CODEWITHSUNDEEP
javascriptjestjs
Andrea Carraro
Andrea Carraro

Reputation: 10419

How to change mock implementation on a per single test basis?

I'd like to change the implementation of a mocked dependency on a per single test basis by extending the default mock's behaviour and reverting it back to the original implementation when the next test executes.

More briefly, this is what I'm trying to achieve:

  1. Mock dependency
  2. Change/extend mock implementation in a single test
  3. Revert back to original mock when next test executes

I'm currently using Jest v21. Here is what a typical test would look like:

// __mocks__/myModule.js

const myMockedModule = jest.genMockFromModule('../myModule');

myMockedModule.a = jest.fn(() => true);
myMockedModule.b = jest.fn(() => true);

export default myMockedModule;

// __tests__/myTest.js

import myMockedModule from '../myModule';

// Mock myModule
jest.mock('../myModule');

beforeEach(() => {
  jest.clearAllMocks();
});

describe('MyTest', () => {
  it('should test with default mock', () => {
    myMockedModule.a(); // === true
    myMockedModule.b(); // === true
  });

  it('should override myMockedModule.b mock result (and leave the other methods untouched)', () => {
    // Extend change mock
    myMockedModule.a(); // === true
    myMockedModule.b(); // === 'overridden'
    // Restore mock to original implementation with no side effects
  });

  it('should revert back to default myMockedModule mock', () => {
    myMockedModule.a(); // === true
    myMockedModule.b(); // === true
  });
});

Here is what I've tried so far:

  1. mockFn.mockImplementationOnce(fn)

    it('should override myModule.b mock result (and leave the other methods untouched)', () => {

  myMockedModule.b.mockImplementationOnce(() => 'overridden');

  myModule.a(); // === true
  myModule.b(); // === 'overridden'
});

    Pros

    • Reverts back to original implementation after first call

    Cons

    • It breaks if the test calls b multiple times
    • It doesn't revert to original implementation until b is not called (leaking out in the next test)

  2. jest.doMock(moduleName, factory, options)

    it('should override myModule.b mock result (and leave the other methods untouched)', () => {

  jest.doMock('../myModule', () => {
    return {
      a: jest.fn(() => true,
      b: jest.fn(() => 'overridden',
    }
  });

  myModule.a(); // === true
  myModule.b(); // === 'overridden'
});

    Pros

    • Explicitly re-mocks on every test

    Cons

    • Cannot define default mock implementation for all tests
    • Cannot extend default implementation forcing to re-declare each mocked method

  3. Manual mocking with setter methods (as explained here)

    // __mocks__/myModule.js

const myMockedModule = jest.genMockFromModule('../myModule');

let a = true;
let b = true;

myMockedModule.a = jest.fn(() => a);
myMockedModule.b = jest.fn(() => b);

myMockedModule.__setA = (value) => { a = value };
myMockedModule.__setB = (value) => { b = value };
myMockedModule.__reset = () => {
  a = true;
  b = true;
};
export default myMockedModule;

    // __tests__/myTest.js

it('should override myModule.b mock result (and leave the other methods untouched)', () => {
  myModule.__setB('overridden');

  myModule.a(); // === true
  myModule.b(); // === 'overridden'

  myModule.__reset();
});

    Pros

    • Full control over mocked results

    Cons

    • Lot of boilerplate code
    • Hard to maintain on long term

  4. jest.spyOn(object, methodName)

    beforeEach(() => {
  jest.clearAllMocks();
  jest.restoreAllMocks();
});

// Mock myModule
jest.mock('../myModule');

it('should override myModule.b mock result (and leave the other methods untouched)', () => {

  const spy = jest.spyOn(myMockedModule, 'b').mockImplementation(() => 'overridden');

  myMockedModule.a(); // === true
  myMockedModule.b(); // === 'overridden'

  // How to get back to original mocked value?
});

    Cons

    • I can't revert mockImplementation back to the original mocked return value, therefore affecting the next tests

Upvotes: 255

Views: 222020

Answers (8)

Ohhh
Ohhh

Reputation: 668

In Jest with Typescript using the jest.mocked api lets us avoid needing to do any casting.

import * as someModule from './somewhere';

jest.mock('./somewhere', () => ({
  functionToMock: jest.fn().mockResolvedValue('defaultValue');
}))

describe('someModule', ()=>{
  it('should return "customValue"',()=>{
    jest.mocked(someModule)
      .functionToMock
      .mockImplementation(jest.fn().mockResolvedValue('customValue');
    
    // Do something that calls `functionToMock` and use the result.
  });
});

Thanks to Ben Butterworth's answer for the inspiration.

Upvotes: 4

Ben Butterworth
Ben Butterworth

Reputation: 28532

I needed to solve the same problem in the title, but I use typescript and vitest (same API as Jest). I was stuck for a while until I found this simple API, mocked (jest docs).

