Mock.mockImplementation() not working

Question

I have a service class

Service.js

class Service {
}
export default new Service();

And I am trying to provide a mock implementation for this. If I use something like this:

jest.mock('./Service', () => { ... my mock stuff });

It works fine, however I'm not able to access any variables declared outside of the mock, which is a bit limiting as I'd like to reconfigure what the mock returns, etc.

I tried this (inspired by this other StackOverflow article: Service mocked with Jest causes "The module factory of jest.mock() is not allowed to reference any out-of-scope variables" error)

import service from './Service';

jest.mock('./Service', () => jest.fn);

service.mockImplementation(() => {
    return { ... mock stuff }
);

Unfortunately when I am trying to run this, I get the below error:

TypeError: _Service2.default.mockImplementation is not a function

It's a React Native project (set up with create-react-native-application, aka expo), Jest is 20.0.4. Node: 7.2.0, Npm: 5.3.0 — Janos, Aug 19 '17 at 13:05

score 48 · Answer 1 · edited Aug 23 '20 at 19:25

48

I had same problem as @Janos, the other answers didn't help either. You could do two things :

If you need to mock only a function from Service, in your test file:

import service from './Service';

jest.mock('./Service', () => jest.fn());

service.yourFunction = jest.fn(() => { /*your mock*/ })

If you need to mock the entire Module:

Say your service.js is in javascript/utils, create a javascript/utils/_mocks_ and inside it create a service.js file, you can then mock the entire class in this file, eg:
```
const myObj = {foo: "bar"}

const myFunction1 = jest.fn(() => { return Promise.resolve(myObj) })

const  myFunction2 = ...

module.exports = {
  myFunction1,
  myFunction2
}
```
then in your test file you just add:
```
jest.mock('./javascript/utils/service')
```
...functions exported from the mockfile will be then hit through your test file execution.

edited Aug 23 '20 at 19:25

Alexander Nied

12,804
4
25
45

answered May 20 '19 at 08:59

guillaume

651
6
14

2

This is the actual answer to the problem. The most voted does not solve the is not a function issue. – Paulo Neves Jan 30 '21 at 11:48
Solution 1 will replace service by a mock which wont be restored between each test, right ? – A.DUPONCHEL Jun 29 '21 at 14:05
if i could upvote this twice, i would. – cfnerd Oct 20 '21 at 22:49

score 17 · Answer 2 · answered Aug 28 '17 at 02:39

17

The mock is equal to jest.fn. You need to call jest.fn to create a mocked function.

So this:

jest.mock('./Service', () => jest.fn);

Should be:

jest.mock('./Service', () => jest.fn());

answered Aug 28 '17 at 02:39

deleteme

349
1
3

4

This did not work either. In this case service.mockImplementation didn't even exist. – Janos Aug 30 '17 at 07:31
8

This didn't work and the asker explicitly mentioned that it didn't work. – Sophie McCarrell Sep 26 '19 at 22:13

score 13 · Answer 3 · edited Sep 16 '22 at 01:37

13

ran into similar issues and resolved it by using .mockImplementationOnce

jest.mock('./Service', () => jest.fn()
  .mockImplementationOnce(() => {
    return { ... mock stuff }
  })
  .mockImplementationOnce(() => {
   return { ... mock other stuff }
  })
);

now when you run another test it will return the second mock object.

edited Sep 16 '22 at 01:37

Penny Liu

15,447
5
79
98

answered Nov 05 '19 at 15:50

Hendrik

139
1
5

THANK YOU VERY MUCH! Maybe not that convenient, but it worked out for me! – O.S.Kaya Oct 14 '21 at 17:29

score 3 · Answer 4 · answered Jan 03 '18 at 22:14

You need to store your mocked component in a variable with a name prefixed by "mock" and make sure you return an object with a default property as you import your Service from the default in your "main.js" file.

// Service.js
class Service {
}
export default new Service();

// main.test.js (main.js contains "import Service from './Service';")

const mockService = () => jest.fn();

jest.mock('./Service', () => {
    return {
        default: mockService
    }
});

score 0 · Answer 5 · answered Jun 15 '20 at 13:17

0

I had similar problem, and the cause was that ".spec.js" file had an

import jest from "jest-mock";

After removing this line, it worked.

answered Jun 15 '20 at 13:17

Somnium

1,059
1
9
34

score 0 · Answer 6 · answered Jan 19 '23 at 00:05

My mistake was that I was resetting the mock before each test. If you do that, be sure to reconfigure the mock implementation.

For example, change this:

  let value;
  let onPropertyChange: OnPropertyChangeCallback = jest.fn((changes: any) => {
      value = changes["testValue"];
    });

  const user = userEvent.setup();

  beforeEach(() => {
    jest.resetAllMocks();
  });

to this:

  let value;
  let onPropertyChange: OnPropertyChangeCallback;

  const user = userEvent.setup();

  beforeEach(() => {
    jest.resetAllMocks();

    onPropertyChange = jest.fn((changes: any) => {
      value = changes["testValue"];
    });
  });

score 0 · Answer 7 · answered Jun 01 '23 at 14:50

0

In my case mockImplementation didn't work because I copy paste tests and forgot to remove "async" keyword in this place it('test description', **async** () => {...}

answered Jun 01 '23 at 14:50

Vadzim Zhukouski

11
2

score 0 · Answer 8 · answered Aug 08 '23 at 09:59

Remove async from your test and DO NOT await expect!

WRONG

it.only('throws error on private visibility', async () => {
    await expect(getSteamData(mockSteamId)).rejects.toThrow(ErrorMessage.PRIVATE_VISIBILITY);
  });

RIGHT

it.only('throws error on private visibility', () => {
     expect(getSteamData(mockSteamId)).rejects.toThrow(ErrorMessage.PRIVATE_VISIBILITY);
  });

Mock.mockImplementation() not working

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