How to properly make mock throw an error in Jest?

Question

I'm testing my GraphQL api using Jest.

I'm using a separate test suit for each query/mutation

I have 2 tests (each one in a separate test suit) where I mock one function (namely, Meteor's callMethod) that is used in mutations.

  it('should throw error if email not found', async () => {
    callMethod
      .mockReturnValue(new Error('User not found [403]'))
      .mockName('callMethod');

    const query = FORGOT_PASSWORD_MUTATION;
    const params = { email: 'user@example.com' };

    const result = await simulateQuery({ query, params });

    console.log(result);

    // test logic
    expect(callMethod).toBeCalledWith({}, 'forgotPassword', {
      email: 'user@example.com',
    });

    // test resolvers
  });

When I console.log(result) I get

{ data: { forgotPassword: true } }

This behaviour is not what I want because in .mockReturnValue I throw an Error and therefore expect result to have an error object

Before this test, however, another is ran

 it('should throw an error if wrong credentials were provided', async () => {
    callMethod
      .mockReturnValue(new Error('cannot login'))
      .mockName('callMethod');

And it works fine, the error is thrown

I guess the problem is that mock doesn't get reset after the test finishes. In my jest.conf.js I have clearMocks: true

Each test suit is in a separate file, and I mock functions before tests like this:

import simulateQuery from '../../../helpers/simulate-query';

import callMethod from '../../../../imports/api/users/functions/auth/helpers/call-accounts-method';

import LOGIN_WITH_PASSWORD_MUTATION from './mutations/login-with-password';

jest.mock(
  '../../../../imports/api/users/functions/auth/helpers/call-accounts-method'
);

describe('loginWithPassword mutation', function() {
...

UPDATE

When I substituted .mockReturnValue with .mockImplementation everything worked out as expected:

callMethod.mockImplementation(() => {
  throw new Error('User not found');
});

But that doesn't explain why in another test .mockReturnValue works fine...

It looks like your mock is *returning* an error object, not _throwing_ it. Without seeing your code that you are testing, I can only share the experience I had. I forgot to mock a function called in my mutation, which caused an error to be thrown unintentionally. Perhaps there is something similar happening for you? — Rhuarc13, May 14 '18 at 13:54

eduardomoroni · Accepted Answer · 2022-03-24T14:38:02.557

345

Change .mockReturnValue with .mockImplementation:

    yourMockInstance.mockImplementation(() => {
      throw new Error();
    });

in case you want to assert

   test('the fetch fails with an error', () => {
     return expect(fetchData()).rejects.toMatch('error');
   });

If it's a promise you can also to .rejects www.jestjs.io/docs/en/asynchronous#resolves--rejects

edited Mar 24 '22 at 14:38

answered Jun 02 '18 at 11:57

eduardomoroni

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1

cool. How to handle it and make and assert? – Dmytro Feb 19 '21 at 23:00
2

it throws the error but then the test fails because it has thrown an error. how do we assert? – schlingel Mar 07 '22 at 07:36

gawkface · Answer 2 · 2021-11-27T03:44:17.670

38

For promises, can use https://jestjs.io/docs/mock-function-api#mockfnmockrejectedvaluevalue

test('async test', async () => {
  const asyncMock = jest.fn().mockRejectedValue(new Error('Async error'));

  await asyncMock(); // throws "Async error"
});

For testing that error was thrown or not, can use https://eloquentcode.com/expect-a-function-to-throw-an-exception-in-jest

const func = () => {
  throw new Error('my error')
}
it('should throw an error', () => {
    expect(func).toThrow()
})

edited Nov 27 '21 at 03:44

answered Jul 29 '21 at 18:59

gawkface

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You would also need a try and catch in your expect otherwise it would not assert correctly. Can you please improve your answer or reply if I am missing something. – MG Developer Nov 24 '21 at 16:57
1

@MGDeveloper we dont need try-catch while unit testing and using toThrow() (https://jestjs.io/docs/expect#tothrowerror). If you try that in your tests, it should work. Can also test in here : https://codesandbox.io/s/jest-playground-forked-euewe?file=/src/sum.test.js:210-217 (sandbox content is transient though). I did edit the terminology from "handling" to "testing" if that was confusing – gawkface Nov 27 '21 at 03:42
1

I found the mockRejectedValue helpful in the case that the asynchronous unit I was testing handled the exception thrown in a specific way that I wanted to test, therefore in that case a catch or toThrow() would not be needed. – CortexCompiler Aug 10 '22 at 19:31

score 17 · Answer 3 · edited Oct 23 '20 at 16:06

17

For Angular + Jest:

import { throwError } from 'rxjs';

yourMockInstance.mockImplementation(() => {
  return throwError(new Error('my error message'));
});

edited Oct 23 '20 at 16:06

Sean W

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answered Apr 02 '20 at 16:14

AngularBoy

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Technically this isn't a `throw` in the pure JS sense. You are configuring the mock to return a RXJS observable object which immediately emits an error notification. Still, maybe handy for folks to see here. The accepted answer certainly *will* make a mock throw an error. In all cases. – Zach Lysobey Sep 16 '20 at 21:57
Only returning `throw Error` should be enough: `yourMockInstance.mockImplementation(() => throwError('my error message'));` – manzapanza Apr 30 '21 at 21:42
Actually using `mockReturnValue` is enough: `mockInstance.mockReturnValue(throwError(() => new Error('my error message')))` – RcoderNY Jun 19 '22 at 05:41

score 0 · Answer 4 · answered Mar 31 '23 at 17:49

make sure to add throw new Error('Network error or something') either in catch block or conditionally.

import fetchApi from '../src'

it("should throw error", async () => {
    const errorMessage: string = "Network Error";
    
    (axios.post as jest.Mock).mockRejectedValueOnce(new Error(errorMessage));
    
    expect(async () => await fetchApi()).rejects.toThrow(
          errorMessage
    );
});

How to properly make mock throw an error in Jest?

4 Answers4

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