How to mock a module multiple times?

[tal-rofe]

In my source code, I use the cosmiconfig module. My code integrates with it, and I'd like to test some cases with it.

Here is my source code:

import { cosmiconfig } from 'cosmiconfig';
import TypeScriptLoader from 'cosmiconfig-typescript-loader';

const explorer = cosmiconfig(CONFIGURATION_MODULE_NAME, {
	searchPlaces: SEARCH_PLACES,
	loaders: {
		'.ts': TypeScriptLoader(),
	},
});

const result = await explorer.search();

I want to mock this cosmiconfig module in order to test some cases.

What's the correct way of doing so?

For example, for checking upon null return value from await explorer.search(), I tried:

import { describe, it, expect, afterEach, vi } from 'vitest';

vi.mock('cosmicocnfig', () => {
	return {
		cosmiconfig: vi.fn(() => {
			return {
				search: () => Promise.resolve(null),
			};
		}),
	};
});

But now I also want to test when await explorer.search() rejects. How can I mock it again?

[sheremet-va]

You can import it and change the returned value:

import { cosmiconfig } from 'cosmiconfig'

vi.mocked(cosmiconfig).mockReturnValue({
  search: () => Promise.reject(null)
})

vi.mocked is just a typehint, btw. It's noop.

[tal-rofe]

@sheremet-va

So, will the following snippet do the job?

import { describe, it, expect, afterEach, vi } from 'vitest';

vi.mock('cosmicocnfig');
it('getConfiguration | Function should return the default when there is no configuration file', async () => {
vi.mocked(cosmiconfig).mockReturnValue({
	search: () => Promise.resolve(null),
	...cosmiconfigFunctions,
});

const result = await getConfiguration();

expect(result).toJsonEqual(DEFAULT_CONFIGURATION);
});

```ts

[sheremet-va]

I think, yes.

[tal-rofe]

@sheremet-va

I still get an error with the following snippet:
https://gist.github.com/tal-rofe/26ec299a1b266d49d880e690781450e4

TypeError: __vite_ssr_import_0__.vi.mocked(...).mockReturnValue is not a function

[sheremet-va]

What gives this error? I see you are doing these:

vi.mocked(validateConfiguration).mockReturnValue({});

but you haven't mocked the module you are importing them from

[tal-rofe]

For example,

TypeError: __vite_ssr_import_0__.vi.mocked(...).mockReturnValue is not a function
 ❯ tests/utils/configuration.spec.ts:144:26
    142|                        };
    143| 
    144|                        vi.mocked(cosmiconfig).mockReturnValue({
       |                          ^
    145|                                search: () =>
    146|                                        Promise.resolve({

[tal-rofe]

@sheremet-va

I know I have a flaw in line 8 in the snippet I've provided. I fixed the flaw but the issue persists

[tal-rofe]

Can confirm the answer was indeed provided, and I have some irrelevant issue

[adaptably]

The following works for me.

myFunction.js

import library from '@whatever/library'

export default () => {
  return library()
}

myFunction.spec.js

import { describe, expect, it, vi } from 'vitest'

// --------------------------------------------

import library from '@whatever/library'
vi.mock('@whatever/library')

// This also seems to work in my environment, but it sounds like this may not be officially supported or
// work in all environments.
// 
// const library = vi.fn()
// vi.mock('@whatever/library', () => ({ default: library }))

// --------------------------------------------

import myFunction from './myFunction.js'

// --------------------------------------------

describe('myFunction', () => {
  it('returns value from library', () => {
    library.mockReturnValue('foo')
    expect(myFunction()).toEqual('foo')

    library.mockReturnValue('bar')
    expect(myFunction()).toEqual('bar')
  })
})

[sheremet-va]

// const library = vi.fn()
// vi.mock('@whatever/library', () => ({ default: library }))

This won't work, because vi.mock is hoisted, and defined before library.

[adaptably]

Perhaps it shouldn't, but it does – at least in my environment. 🙃 I've just tested it.

vi.mock throws the expected ReferenceError if the library variable is used within the call to vi.mock, i.e.

const library = vi.fn()
vi.mock('svelte', library)

However, if the library variable is referenced inside the anonymous function passed to vi.mock as above, it works just fine for me.

Regardless, it sounds like the best practice would be the pattern suggested by @sheremet-va. 👍🏼 Updated my answer to include this caveat.

[RossRogers]

See also #4124