Why Are My Cookies Mocked in Jest Not Updating Between Tests?

So, you’re working on a React+NextJS project and need to test whether a component is rendering correctly based on the values of cookies, using the cookies-next package for handling cookies. You’ve put together a Jest test suite to mock cookie values and assert that the output is as expected. However, you’ve run into a problem: when you change the cookie values in a subsequent test, the component still renders using the old cookie values. What’s going wrong here?

Let’s start by examining the code and understanding the logic piece by piece.

Initial Test Setup

You have a component called MyComponent that checks the values of cookies and renders different HTML based on those values. Here’s a snippet of your code:

import { getCookies } from 'cookies-next';

const MyComponent = () => {
    const { locale, countryCode } = getCookies();

    let outputHtml;

    if (locale === 'en-us' && countryCode === 'US') {
        outputHtml = <div>US</div>;
    } else if (locale === 'en-gb' && countryCode === 'UK') {
        outputHtml = <div>UK</div>;
    } else {
        outputHtml = <div>Others</div>;
    }

    return outputHtml;
};

export default MyComponent;

You have written a Jest test to mock the cookie values using jest.mock and jest.fn(). Here is the initial test:

jest.mock('cookies-next', () => ({
  __esModule: true,
  getCookies: jest.fn()
}));

const mockedGetCookies = getCookies as jest.Mock;

describe('Testing the component', () => {
  it('will render correct HTML based on cookies', () => {
    mockedGetCookies.mockReturnValue({
      locale: 'en-us',
      countryCode: 'US'
    });

    // Render the component
    render(<MyComponent />);

    // assertions
    expect(screen.getByText('US')).toBeInTheDocument();
  });
});

This test checks if your component correctly renders <div>US</div> when the locale is ‘en-us’ and the countryCode is ‘US’. The test should pass, as you’ve confirmed.

Adding a New Test

Then, you added another test to check if the component correctly renders for a different set of cookie values:

describe('Testing the component', () => {
  it('will render correct HTML based on cookies', () => {
    mockedGetCookies.mockReturnValue({
      locale: 'en-gb',
      countryCode: 'UK'
    });

    // Render the component
    render(<MyComponent />);

    // assertions
    expect(screen.getByText('UK')).toBeInTheDocument();
  });
});

However, when running both tests, you find that the second test still renders <div>US</div>, implying that the cookie values are not being updated between tests.

The Issue

The problem is that the mockReturnValue in Jest doesn’t reset between tests unless explicitly done so. Therefore, the first mocked value persists, causing both tests to render the same output.

The Solution

To ensure that each test gets its own set of mocked values, you should reset mocks between tests. Jest provides utilities that you can use in your test suite for this purpose. You can use jest.resetAllMocks or mockedGetCookies.mockReset() before each test to reset the mock function.

Here’s the revised test suite with the necessary reset:

jest.mock('cookies-next', () => ({
  __esModule: true,
  getCookies: jest.fn()
}));

const mockedGetCookies = getCookies as jest.Mock;

describe('Testing the component', () => {

  beforeEach(() => {
    // Reset the mock before each test
    mockedGetCookies.mockReset();
  });

  it('will render correct HTML based on cookies', () => {
    mockedGetCookies.mockReturnValue({
      locale: 'en-us',
      countryCode: 'US'
    });

    // Render the component
    render(<MyComponent />);

    // assertions
    expect(screen.getByText('US')).toBeInTheDocument();
  });

  it('will render correct HTML based on different cookies', () => {
    mockedGetCookies.mockReturnValue({
      locale: 'en-gb',
      countryCode: 'UK'
    });

    // Render the component
    render(<MyComponent />);

    // assertions
    expect(screen.getByText('UK')).toBeInTheDocument();
  });
});

In this revised test suite, mockedGetCookies.mockReset() is called before each test case, ensuring that each test gets a fresh mock setup. This allows you to test the component behavior with different cookie values accurately.

Running the Fix

Make sure to run the updated test suite to verify that it works as expected. By resetting the mocks between tests, you can be confident that each test is isolated and the component renders based on the correct cookie values.

And there you have it! By properly resetting your mocks, you ensure accurate and isolated test cases that truly reflect different scenarios based on cookie values. Happy testing!