValueFrom转Promise（RxJS 7+） #

利用async/await让jest等待Promise完成：

import { firstValueFrom } from 'rxjs';

it.each([500, 501])('should handle 500/501 errors', async () => {
  const response = new HttpErrorResponse({ status: statusCode });
  const request = new HttpRequest('GET', '/fakeUrl');
  const mockHandler = { handle: jest.fn(() => throwError(() => response )) };
  const result = errorInterceptor.intercept(request, mockHandler);

  try {
    await firstValueFrom(result);
    fail('error should be thrown');
  } catch (err) {
    expect(err).toBeTruthy();
  }
});

方案3：使用TestScheduler同步测试（适合复杂场景） #

用RxJS的测试调度器把异步操作转为同步执行：

import { TestScheduler } from 'rxjs/testing';

let testScheduler: TestScheduler;

beforeEach(() => {
  errorInterceptor = new ErrorInterceptor();
  testScheduler = new TestScheduler((actual, expected) => {
    expect(actual).toEqual(expected);
  });
});

it.each([500, 501])('should handle 500/501 errors', (statusCode) => {
  testScheduler.run((helpers) => {
    const { cold, expectObservable } = helpers;
    const response = new HttpErrorResponse({ status: statusCode });
    const request = new HttpRequest('GET', '/fakeUrl');
    const mockHandler = { handle: jest.fn(() => cold('#', null, response)) };
    const result = errorInterceptor.intercept(request, mockHandler);

    expectObservable(result).toBe('#', null, response);
  });
});

内容的提问来源于stack exchange，提问作者matt-rock