How to change the behaviour of a mocked import?

Question

I am quite confused with mocking in Jest an how to unit test the implementations. The thing is i want to mock different expected behaviours.

Is there any way to achieve this? as imports can be only on the top of the file and to be able to mock something it must be declared before the import. I have also tried to pass a local function so I could overwrite the behaviour but jest complains you are not allowed to pass anything local.

jest.mock('the-package-to-mock', () => ({
  methodToMock: jest.fn(() => console.log('Hello'))
}));

import * as theThingToTest from '../../../app/actions/toTest'
import * as types from '../../../app/actions/types'

it('test1', () => {
  expect(theThingToTest.someAction().type).toBe(types.SOME_TYPE)
})

it('test2', () => {
  //the-package-to-mock.methodToMock should behave like something else
  expect(theThingToTest.someAction().type).toBe(types.SOME_TYPE)
})

internally as you can imagine theThingToTest.someAction() uses the-package-to-mock.methodToMock

Solution 1

You can mock with a spy and import the mocked module. In your test you set how the mock should behave using mockImplementation:

jest.mock('the-package-to-mock', () => ({
  methodToMock: jest.fn()
}));
import { methodToMock } from 'the-package-to-mock'

it('test1', () => {
  methodToMock.mockImplementation(() => 'someValue')
})

it('test2', () => {
  methodToMock.mockImplementation(() => 'anotherValue')
})

Solution 2

I use the following pattern:

'use strict'

const packageToMock = require('../path')

jest.mock('../path')
jest.mock('../../../../../../lib/dmp.db')

beforeEach(() => {
  packageToMock.methodToMock.mockReset()
})

describe('test suite', () => {
  test('test1', () => {
    packageToMock.methodToMock.mockResolvedValue('some value')
    expect(theThingToTest.someAction().type).toBe(types.SOME_TYPE)

  })
  test('test2', () => {
    packageToMock.methodToMock.mockResolvedValue('another value')
    expect(theThingToTest.someAction().type).toBe(types.OTHER_TYPE)
  })
})

Explanation:

You mock the class you are trying to use on test suite level, make sure the mock is reset before each test and for every test you use mockResolveValue to describe what will be return when mock is returned

Solution 3

Another way is to use jest.doMock(moduleName, factory, options).

E.g.

the-package-to-mock.ts:

export function methodToMock() {
  return 'real type';
}

toTest.ts:

import { methodToMock } from './the-package-to-mock';

export function someAction() {
  return {
    type: methodToMock(),
  };
}

toTest.spec.ts:

describe('45006254', () => {
  beforeEach(() => {
    jest.resetModules();
  });
  it('test1', () => {
    jest.doMock('./the-package-to-mock', () => ({
      methodToMock: jest.fn(() => 'type A'),
    }));
    const theThingToTest = require('./toTest');
    expect(theThingToTest.someAction().type).toBe('type A');
  });

  it('test2', () => {
    jest.doMock('./the-package-to-mock', () => ({
      methodToMock: jest.fn(() => 'type B'),
    }));
    const theThingToTest = require('./toTest');
    expect(theThingToTest.someAction().type).toBe('type B');
  });
});

unit test result:

 PASS  examples/45006254/toTest.spec.ts
  45006254
    ✓ test1 (2016 ms)
    ✓ test2 (1 ms)

-----------|---------|----------|---------|---------|-------------------
File       | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
-----------|---------|----------|---------|---------|-------------------
All files  |     100 |      100 |     100 |     100 |                   
 toTest.ts |     100 |      100 |     100 |     100 |                   
-----------|---------|----------|---------|---------|-------------------
Test Suites: 1 passed, 1 total
Tests:       2 passed, 2 total
Snapshots:   0 total
Time:        3.443 s

source code: https://github.com/mrdulin/jest-v26-codelab/tree/main/examples/45006254

Solution 4

spyOn worked best for us. See previous answer:

https://stackoverflow.com/a/54361996/1708297

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