How to test promise with try and catch a function like this example

Question

I cannot test the reject for some reason. Here is the function and I want to test it 100%. now it is like 92% and complain about reject(e) is not tested..

  public resolve(): Promise<{ [key: string]: boolean }> {
    return new Promise<{ [key: string]: boolean }>(async (resolve, reject) => {
      try {
        for await (const setting of settings) {
          // ObjectStore that acts like hashMap
          this.store.set(setting.key, setting.value);
        }
        resolve();
      }
      catch (e) {
        reject(e);
      }
    });
  }

Updates:

I have to make another Mock to make the catch statement happening first. And TestBed.overrideProvider is not really working for me, so that I have to

  it('should return a rejected promise', async() => {
    TestBed.resetTestingModule();
    TestBed.configureTestingModule({ 
//inserting the new mock provider to trigger the catch 
... 
});

and then use the answer below (Thank you @Apoorva Chikara) and works for me

If there is easier way to accomplish this, please let me know

Here is my updated code for testing.

  it('should return a rejected promise', async() => {
    TestBed.resetTestingModule();
    TestBed.configureTestingModule({
      providers: [
        {
          provide: ObjectStore,
          useValue: new ObjectStore('test'),
        },
        {
          // Making this client invalid to trigger catch and throw error
          // I think writing a mock class to throw error also can be done
          // this is the "settings" from azure listConfigurationSettings 
          provide: AppConfigurationClient,
          useValue: {},
        }
      ]
    });
    service = TestBed.inject(FeatureFlagResolver);
    store = TestBed.inject(ObjectStore);
    const rejection = async () => {
      await service.resolve().catch();
    }
    await expect(rejection()).rejects.toThrow();
  });

The "settings" is listConfigurationSettings from MS azure. we used to read from azure for configuration settings. For example, we want to enable certain thing(s) in the Angular site, we can go to azure portal to change app-setting ("chatbot", 'on'), so that the chatbot will be on the page.. Just app settings that people to toggle on, off for now.

Sorry I should have included more information in the first place.

Replied above in the main thread – Zeo Oct 24 '21 at 22:37 — Zeo, Oct 24 '21 at 22:37

Bergi · Answer 1 · 2021-10-24T22:45:29.810

Never pass an async function as the executor to new Promise! Your function should be simplified to

public async resolve(): Promise<{ [key: string]: boolean }> {
//     ^^^^^
  for await (const setting of settings) {
    // ObjectStore that acts like hashMap
    this.store.set(setting.key, setting.value);
  }
}

and then you won't need a separate test for the code path that doesn't even exist.

~~Furthermore, you probably don't want to use for await, unless settings is an asynchronous generator.~~ Given the updated details, it actually is a PagedAsyncIterableIterator. I would however recommend not to store that in a global variable, but do the iteration in the same function where the .listConfigurationSettings() call is done.

There are some readings in the link(s) you provided.. I will go over them. It is good to learn more.. I have senior in my company that the person doesnt like their code being "fixed".... But thank you that I know more about the pattern.. — Zeo, Oct 25 '21 at 12:58

score 0 · Answer 2 · answered Oct 24 '21 at 13:50

0

You can catch the error and test it. It is similar to testing the resolving.

  describe('Test Case', () => {
    describe('Testing Sample 1', () => {
      it('should return a rejected promise', () => {
        service.resolve()
          .catch((error) => {
            expect(error).toEqual('the error string');
          });
      });
    });
  });

answered Oct 24 '21 at 13:50

Apoorva Chikara

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If that works, you can accept the answer for future reference for other users. – Apoorva Chikara Oct 24 '21 at 15:40
You'll want to use `await expect(service.resolve()).rejects()`, see https://jestjs.io/docs/asynchronous. Notice that in the above code, the test actually does succeed when the promise does fulfill! – Bergi Oct 24 '21 at 16:32
Thank you @Bergi. Although I used another way to do it but I think we are accomplishing the similar thing here. – Zeo Oct 24 '21 at 23:20

How to test promise with try and catch a function like this example

2 Answers2