Mocking up static methods in jest

I am having trouble mocking up a static method in jest. Immagine you have a class A with a static method:

export default class A {
  f() {
    return 'a.f()'
  }

  static staticF () {
    return 'A.staticF()'
  }
}

And a class B that imports A

import A from './a'

export default class B {
  g() {
    const a = new A()
    return a.f()  
  }

  gCallsStaticF() {
    return A.staticF()
  }
}

Now you want to mock up A. It is easy to mock up f():

import A from '../src/a'
import B from '../src/b'

jest.mock('../src/a', () => {
  return jest.fn().mockImplementation(() => {
    return { f: () => { return 'mockedA.f()'} }
  })
})

describe('Wallet', () => {
  it('should work', () => {
    const b = new B()
    const result = b.g()
    console.log(result) // prints 'mockedA.f()'
  })
})

However, I could not find any documentation on how to mock up A.staticF. Is this possible?

You can just assign the mock to the static method

import A from '../src/a'
import B from '../src/b'

jest.mock('../src/a')

describe('Wallet', () => {
    it('should work', () => {
        const mockStaticF = jest.fn().mockReturnValue('worked')

        A.staticF = mockStaticF

        const b = new B()

        const result = b.gCallsStaticF()
        expect(result).toEqual('worked')
    })
})

Hope this will help you

// code to mock
export class AnalyticsUtil {
    static trackEvent(name) {
        console.log(name)
    }
}

// mock
jest.mock('../src/AnalyticsUtil', () => ({
    AnalyticsUtil: {
        trackEvent: jest.fn()
    }
}))

// code to mock
export default class Manager {
    private static obj: Manager
    
    static shared() {
        if (Manager.obj == null) {
            Manager.obj = new Manager()
        }
        return Manager.obj
    }
    
    nonStaticFunc() {
    }
}

// mock
jest.mock('../src/Manager', () => ({
    shared: jest.fn().mockReturnValue({
        nonStaticFunc: jest.fn()
    })
}))

// usage in code
someFunc() {
    RNDefaultPreference.set('key', 'value')
}

// mock RNDefaultPreference
jest.mock('react-native-default-preference', () => ({
    set: jest.fn()
}))

// code to mock
export namespace NavigationActions {
    export function navigate(
      options: NavigationNavigateActionPayload
    ): NavigationNavigateAction;
}

// mock
jest.mock('react-navigation', () => ({
    NavigationActions: {
        navigate: jest.fn()
    }
}))

I managed to mock it in a separate file in the __mocks__ folder using prototyping. So you would do:

function A() {}
A.prototype.f = function() {
    return 'a.f()';
};
A.staticF = function() {
    return 'A.staticF()';
};
export default A;

We need to create a mock and give visibility for the mocked method to the test suite. Below full solution with comments.

let mockF; // here we make variable in the scope we have tests
jest.mock('path/to/StaticClass', () => {
  mockF = jest.fn(() => Promise.resolve()); // here we assign it
  return {staticMethodWeWantToMock: mockF}; // here we use it in our mocked class
});

// test
describe('Test description', () => {
  it('here our class will work', () => {
    ourTestedFunctionWhichUsesThisMethod();
    expect(mockF).toHaveBeenCalled(); // here we should be ok
  })
})