How to mock uuid with Jest

Question

so on my search for an answer to my problem I found this post: Jest: How to globally mock node-uuid (or any other imported module)

I already tried the answer but I can't seem to use it properly, as it´s giving me an undefined error. I'm new to the testing scene so please excuse any major errors:

This was my first approach

const mockF = jest.mock('uuid'); 

mockF.mockReturnValue('12345789'); 

But it wouldn't recognize the functions.

"mockF.mockReturnValue is not a function" among others I tried.

Then I tried to manually mock as the post suggested but can' seem to make it work, can you help me? Thanks

Here's the entire test if it helps:

    const faker = require('faker');
    const storageUtils = require('../../storage/utils');
    const utils = require('../utils/generateFile');
    const { generateFileName } = storageUtils;
    const { file } = utils;

    test('should return a valid file name when provided the correct information', () => {
        // ARRANGE
        // create a scope
        const scope = {
            type: 'RECRUITER',
            _id: '987654321',
        };
        // establish what the expected name to be returned is
        const expectedName = 'r_987654321_123456789.png';

        jest.mock('uuid/v4', () => () => '123456789');

        // ACTION
        const received = generateFileName(file, scope);

        // ASSERT
        // expect the returned value to equal our expected one
        expect(received).toBe(expectedName);
    });

Answer 1

Mock it by using mockImplementation.

import uuid from 'uuid/v4';
jest.mock('uuid/v4');

describe('mock uuid', () => {
  it('should return testid }', () => {
    uuid.mockImplementation(() => 'testid');
    ...
  });  
});

Make sure you import uuid correctly (with correct version reference e.g. v4, v3...)

Answer 2

This worked for me. In my case I only wanted to verify that the function was called rather than asserting on a specific value.

import * as uuid from 'uuid';
jest.mock('uuid');

const uuidSpy = jest.spyOn(uuid, 'v4');

// in a test block
expect(uuidSpy).toHaveBeenCalledTimes(1);

Answer 3

I figure I might as well add my solution here, for posterity. None of the above was exactly what I needed:

jest.mock('uuid', () => ({ v4: () => '123456789' }));

By the way, using a variable instead of '123456789' breaks it.

This should be mocked where we have the imports and it does not require you to explicitely import uuid in your spec file.

Answer 4

for my case I used the answer of this Github issue

jest.mock('uuid/v4', () => {
 let value = 0;
 return () => value++;
});