Jest offers three effective methods to mock modules in your tests, enabling you to isolate and control external dependencies for focused testing. Here's a detailed breakdown of each approach:
- Direct Mocking in the Test File (Using
jest.mock):
Ideal for mocking specific modules within the context of a single test file.
Syntax:
jest.mock('module-to-mock', () => ({
// Mock implementation for the module
}));
Example:
import { useSite } from 'src/context/SiteContext'; // Import the mocked module
jest.mock('src/context/SiteContext', () => ({
useSite: jest.fn(),
}));
const useSiteMockImpelmetation = (market: string) => {
(useSite as any).mockImplementation(() => {
return {
config: {
market,
},
};
});
};
it('should show 4 list item if is spanish market', () => {
mockMarket('es');
renderComponent();
const items = screen.getAllByRole('listitem');
expect(items).toHaveLength(4);
});
- Global Mocking in jest.setup.ts:
Suitable for globally mocking modules across all test files.
Location: Create a file named jest.setup.ts (or .js) in your project's root directory.
Syntax:
jest.mock('module-to-mock', () => ({
// Mock implementation for the module
}));
Example (in jest.setup.ts):
jest.mock('next-i18next', () => ({
useTranslation: () => ({
t: (key: string) => key,
}),
}));
- Manual Mocking in the
__mocks__Folder: (Manual Mocks · Jest )
Useful when the mocked module has complex logic or requires custom configuration.
-
Steps:
- Create a directory named mocks within the same directory as the module you want to mock.
- Inside
__mocks__, create a file with the same name as the module (including extension). This file will contain your mock implementation.
-
Example (assuming
src/context/SiteContext.tsxis the module to mock):- Create mocks/src/context/SiteContext.tsx.
- Implement your mock logic in this file.
jest.mock('src/context/SiteContext', () => ({
useSite: jest.fn(() => ({
config: {
market:'us'
}
})),
}));
Then in the test file, we can use it:
jest.mock('src/context/SiteConfigContext');
- The useSite mock will automatically have the mock value that defined in the
__mocks__/src/context/SiteContext.tsxfile
Props and cons:
| Method | Pros | Cons |
|---|---|---|
Direct Mocking (jest.mock in Test File) |
Targeted: Mocks apply only to a specific test file. - Simple: Easy to set up for basic mocking needs. - Clear: Mock behavior stays within the test case. | Limited Scope: Mocks are not shared across other tests. - Repetition: May lead to code duplication if mocking the same module in multiple tests. |
| Global Mocking (jest.setup.ts) | Consistency: Mocks are applied consistently across all tests. - Convenience: No need to repeat mock definitions for each test. | Global Impact: Accidental side effects in mocks can affect all tests. - Less Control: Less granular control over mock behavior compared to direct mocking. |
| Manual Mocking (mocks Folder) | Flexibility: Suitable for complex mocking logic or custom configurations. - Organization: Keeps mock code separate for better maintainability. | Setup: Requires creating a dedicated folder structure. - Potential Issues: May be harder to debug if mock logic becomes intricate. |
Choosing the Right Method:
For basic mocks within a single test: Direct mocking with jest.mock is straightforward and keeps code focused.
For consistent global mocking: Use jest.setup.ts when you need the same mock behavior across all tests.
For complex or reusable mock logic: Manual mocking with the mocks folder offers flexibility and organization.
We always can use jest.mock('path') to unmock the mock file
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