TEST_RECIPES.md

Mock, already globally mocked native module

Let's say we have mocked react-native-touch-id in setup.js. This is how we mocked it in setup.js

jest.mock('react-native-touch-id', () => {
  return {
    authenticate: jest.fn(message => Promise.resolve()),
  }
})

If you notice here, we are always resolving module. If we run our test using TouchId module, then we can't test error conditions.

To mock this implementation again which rejects this promise and allow us to test error condition. We can do something as follows:

import TouchId from 'react-native-touch-id'
...
it('test error condition', async () => {
  TouchId.authenticate.mockImplementation(_ => Promise.reject())
  // authenticate is internally using TouchId, we test component interface
  await component.authenticate()
  // make some assertions
})

Error encountered: Invariant Violation: Native module cannot be null.

We are missing the mocks for native module. Open <rootDir>/__mocks__/setup.js and create a mock for native module which is throwing error. You can check which native module is throwing error from call stack printed in terminal