prefer-spy-on.md

Suggest using jest.spyOn() (prefer-spy-on)

🔧 This rule is automatically fixable by the --fix CLI option.

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.

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);