Mocking a Class Dependency in Jest

Question

Mocking a Class Dependency in Jest

I have 4 classes available, and I utilize 3 of them to create an instance of another class. This is how it's done:

 const repo = new PaymentMessageRepository();
 const gorepo = new GoMessageRepository();
 const sqsm = new PaymentMessageQueueManager(process.env.SOME_ENV, process.env.ANOTHER_ENV);
 const systemUnderTest = PaymentProcessorService(repo,gorepo,sqsm);

The test scenario I want to explore involves testing if, in the event that an Error is triggered by a method such as repo.methodOne(), systemUnderTest should return a specific response. To accomplish this, I need to mock the PaymentMessageRepository class. How can I go about doing this?

All my classes are associated with Joi schema. I encountered challenges in this aspect but managed to resolve it using: jest.mock('joi').

In addition, is it possible to apply mocking for only one particular test? Whenever I attempt to implement mocking, it seems like all my tests are affected by the mock. Should I separate them somehow?

Here is what I have implemented thus far:

import { PaymentMessageQueueManager } from '../Queues/PaymentMessageQueueManager';
import { PaymentMessageRepository, PaymentMessageRepository_Schema } from '../Repository/PaymentMessageRepository'
import { GoMessageRepository } from '../Services/GoMessageRepository';
import {PaymentProcessorService} from '../Services/PaymentProcessorService'

jest.mock('joi');

jest.mock('../Repository/PaymentMessageRepository', () => ({
    PaymentMessageRepository : jest.fn().mockImplementation( () => ({
        putItem: jest.fn(),
    })),
    PaymentMessageRepository_Schema: {
        required : jest.fn(),
    },
})); 

jest.mock('../Services/GoMessageRepository', () => ({
    GoMessageRepository: jest.fn().mockImplementation( () => ({
        putItem: jest.fn(),
    })),
    GoMessageRepository_Schema: {
        required: jest.fn(),
    },    
}));

jest.mock('../Queues/PaymentMessageQueueManager', () => ({
    PaymentMessageQueueManager: jest.fn().mockImplementation( () => ({
        sendToQ: jest.fn(),
    })),
    PaymentMessageQueueManager_Schema: {
            required: jest.fn(),
        }
}));


describe('PaymentProcessorService unit tests', () => {
    const env = process.env
    beforeEach(() => {
        jest.resetModules()
        process.env = { ...env }
    });

    test('When a dependency is null, processor service should not be created', () => {
        process.env.PAYMENT_MESSAGE_TABLE_NAME = 'MOCK_TABLE_NAME';
        
        const repo = new PaymentMessageRepository();
        const gorepo = new GoMessageRepository();
        const sqsm = new PaymentMessageQueueManager(process.env.SOME_ENV, process.env.ANOTHER_ENV);
       
        expect ( () => new PaymentProcessorService(repo, undefined, sqsm)).toThrow(Error);
    })
    afterEach(() => {
        process.env = env
    });
    
    test('When mocked dependencies, processor should return no error', () => {
        const repo_mocked = new GoMessageRepository(); // <--- How to mock this so that repo_mocked.some_method() either throws or returns 0?

    })
})

Currently, the issue I am facing is that the first test fails because when mocking 'Joi', it overrides the .assert() method and alters its behavior, causing it to return a different result. As a result, even if there is a missing dependency, the overridden 'Joi' returns okay and causes the assertion to fail. If I remove all mocks, the test functions as intended.

Am I approaching the mocking process incorrectly? My main goal is to be able to mock a specific method to return a certain response or trigger an exception, and then assert the outcome accordingly. Achieving this seamlessly in C# with XUnit is straightforward, but I find it challenging in TS/JS with Jest (my source files are in TypeScript while my test files are in JavaScript).

