Jest: Mocking static class methods

[samzhang111]

I ran into an issue trying to mock a static class method in Jest. Is this behavior supported?

Cross-posted at http://stackoverflow.com/questions/38206478/mocking-methods-in-jest

I have a very simple unit test that looks like this:

import ApiWrapper from '../../services/api_wrapper';
jest.unmock('../helper')

describe('Helper', () => {
    let Helper;

    beforeEach(() => {
        Helper = require('../helper').default;
    });

    it('calls the Api Wrapper', () => {
        Helper.help()

        expect(ApiWrapper.help).toHaveBeenCalled();
    });

});

Where Helper looks like this:

import ApiWrapper from '../services/api_wrapper'
class Helper {
    help() {
        ApiWrapper.help()
    }
}

export default new Helper();

And ApiWrapper looks like this:

class ApiWrapper {
  static help () {
     console.log('help!')
  }
}
export default ApiWrapper;

ApiWrapper.help() gets mocked by Jest so 'help!' never gets printed, yet the expectation in the test fails. This still fails if we rewrite ApiWrapper to just be a plain Javascript object like such:

export default {
    help: () => { console.log('help!'); }
}

It works, however, if we change the imports in the spec (so ApiWrapper is imported in the beforeEach), and rewrite ApiWrapper to be a Singleton class, like so:

class ApiWrapper {
   help() {
      console.log('help!');
   }
}();

export default new ApiWrapper();

What is it about Jest's mocking behavior that makes this happen?

[cpojer]

The correct assertion should be toBeCalled. Can you try that?

[samzhang111]

Sorry, that's what I meant. I typed the example in Stackoverflow directly.

[cpojer]

Jest resets the module registry on every call to it. You are requiring ApiWrapper on the top and Helper inside beforeEach, so Helper likely receives a different copy of ApiWrapper. To fix it you can:

• Import both classes on the top.
• Require both classes in beforeEach

Simplified explanation:

var A = require('A');
jest.resetModuleRegistry();
A !== require('A'); // true, A is not the exact same module as before we reset the registry.

Let me know if that doesn't fix your problem.