ts-jest-mocker

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

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Table of Contents

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

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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,
        });
    });
});

Configuring ts-jest-mocker

In order to be more flexible and allow covering more complex testing scenarios, ts-jest-mocker now allows specifying mocking configuration. Configuration can be global, to cover all test cases (mocks) at once, or local, to cover and change behavior only in case of specific tests cases (mocks);

Local configuration

mock(MyClass, {
    // config values
});

Global config

In order for configuration to be properly passed to test environments, you can define ts-jest-mocker.setup.ts configuration file and specify it in setupFiles array in Jest configuration.

ts-jest-mocker.setup.ts

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

TsJestMocker.setConfig({
    failIfMockNotProvided: false,
});

jest.config.ts

import type { Config } from 'jest';

const config: Config = {
    verbose: true,
    setupFiles: ['./ts-jest-mocker.setup.ts'],
};

export default config;

Configuration options

excludeMethodNames

Default value (classes): [] (empty array - no excluding) Default value (interfaces): ['then', Symbol.iterator] (then method - to cover "thenable" in promises)

Description: Methods specified on the exclusion list will be automatically excluded from mocking.

Examples:

// mock with local config
const mock = mock(MyClass, {
    excludeMethodNames: ['schedule', 'then'],
});

// global config
TsJestMocker.setConfig({
    excludeMethodNames: ['schedule', 'then'],
});

includeMethodNames

Default value (classes): [] (empty array - no additional including) