Jest mock node module not working with typescript

Question

I try to mock the module uuid/v4 coming from npm. To do that, I have created a mock folder as suggested by jest : https://jestjs.io/docs/en/manual-mocks

My folder structure :

├──__mocks__ 
|  └──uuid
|       └──v4.ts
├──src
│   └──__tests__
│   └── ...
├──node_modules

The mock node module file v4.ts :

module.exports = jest.fn();

When I try to import uuid/v4 in my test file, jest should normally import the mock and I should be able to play with it.

Here my test file :

import uuidv4 from 'uuid/v4';

it('should create a job', () => {
    const jobId = 'fake-job-id';
    uuidv4.mockReturnValue(jobId); 
    ...
}

Unfortunately, the mock import seems not working because I can't add the mockReturnValue provided by jest and I have the following typescript error : property 'mockReturnValue' does not exist on type v4. ts(2339)

Do you have any idea how I can fix that please ? Thanks by advance.

Brian Adams · Accepted Answer · 2019-09-04 16:50:29Z

A common way to handle a situation like this is to auto-mock the module in the test, then tell TypeScript the module is auto-mocked by using jest.Mocked like this:

jest.mock('uuid/v4');  // <= auto-mock uuid/v4

import uuidv4 from 'uuid/v4';
const mocked = uuidv4 as jest.Mocked<typeof uuidv4>;  // <= tell TypeScript it's an auto-mock

test('hi', () => {
  mocked.mockReturnValue('mocked result');  // <= use the properly typed mock
  expect(uuidv4()).toBe('mocked result');  // Success!
})

Unfortunately the typing for uuid/v4 doesn't seem to work right with this approach.

As a workaround you can use a type assertion:

jest.mock('uuid/v4');  // <= auto-mock uuid/v4

import uuidv4 from 'uuid/v4';

test('hi', () => {
  (uuidv4 as jest.Mock).mockReturnValue('mocked result');  // <= use a type assertion
  expect(uuidv4()).toBe('mocked result');  // Success!
})

Thank you Brian it works perfectly ! I have noted that it works also just with : jest.mock('uuid/v4'); const uuidv4 = require('uuid/v4'); and without need of type assertion.
Awesome, good to hear! Yeah, using jest.Mocked is ideal since it maintains type safety, but since the typing doesn't work right in this case either the type assertion (some type safety) or const uuidv4 = require('uuid/v4'); (uuidv4 is type any) work fine as workarounds.

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