// A default export
import getProducts from '../../product-service';
// Or not:
import {getCategories, deleteProduct} from '../../product-service');

vi.mock('../../product-service');

describe('ProductManager', () => {
  it('will not finish when an error is returned', async () => {
    vi.mocked(getProducts).mockRejectedValue(null);
    vi.mocked(getCategories).mockRejectedValue(null);
    // do something and assert the results
  })

  it('will not finish when an error is returned', async () => {
    vi.mocked(getProducts).mockResolvedValue(true);
    // do something and assert the results
  })
}

Upvotes: 4

manna
manna

Reputation: 198

It's a very cool way I've discovered on this blog https://mikeborozdin.com/post/changing-jest-mocks-between-tests/

import { sayHello } from './say-hello';
import * as config from './config';

jest.mock('./config', () => ({
  __esModule: true,
  CAPITALIZE: null
}));

describe('say-hello', () => {
  test('Capitalizes name if config requires that', () => {
    config.CAPITALIZE = true;

    expect(sayHello('john')).toBe('Hi, John');
  });

  test('does not capitalize name if config does not require that', () => {
    config.CAPITALIZE = false;

    expect(sayHello('john')).toBe('Hi, john');
  });
});

Upvotes: 1

Foxlab
Foxlab

Reputation: 932

I did not manage to define the mock inside the test itself so I discover that I could mock several results for the same service mock like this :

jest.mock("@/services/ApiService", () => {
    return {
        apiService: {
            get: jest.fn()
                    .mockResolvedValueOnce({response: {value:"Value", label:"Test"}})
                    .mockResolvedValueOnce(null),
        }
    };
});

I hope it'll help someone :)

Upvotes: 1

user1095118
user1095118

Reputation: 4496

A nice pattern for writing tests is to create a setup factory function that returns the data you need for testing the current module.

Below is some sample code following your second example although allows the provision of default and override values in a reusable way.


const spyReturns = returnValue => jest.fn(() => returnValue);

describe("scenario", () => {
  beforeEach(() => {
    jest.resetModules();
  });

  const setup = (mockOverrides) => {
    const mockedFunctions =  {
      a: spyReturns(true),
      b: spyReturns(true),
      ...mockOverrides
    }
    jest.doMock('../myModule', () => mockedFunctions)
    return {
      mockedModule: require('../myModule')
    }
  }

  it("should return true for module a", () => {
    const { mockedModule } = setup();
    expect(mockedModule.a()).toEqual(true)
  });

  it("should return override for module a", () => {
    const EXPECTED_VALUE = "override"
    const { mockedModule } = setup({ a: spyReturns(EXPECTED_VALUE)});
    expect(mockedModule.a()).toEqual(EXPECTED_VALUE)
  });
});

It's important to say that you must reset modules that have been cached using jest.resetModules(). This can be done in beforeEach or a similar teardown function.

See jest object documentation for more info: https://jestjs.io/docs/jest-object.

Upvotes: 100

A Jar of Clay
A Jar of Clay

Reputation: 6298

Use mockFn.mockImplementation(fn).

import { funcToMock } from './somewhere';
jest.mock('./somewhere');

beforeEach(() => {
  funcToMock.mockImplementation(() => { /* default implementation */ });
  // (funcToMock as jest.Mock)... in TS
});

test('case that needs a different implementation of funcToMock', () => {
  funcToMock.mockImplementation(() => { /* implementation specific to this test */ });
  // (funcToMock as jest.Mock)... in TS

  // ...
});

Upvotes: 145

Denis P
Denis P

Reputation: 796

When mocking a single method (when it's required to leave the rest of a class/module implementation intact) I discovered the following approach to be helpful to reset any implementation tweaks from individual tests.

I found this approach to be the concisest one, with no need to jest.mock something at the beginning of the file etc. You need just the code you see below to mock MyClass.methodName. Another advantage is that by default spyOn keeps the original method implementation but also saves all the stats (# of calls, arguments, results etc.) to test against, and keeping the default implementation is a must in some cases. So you have the flexibility to keep the default implementation or to change it with a simple addition of .mockImplementation as mentioned in the code below.

The code is in Typescript with comments highlighting the difference for JS (the difference is in one line, to be precise). Tested with Jest 26.6.

describe('test set', () => {
    let mockedFn: jest.SpyInstance<void>; // void is the return value of the mocked function, change as necessary
    // For plain JS use just: let mockedFn;

    beforeEach(() => {
        mockedFn = jest.spyOn(MyClass.prototype, 'methodName');
        // Use the following instead if you need not to just spy but also to replace the default method implementation:
        // mockedFn = jest.spyOn(MyClass.prototype, 'methodName').mockImplementation(() => {/*custom implementation*/});
    });

    afterEach(() => {
        // Reset to the original method implementation (non-mocked) and clear all the mock data
        mockedFn.mockRestore();
    });

    it('does first thing', () => {
        /* Test with the default mock implementation */
    });

    it('does second thing', () => {
        mockedFn.mockImplementation(() => {/*custom implementation just for this test*/});
        /* Test utilising this custom mock implementation. It is reset after the test. */
    });

    it('does third thing', () => {
        /* Another test with the default mock implementation */
    });
});

Upvotes: 6

Thomas Hagstr&#246;m
Thomas Hagstr&#246;m

Reputation: 4528

Little late to the party, but if someone else is having issues with this.

We use TypeScript, ES6 and babel for react-native development.

We usually mock external NPM modules in the root __mocks__ directory.

I wanted to override a specific function of a module in the Auth class of aws-amplify for a specific test.

    import { Auth } from 'aws-amplify';
    import GetJwtToken from './GetJwtToken';
    ...
    it('When idToken should return "123"', async () => {
      const spy = jest.spyOn(Auth, 'currentSession').mockImplementation(() => ({
        getIdToken: () => ({
          getJwtToken: () => '123',
        }),
      }));

      const result = await GetJwtToken();
      expect(result).toBe('123');
      spy.mockRestore();
    });

Gist: https://gist.github.com/thomashagstrom/e5bffe6c3e3acec592201b6892226af2

Tutorial: https://medium.com/p/b4ac52a005d#19c5

Upvotes: 69

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