Powerful, lightweight, TypeScript-friendly and 100% Jest API compatible library which simplifies classes and interfaces mocking.

Table of Contents

• Getting started
  • Mocking classes
  • Mocking interfaces
• More advanced example
• Jest API compatibility
• Why to use ts-jest-mocker

Getting started

Install ts-jest-mocker using npm:

npm install --save-dev ts-jest-mocker

Mocking classes

import { mock } from 'ts-jest-mocker';

const serviceMock = mock(YourService); // automatically mocks all methods

serviceMock.yourMethod.mockReturnValue('Test');

Mocking interfaces

import { mock } from 'ts-jest-mocker';

const interfaceMock = mock<YourInterface>(); // automatically mocks all interface methods

interfaceMock.yourMethod.mockReturnValue('Test');

Using Mock type

import { Mock, mock } from 'ts-jest-mocker';

let serviceMock: Mock<YourService>;

serviceMock = mock(YourService);

serviceMock.yourMethod.mockReturnValue('Test');

More advanced example

As an example let's create service UserService used for managing your application users. This service has dependency to UsersRepository which is used to load users from your database.

users-repository.ts file:

export interface User {
    name: string;
    age: number;
}

export class UsersRepository {
    getUsers(): Array<User> {
        return [
            {
                name: 'User1',
                age: 30,
            },
            {
                name: 'User2',
                age: 40,
            },
        ];
    }
}

users-service.ts file:

import { User, UsersRepository } from './users-repository';

export class UsersService {
    constructor(private readonly usersRepository: UsersRepository) {}

    getUsers(): Array<User> {
        return this.usersRepository.getUsers();
    }
}

Now let's create file users-service.test.ts and write some unit tests using ts-jest-mocker. We will mock UsersRepository using mock() function. All repository methods will be mocked automatically using jest.fn() internally and all type-checking will work out-of-the-box.

users-service.test.ts file:

import { mock } from 'ts-jest-mocker';
import { UsersRepository } from './users-repository';
import { UsersService } from './users-service';

describe('UsersService', () => {
    it('should return all users', () => {
        // GIVEN
        const repositoryMock = mock(UsersRepository);
        repositoryMock.getUsers.mockReturnValue([
            {
                name: 'Mocked user 1',
                age: 40,
            },
        ]);
        const service = new UsersService(repositoryMock);

        // WHEN
        const users = service.getUsers();

        // THEN
        expect(users).toBeDefined();
        expect(users.length).toBe(1);
        expect(users[0]).toEqual({
            name: 'Mocked user 1',
            age: 40,
        });
    });
});

Jest API compatibility

ts-jest-mocker is not an alternative to Jest and does not provide an alternative API. It is utility which main purpose is to add additional capability on top of Jest to simplify writing mocks and keep all the benefits of data types.

While using ts-jest-mocker you don't need to use any custom calls to reset mock or anything. You call for example jest.resetAllMocks() as you usually do.

Why to use ts-jest-mocker?

Do you often catch yourself writing mocks manually using jest.fn()? Do you maybe omit defining jest.Mock type generics and implicitly end up using any, or what's worse, you need to explicitly convert your mocks to any to be able to use them with classes under test? It is hard to do refactoring and keeping unit tests up-to-date?

The above :point_up_2: sounds familiar to you? Stop doing that! ts-jest-mocker will help you do that!

❌ Don'ts

const mockUserRepository = {
    yourMethod1: jest.fn(),
    yourMethod2: jest.fn(),
    yourMethod3: jest.fn(),
    yourMethod4: jest.fn(),
    yourMethod5: jest.fn(),
    yourMethod6: jest.fn(),

    // ...

    // ❌️ you have to mock all the methods
    // so mock and UsersRepository are compatible?
    yourMethod20: jest.fn(),
};

const userService = new UserService(mockUserRepository);

const mockUserRepository = {
    yourMethod1: jest.fn(),
    yourMethod2: jest.fn(),
} as any;
// ❌ you mock only what you need and then cast explicitly to any
// and loose benefits from compilation phase?

const userService = new UserService(mockUserRepository);

const mockUserRepository = {
    yourMethod1: jest.fn(),
};

// ❌️ You often skip specifying mock types like jest.fn<User, [User]>() and
// then need to check over and over again in the code what actually
// mocked methods should return?
mockUserRepository.yourMethod1.mockReturnedValue({
    name: 'User1',
    age: 20,
});

const userService = new UserService(mockUserRepository as any);

✅ Do's

// ✅ simply use mock() function and ts-jest-mocker will
// provide mocks for all the methods for you
const mockUserRepository = mock(UsersRepository);

mockUserRepository.yourMethod1.mockReturnedValue({
    name: 'User1',
    age: 20,
}); // ✅ return type is automatically checked while compilation

mockUserRepository.yourMethod1.mockReturnedValue({
    name: 'User1',
}); // ❗ [compilation error] - you will catch incorrect types

mockUserRepository.yourMethod1.mockReturnedValue({
    age: 20,
}); // ❗ [compilation error] - you will catch incorrect types

// ❗ [compilation error] - you will catch incorrect types
mockUserRepository.yourMethod1.mockReturnedValue(true);

const userService = new UserService(mockUserRepository);