Jest mock promise rejection to enforce rejection in calling function-CodePudding

I'm trying to test the following get function using Jest. How can test / mock the Promise rejection in localForage.getItem, so that I can test the get catch block?

async get<T>(key: string): Promise<T | null> {
  if (!key) {
    return Promise.reject(new Error('There is no key to get!'));
  }

  try {
    return await this.localForage.getItem(key);
  } catch (err) {
    throw new Error('The key ('   key   ") isn't accessible.");
  }
}

I tried the following:

  test('test get promise rejection', async () => {
    const expectedError = new Error(
      'The key ('   'fghgdfghfghfdh'   ") isn't accessible."
    );
    jest.fn(localforage.getItem).mockRejectedValue(new Error());
    expect(get('fghgdfghfghfdh')).rejects.toThrow(expectedError);
  });

But I get the following error:

node:internal/process/promises:246
          triggerUncaughtException(err, true /* fromPromise */);
          ^

[UnhandledPromiseRejection: This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). The promise rejected with the reason "Error: expect(received).rejects.toThrow()

Received promise resolved instead of rejected
Resolved to value: null".] {
  code: 'ERR_UNHANDLED_REJECTION'
}

CodePudding user response：

Well... we remove the await keyword from this line

expect(await get('fghgdfghfghfdh')).rejects.toThrow(expectedError);

Because the error clearly states

received value must be a promise or a function returning a promise

Then the test fails because it expected to be rejected and insted is resolved with null value

So, either invoke get without a key

 expect(get()).rejects.toThrow(expectedError);

Or make get function more defensive like this

async get<T>(key: string): Promise<T | null> {
  if (!key) {
    return Promise.reject(new Error('There is no key to get!'));
  }

  try {
    const result = await this.localForage.getItem(key);
    if (result) return result;
    throw new Error('empty value');
  } catch (err) {
    throw new Error('The key ('   key   ") isn't accessible: ");
  }
}

Which approach to use? I think both... anyhow I do hope you manage with your tests!

CodePudding user response：

I got it working, I had to replace localforage.getItem with jest.fn().mockRejectedValue:

test('test get promise rejection', async () => {
  localforage.getItem = jest.fn().mockRejectedValue(new Error());

  const expectedError = new Error(
    'The key ('   'fghgdfghfghfdh'   ") isn't accessible."
  );
  expect(handler.get('fghgdfghfghfdh')).rejects.toThrow(expectedError);
});