javascript typescript dependency-injection jestjs

Answer 1

Answer №1

After thorough investigation and exploration, I have successfully figured out how to mock methods of the injected classes. Although I cannot guarantee that this is the most optimal or elegant solution, it is what I have devised:

test('When a dependency is mocked, and message is ok, proccesor service should be created', async () =>{
        process.env.PAYMENT_MESSAGE_TABLE_NAME = 'MOCK_TABLE_NAME';
        process.env.GO_MESSAGE_TABLE_NAME = 'GO_MSG_TABLE_NAME';
        const repo = new PaymentMessageRepository();
        const sqsm = new PaymentMessageQueueManager(process.env.JAIMITO_SQS_URL_DEPOSIT, process.env.JAIMITO_SQS_URL_WITHDRA);
        const gorepo = new GoMessageRepository();

        const goRepoPutItemMock = jest
        .spyOn(GoMessageRepository.prototype, 'putItem')
        .mockImplementation(() => {
            console.log('Put Item in Go Table');
        }); // comment this line if just want to "spy"

        const repoPutItemMock = jest
        .spyOn(PaymentMessageRepository.prototype, 'putItem')
        .mockImplementation( () => {
            console.log('Put Item in Payment Table');
        });

        const queueManagerPutMessageMock = jest
        .spyOn(PaymentMessageQueueManager.prototype, 'sendToQ')
        .mockImplementation( () => {
            console.log('Sending Payment to Q');
        });

        const message = "<TransferNotification>MESSAGE_HERE</TransferNotification>";
      
        let systemUnderTest = new PaymentProcessorService(repo, gorepo, sqsm);

        let resp = await systemUnderTest.processMessages(  parsePaymentsFromEvent(constructSQSEvent(message)) );
    
        expect(repoPutItemMock).toHaveBeenCalled();
        expect(goRepoPutItemMock).not.toHaveBeenCalled();
        expect(queueManagerPutMessageMock).toHaveBeenCalled();
        expect(resp.batchItemFailures.length == 0).toBe(true);
    });

This approach allows me to mock goRepoPutItemMock, for instance, to throw specific errors or return customized values that can be handled within my systemUnderTest implementation.

Answer 2

After thorough investigation and exploration, I have successfully figured out how to mock methods of the injected classes. Although I cannot guarantee that this is the most optimal or elegant solution, it is what I have devised:

test('When a dependency is mocked, and message is ok, proccesor service should be created', async () =>{
        process.env.PAYMENT_MESSAGE_TABLE_NAME = 'MOCK_TABLE_NAME';
        process.env.GO_MESSAGE_TABLE_NAME = 'GO_MSG_TABLE_NAME';
        const repo = new PaymentMessageRepository();
        const sqsm = new PaymentMessageQueueManager(process.env.JAIMITO_SQS_URL_DEPOSIT, process.env.JAIMITO_SQS_URL_WITHDRA);
        const gorepo = new GoMessageRepository();

        const goRepoPutItemMock = jest
        .spyOn(GoMessageRepository.prototype, 'putItem')
        .mockImplementation(() => {
            console.log('Put Item in Go Table');
        }); // comment this line if just want to "spy"

        const repoPutItemMock = jest
        .spyOn(PaymentMessageRepository.prototype, 'putItem')
        .mockImplementation( () => {
            console.log('Put Item in Payment Table');
        });

        const queueManagerPutMessageMock = jest
        .spyOn(PaymentMessageQueueManager.prototype, 'sendToQ')
        .mockImplementation( () => {
            console.log('Sending Payment to Q');
        });

        const message = "<TransferNotification>MESSAGE_HERE</TransferNotification>";
      
        let systemUnderTest = new PaymentProcessorService(repo, gorepo, sqsm);

        let resp = await systemUnderTest.processMessages(  parsePaymentsFromEvent(constructSQSEvent(message)) );
    
        expect(repoPutItemMock).toHaveBeenCalled();
        expect(goRepoPutItemMock).not.toHaveBeenCalled();
        expect(queueManagerPutMessageMock).toHaveBeenCalled();
        expect(resp.batchItemFailures.length == 0).toBe(true);
    });

This approach allows me to mock goRepoPutItemMock, for instance, to throw specific errors or return customized values that can be handled within my systemUnderTest implementation.

Mocking a Class Dependency in Jest

Answer №1

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