How to mock object in jest

Question

I'm trying to mock a function of an object with Jest and Typescript and it just does not work. Here is a short version of what I have:

// myModule.ts

export const Foo = {
  doSomething: () => {
    // ... does something
    console.log('original implementation');
  }
}

Then in my test:

jest.mock('.myModule.ts', () => {
  return {
    Foo: {
      doSomething: jest.fn().mockImplementation(() => {
        console.log('mock implementation')
      })
    }
  }
})

// .. further down in the test
Foo.doSomething();

Shouldn't the console.log('mock implementation') be called when I call Foo.doSomething()? It doesn't error and it's not calling the original implementation anymore, but it's also not calling my mockImplementation.

LoremIpsum · Accepted Answer · 2020-05-05 18:26:12Z

1

Try this:

import { Foo } from './foo';

jest
  .spyOn(Foo, 'doSomething')
  .mockImplementation(() => console.log('mock implementation'));

Foo.doSomething();

answered May 5, 2020 at 18:26

LoremIpsum

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1 Comment

overdub60 Over a year ago

Awesome, thanks, that worked right away and was so simple!

Chris · Accepted Answer · 2020-11-30 22:43:20Z

0

After switching to jest 26.6, I found that none of these worked outside of tests (had been doing it directly below my imports, outside of any describe or it call). If I wanted any kind of mock implementation, I had to set it in the test, or in a beforeEach. If anyone can supply a link to the documentation on this, I'd be obliged.

answered Nov 30, 2020 at 22:43

Chris

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