42 lines
1.3 KiB
Markdown
42 lines
1.3 KiB
Markdown
|
# 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](https://jestjs.io/docs/en/mock-function-api).
|
||
|
|
|
||
|
|
```js
|
||
|
|
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:
|
||
|
|
|
||
|
|
```js
|
||
|
|
Date.now = jest.fn();
|
||
|
|
Date.now = jest.fn(() => 10);
|
||
|
|
```
|
||
|
|
|
||
|
|
These patterns would not be considered warnings:
|
||
|
|
|
||
|
|
```js
|
||
|
|
jest.spyOn(Date, 'now');
|
||
|
|
jest.spyOn(Date, 'now').mockImplementation(() => 10);
|
||
|
|
```
|