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1.3 KiB
1.3 KiB
Suggest using jest.spyOn() (prefer-spy-on)
When mocking a function by overwriting a property you have to manually restore
the original implementation when cleaning up. When using jest.spyOn() Jest
keeps track of changes, and they can be restored with jest.restoreAllMocks(),
mockFn.mockRestore() or by setting restoreMocks to true in the Jest
config.
Note: The mock created by jest.spyOn() still behaves the same as the original
function. The original function can be overwritten with
mockFn.mockImplementation() or by some of the
other mock functions.
Date.now = jest.fn(); // Original behaviour lost, returns undefined
jest.spyOn(Date, 'now'); // Turned into a mock function but behaviour hasn't changed
jest.spyOn(Date, 'now').mockImplementation(() => 10); // Will always return 10
jest.spyOn(Date, 'now').mockReturnValue(10); // Will always return 10
Rule details
This rule triggers a warning if an object's property is overwritten with a jest mock.
Default configuration
The following patterns are considered warnings:
Date.now = jest.fn();
Date.now = jest.fn(() => 10);
These patterns would not be considered warnings:
jest.spyOn(Date, 'now');
jest.spyOn(Date, 'now').mockImplementation(() => 10);