Jest: Mastering the art of testing a MongoDB-powered JavaScript service

Question

Jest: Mastering the art of testing a MongoDB-powered JavaScript service

As a newcomer to Jest, I am navigating my way through using Dependency Injection with a UserService.

public async getAll() {
  const userRecords = await this.userModel.find().select('name').catch((e) => {
    throw new HttpException(500, 'Error while fetching users.', e)
  });
  return <[IUser]>userRecords;
}

I am looking to test this feature and have identified three possible tests:

Verify the JSON response after calling the route
Check the accuracy of DB content retrieval
Test the functionality of the getAll function independently

While tests 1 and 2 are straightforward in terms of covering request and DB aspects, how can I effectively isolate and test only the getAll function?

In attempting to do so, I experimented with the following code snippet:

const userModel = {
  find: (user) => {
    return [
      { id: 'user1' },
      { id: 'user2' }
    ]
  },
};
const userService = new UserService(userModel);
const userRecords = await userService.getAll();

expect(argumentRecord).toBeDefined();

However, the test failed due to the error stating that select is undefined.

Should I consider mocking select() as well? Would restructuring my code help resolve this issue?

javascript mongodb testing jestjs

Answer 1

Answer №1

If I were to create a test scenario, I would utilize the jest.fn(implementation) method to mock functions and ensure that function calls meet expectations.

const userQuery = {
    select: jest.fn(() => Promise.resolve([]))
};

const userModel = {
    find: jest.fn(() => userQuery)
};

const userService = new UserService(userModel);
const userRecords = await userService.getAll();

expect(userRecords).toEqual([]);
expect(userModel.find).toHaveBeenCalled();
expect(userQuery.select).toHaveBeenCalledWith('name');

Although it may seem excessive, enforcing expectations on function calls explicitly confirms that the mock is being utilized by getAll.

In addition, I would organize the tests in such a manner that allows for testing different code paths without fully re-implementing the entire mock.

describe('getAll()', () => {

    let userQuery, userModel, userService;
    beforeEach(() => {
        userQuery = {
            select: jest.fn(() => Promise.resolve([]))
        };

        userModel = {
            find: jest.fn(() => userQuery)
        };

        userService = new UserService(userModel);
    });

    afterEach(() => {
        expect(userModel.find).toHaveBeenCalled();
        expect(userQuery.select).toHaveBeenCalledWith('name');
    });

    it('should retrieve user names', async () => {
        const users = [{
            name: 'john'
        }, {
            name: 'jane'
        }];
        userQuery.select.mockImplementation(() => Promise.resolve(users));

        await expect(userService.getAll()).resolves.toBe(users);
    });

    it('should manage errors', async () => {
        const error = new Error('Fake model error');
        userQuery.select.mockImplementation(() => Promise.reject(error));

        await expect(userService.getAll()).rejects.toMatch({
            status: 500,
            message: 'Error while fetching users.',
            cause: error
        });
    });
});

This code has not been tested, so there might be issues with its functionality, but hopefully, it conveys the concept effectively.

While this is slightly off-topic from your query, I recommend avoiding mixing async/await with traditional promise handling.

public async getAll() {
    try {
        return <[IUser]> await this.userModel.find().select('name');
    } catch (e) {
        throw new HttpException(500, 'Error while fetching users.', e)
    }
}

Answer 2

If I were to create a test scenario, I would utilize the jest.fn(implementation) method to mock functions and ensure that function calls meet expectations.

const userQuery = {
    select: jest.fn(() => Promise.resolve([]))
};

const userModel = {
    find: jest.fn(() => userQuery)
};

const userService = new UserService(userModel);
const userRecords = await userService.getAll();

expect(userRecords).toEqual([]);
expect(userModel.find).toHaveBeenCalled();
expect(userQuery.select).toHaveBeenCalledWith('name');

Although it may seem excessive, enforcing expectations on function calls explicitly confirms that the mock is being utilized by getAll.

