Jest - Mock A Constant Property From A Module For A Specific Test

So, I'm attempting to do something which on the surface should be very simple... I have some constants defined in: ` //constants.js module.exports = { MY_CONSTANT: 'TEST' } `

Solution 1:

To mock for one test only:

jest.mock('./constants.js', () => ({
  MY_CONSTANT: 'something fake'
}));

https://jestjs.io/docs/en/manual-mocks

---

To provide a mock for every test:

1. create a __mocks__ directory adiacent to the module you desire to mock
2. provide implementation
3. call jest.mock('./moduleName') in your test

https://jestjs.io/docs/en/manual-mocks#mocking-user-modules

Solution 2:

File that exports a constant value that I want to mock:

// utils/deviceTypeUtils fileimportDeviceInfofrom'react-native-device-info';

exportconst isTablet = DeviceInfo.isTablet();

In my test file, I use this code to mock the constant isTablet:

// file: deviceTypeUtils.specconstDeviceTypeUtilsMock = jest.requireMock('../utils/deviceTypeUtils');
jest.mock('../utils/deviceTypeUtils', () => ({
  isTablet: false,
}));

describe('mock const example', () => {
  it('mock const `isTablet` to the value `true`', () => {
    DeviceTypeUtilsMock.isTablet = true;
  });

  it('mock const `isTablet` to the value `false`', () => {
    DeviceTypeUtilsMock.isTablet = false;
  });
});

Solution 3:

Follow the guidance of the doc on jest.doMock()

The config.js file:

export const env = 'test';

The myComponent.js file:

import {env} from'./config.js';

exportconstMyComponent = () => {
  if (env === 'prod') {
    return (<Text>Prod Env</Text>);
  } elseif (env === 'test'){
    return (<Text>Test Env</Text>);
  } else {
    return (<Text>Dev Env</Text>);
  }
};

The myComponent.test.js file:

describe('Test MyComponent', () => {
  test('Test in prod env', () => {
    jest.doMock('./config', () => ({env: 'prod'}));
    const {MyComponent} = require('./myComponent.js');
    const myComp = mount(<MyComponent />);
    expect(myComp.find('Text')).toHaveText('Prod Env');
  });

  test('Test in test env', () => {
    jest.doMock('./config', () => ({env: 'test'}));
    const {MyComponent} = require('./myComponent.js');
    const myComp = mount(<MyComponent />);
    expect(myComp.find('Text')).toHaveText('Test Env');
  });

  test('Test in dev env', () => {
    jest.doMock('./config', () => ({env: 'dev'}));
    const {MyComponent} = require('./myComponent.js');
    const myComp = mount(<MyComponent />);
    expect(myComp.find('Text')).toHaveText('Dev Env');
  });
});

Solution 4:

I have been able to do something similar by following the post at https://mikeborozdin.com/post/changing-jest-mocks-between-tests/

This creates a mock and then changes it for each test.

import * as constants from'./constants'

jest.mock('./constants', () => ({
  __esModule: true,
  MY_CONSTANT: 'SOME OTHER DEFAULT',
}))

it("Should do something when MY_CONSTANT === "TEST, () => {
  constant.MY_CONSTANT = 'TEST'expect(constant.MY_CONSTANT).toBe('TEST')
})

it("Should do something else when MY_CONSTANT !== "TEST, () => {
  constant.MY_CONSTANT = 'NOT A TEST'expect(constant.MY_CONSTANT).toBe('NOT A TEST')
})

If you are using typescript then the mock needs to be cast to modify the mock values:

const mockConstants = constants as { MY_CONSTANT: string }
mockConstants.MY_CONSTANT = 'TEST'

Solution 5:

I just copy constants and assign them after each test case.

const constants = require('./constants');
const original = {...constants};
afterEach(() => {
    Object.assign(constants, original);
});
test('test1', () => {
    constants.ABC = 'whatever';
});
test('test2', () => {
    // constants.ABC is set back to original
});