Is it possibly for JEST mocks to be overwritten by each other?

user_13813567 0
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I have two tests that look something like:

import someThing from '../someThing';
jest.mock('../someThing');

describe('group', () => {
    it('test #1', async () => {
        someThing.someFunction.mockImplementation(() => 123);
    });
    it('test #2', async () => {
        someThing.someFunction.mockImplementation(() => 456);
    });
});

I'm seeing an issue where it appears the mockImplementation from the first test is being called in the second test. If I disable test #1 test# 2 works as expected. (My actual code is more complicated than this).

The reason I need a separate mock for both is that one is supposed to return a failure result and one is supposed to return a success one given the same input.

From what I understand, JEST runs all test in parallel so I'm assuming what's happening is that the mockImplementation is being modified by both tests at the same time as it's the same import.

jestjs testing mocking javascript
2 Answers

From the Mock Implementations docs:

The mockImplementation method is useful when you need to define the default implementation of a mock function that is created from another module

When you need to recreate a complex behavior of a mock function such that multiple function calls produce different results, use the mockImplementationOnce method

For your case:

import someThing from '../someThing';
jest.mock('../someThing');

describe('group', () => {
    it('test #1', async () => {
        someThing.someFunction.mockImplementationOnce(() => 123);
    });
    it('test #2', async () => {
        someThing.someFunction.mockImplementationOnce(() => 456);
    });
});
user_64635580

It's virtually never desired for tests to affect each other.

As a rule of thumb, Jest spies need to be restored to original state between tests:

beforeEach(() => {
  jest.restoreAllMocks();
  jest.clearAllMocks();
});

Or preferably applied to all tests unconditionally with Jest restoreMocks and clearMocks configuration options.

Not doing this results in test cross-contamination.

A workaround is to use *Once methods, as another answer describes. When used together with *restore*, they are only needed for sequential mocks within a single test.

From what I understand, JEST runs all test in parallel

Jest runs test files in parallel. As for single tests, this isn't the case, unless concurrent tests are in use.

user_37315010
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