In addition, I would organize the tests in such a manner that allows for testing different code paths without fully re-implementing the entire mock.

describe('getAll()', () => {

    let userQuery, userModel, userService;
    beforeEach(() => {
        userQuery = {
            select: jest.fn(() => Promise.resolve([]))
        };

        userModel = {
            find: jest.fn(() => userQuery)
        };

        userService = new UserService(userModel);
    });

    afterEach(() => {
        expect(userModel.find).toHaveBeenCalled();
        expect(userQuery.select).toHaveBeenCalledWith('name');
    });

    it('should retrieve user names', async () => {
        const users = [{
            name: 'john'
        }, {
            name: 'jane'
        }];
        userQuery.select.mockImplementation(() => Promise.resolve(users));

        await expect(userService.getAll()).resolves.toBe(users);
    });

    it('should manage errors', async () => {
        const error = new Error('Fake model error');
        userQuery.select.mockImplementation(() => Promise.reject(error));

        await expect(userService.getAll()).rejects.toMatch({
            status: 500,
            message: 'Error while fetching users.',
            cause: error
        });
    });
});

This code has not been tested, so there might be issues with its functionality, but hopefully, it conveys the concept effectively.

While this is slightly off-topic from your query, I recommend avoiding mixing async/await with traditional promise handling.

public async getAll() {
    try {
        return <[IUser]> await this.userModel.find().select('name');
    } catch (e) {
        throw new HttpException(500, 'Error while fetching users.', e)
    }
}

Answer 3

Answer №2

A great practice is to always mock the select method when testing your functions. It's recommended to also test every part of the function to ensure proper execution. One way to achieve this is by following these steps:

class SomeClass {
    public async getAll() {
      const userRecords = await this.userModel.find().select('name').catch(this.errorHandler);

      return <[IUser]>userRecords;
    }

    public errorHandler(e) {
        throw new HttpException(500, 'Error while fetching users.', e);
    }
}

// In this example, let's assume our output should match a random value
const whatever = Math.random();

// Mock the catch method using jest.fn()
const fakeCatch = jest.fn(() => whatever);

// Mock the select method
const fakeSelect = jest.fn(() => {
    return {
        catch: fakeCatch
    }
});

// Mock the find method
const fakeFind = jest.fn(() => {
    return {
        select: fakeSelect
    };
});

// Create a fake user model object
const fakeUserModel = {
    find: fakeFind,
}

// Instantiate the UserService with the fake user model
const userService = new UserService(fakeUserModel);
const userRecords = await userService.getAll();

// Ensure that the result matches the expected value
expect(userRecords).toEqual(whatever);

// Test if the find method is executed
expect(fakeFind).toHaveBeenCalledTimes(1);

// Check if select is called with the 'name' parameter
expect(fakeSelect).toHaveBeenCalledTimes(1);
expect(fakeSelect).toHaveBeenCalledWith('name');

// Verify that catch calls the errorHandler function
expect(fakeCatch).toHaveBeenCalledWith(userService.errorHandler);

Answer 4

A great practice is to always mock the select method when testing your functions. It's recommended to also test every part of the function to ensure proper execution. One way to achieve this is by following these steps:

class SomeClass {
    public async getAll() {
      const userRecords = await this.userModel.find().select('name').catch(this.errorHandler);

      return <[IUser]>userRecords;
    }

    public errorHandler(e) {
        throw new HttpException(500, 'Error while fetching users.', e);
    }
}

// In this example, let's assume our output should match a random value
const whatever = Math.random();

// Mock the catch method using jest.fn()
const fakeCatch = jest.fn(() => whatever);

// Mock the select method
const fakeSelect = jest.fn(() => {
    return {
        catch: fakeCatch
    }
});

// Mock the find method
const fakeFind = jest.fn(() => {
    return {
        select: fakeSelect
    };
});

// Create a fake user model object
const fakeUserModel = {
    find: fakeFind,
}

// Instantiate the UserService with the fake user model
const userService = new UserService(fakeUserModel);
const userRecords = await userService.getAll();

// Ensure that the result matches the expected value
expect(userRecords).toEqual(whatever);

// Test if the find method is executed
expect(fakeFind).toHaveBeenCalledTimes(1);

// Check if select is called with the 'name' parameter
expect(fakeSelect).toHaveBeenCalledTimes(1);
expect(fakeSelect).toHaveBeenCalledWith('name');

// Verify that catch calls the errorHandler function
expect(fakeCatch).toHaveBeenCalledWith(userService.errorHandler);

Jest: Mastering the art of testing a MongoDB-powered JavaScript service

Answer №1

Answer №2

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