Mock not working when using ts-jest

[kb-ig]

I'm refactoring a JS project over to TS. During this process, I will need a test workflow that runs both JS and TS tests. ts-jest seems like a good candidate for this.

While migrating tests over to ts-jest, I've noticed that some tests are now failing because jest.mock doesnt seem to be running(?), and as such, the code being tested is executing the dependency's actual code rather than it's mocked code.

To demonstrated this, I've created an MRP here. To use this, delete/rename jest.config.js and run npm test to execute the test with plain node and it will pass, then restore jest.config.js and run npm test again and the test will fail because the mock is no longer being called. We can also see that the actual code is being executed because of the log it writes.

Anyone have any ideas what my jest.mock doesnt seem to be running, and how I can actually get this to work with ts-jest?

---

Code being mocked:

class Thing {
    doStuff() {
        console.log("Thing is doing stuff");
    }
}

module.exports = Thing;

Code being tested:

const Thing = require('./Thing');

module.exports = {
    process: async () => {
        const thing = new Thing();
        thing.doStuff();
    }
}

Test

const processor = require('../src/procesor');

const mockThing = {
    doStuff: jest.fn()
};

// When we call `new Thing()`, mockThing should be returned
jest.mock('../src/Thing.js', () => jest.fn().mockImplementation(() => mockThing));

test('it should work', async () => {
    processor.process();
    expect(mockThing.doStuff).toBeCalledTimes(1);
})

[kb-ig]

In the meantime, I've decided to split the testing so that JS files are run by jest and TS files run by ts-jest. This was the only way I could get things working.