Jest: Mock implementation not working

[zaguiini]

Do you want to request a _feature_ or report a _bug_?

A bug, maybe?

What is the current behavior?

I'm trying to have multiple implementations of a mock to test different cases, but I'm not having success.

Consider the following snippet (parts omitted for brevity):

import { initRealm, startQueue } from '@/lib/realm'

class Main extends React.PureComponent {
  async componentDidMount() {
    await initRealm()
  }
}

And the test:

import React from 'react'

import { shallow } from 'enzyme'
import { initRealm } from '@/lib/realm'

import Main from './'

jest.mock('@/lib/realm', () => ({
  initRealm: jest.fn(),
})

describe('Main', () => {
  it('should successfully start realm', () => {
    initRealm.mockImplementation(() => Promise.resolve(true))
    const wrapper = shallow(<Main />)
  })

  it('should fail to init realm', () => {
    initRealm.mockImplementation(() => Promise.reject(new Error('failed to start realm')))
    const wrapper = shallow(<Main />)
  })
})

What is the expected behavior?

The mock implementation function to replace the jest.fn, so that the exception is thrown. But, instead, the test succeeds. I want to have multiple implementations of my mock so I can test both successful and fail cases.

Please provide your exact Jest configuration

"jest": {
  "preset": "react-native",
  "collectCoverage": true,
  "coverageDirectory": "./__coverage__",
  "coverageReporters": [
    "lcov"
  ],
  "collectCoverageFrom": [
    "src/**/*.{js,jsx,mjs}"
  ],
  "setupFiles": [
    "./__setup__"
  ],
  "transformIgnorePatterns": [
    "node_modules/(?!(jest-)?react-native|react-navigation)"
  ]
}

Environment:
  OS: Linux 4.13
  Node: 9.11.1
  Yarn: 1.5.1
  npm: 5.6.0
  Watchman: Not Found
  Xcode: N/A
  Android Studio: 3.0 AI-171.4443003

Am I missing something?

[zaguiini]

If I do that way:

jest.mock('@/lib/realm')

initRealm.mockImplementationOnce(() => Promise.resolve('works'))
initRealm.mockImplementationOnce(() => Promise.reject(new Error('error thrown')))

describe('Main', () => {
  ...

It works. Why? And why it doesn't work inside the test?

Also: will Jest respect the order that the tests are defined in the describe clause, so these tests will not be flaky?

[SimenB]

Tests are run in the order they are declared (unless you use test.concurrent).

Mind setting up a reproduction repo we can pull down to see the failure?

[smakazmi]

should probably test with expect(initRealm).toThrow();

[bcass]

I was having this issue when I was assigning the mocked method with someObj.mockedMethod = jest.fn().mockReturnValue(someVal) inside a data-driven test (using test.each). Calling someObj.mockedMethod.mockReset() at the end of the test (before recreating the mock in the next test run) fixed this issue for me.

[avner-hoffmann]

Anything new with this? It is really a hassle

[sibelius]

I'm mocking s3 package like this

jest.mock('aws-sdk/clients/s3');

using a __mock__/aws-sdk/clients/s3.js

s3.js __mock__

const S3 = jest.fn(() => ({
  putObject: jest.fn((data, callback) => {
    callback(null, {});
  }),
  upload: jest.fn((params, callback) => {
    callback(null, {});
  }),
  getSignedUrl: jest.fn((operation, params, callback) => {
    callback(null, '');
  }),
  getObject: jest.fn((params, callback) => {
    callback(null, '');
  }),
  headObject: jest.fn((params, callback) => {
    callback(null, true);
  }),
  createPresignedPost: jest.fn((params, callback) => {
    const { Bucket = process.env.BUCKET_MEDIA, Fields } = params;
    callback(null, {
      url: `https://s3.amazonaws.com/${Bucket}`,
      fields: {
        key: Fields.key,
        bucket: Bucket,
        success_action_status: 200,
        error: null,
      },
    });
  }),
}));

export default S3;

I'd like to override the implementation of getObject in some tests like this:

s3.getObject.mockImplementationOnce((params, callback) => {
    callback(null, {
      Body,
    });
  });

but mockImplementationOnce is never called

How should I proceed to fix this?

[sibelius]

I've solved making s3 lazy instantiation

it looks like mockImplementation should be called before