45 lines
1.4 KiB
Markdown
45 lines
1.4 KiB
Markdown
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# Suggest using `jest.spyOn()` (`prefer-spy-on`)
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🔧 This rule is automatically fixable by the
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[`--fix` CLI option](https://eslint.org/docs/latest/user-guide/command-line-interface#--fix).
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<!-- end auto-generated rule header -->
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When mocking a function by overwriting a property you have to manually restore
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the original implementation when cleaning up. When using `jest.spyOn()` Jest
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keeps track of changes, and they can be restored with `jest.restoreAllMocks()`,
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`mockFn.mockRestore()` or by setting `restoreMocks` to `true` in the Jest
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config.
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Note: The mock created by `jest.spyOn()` still behaves the same as the original
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function. The original function can be overwritten with
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`mockFn.mockImplementation()` or by some of the
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[other mock functions](https://jestjs.io/docs/en/mock-function-api).
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```js
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Date.now = jest.fn(); // Original behaviour lost, returns undefined
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jest.spyOn(Date, 'now'); // Turned into a mock function but behaviour hasn't changed
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jest.spyOn(Date, 'now').mockImplementation(() => 10); // Will always return 10
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jest.spyOn(Date, 'now').mockReturnValue(10); // Will always return 10
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```
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## Rule details
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This rule triggers a warning if an object's property is overwritten with a jest
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mock.
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The following patterns are considered warnings:
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```js
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Date.now = jest.fn();
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Date.now = jest.fn(() => 10);
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```
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These patterns would not be considered warnings:
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```js
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jest.spyOn(Date, 'now');
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jest.spyOn(Date, 'now').mockImplementation(() => 10);
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```
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