#prefer-spy-on

#Configuration

import { defineConfig, jestPlugin } from '@rslint/core';

export default defineConfig([
  jestPlugin.configs.recommended,
  {
    rules: {
      'jest/prefer-spy-on': 'error',
    },
  },
]);

#Rule Details

When a function is mocked by assigning jest.fn() to an object property, the original implementation is lost and must be restored manually during cleanup. jest.spyOn() wraps the existing method as a mock while preserving its behavior, and Jest can restore it with jest.restoreAllMocks(), mockFn.mockRestore(), or the restoreMocks config option.

The mock returned by jest.spyOn() behaves like the original function until you change it with mockImplementation(), mockReturnValue(), or another mock API.

Examples of incorrect code for this rule:

Date.now = jest.fn();
Date.now = jest.fn(() => 10);
obj.a = jest.fn();
window.fetch = jest.fn(() => ({})).mockReturnValue('ok');

Examples of correct code for this rule:

jest.spyOn(Date, 'now');
jest.spyOn(Date, 'now').mockImplementation(() => 10);
jest.spyOn(obj, 'a').mockImplementation();
jest.spyOn(window, 'fetch').mockReturnValue('ok');

#Original Documentation

• jest/prefer-spy-on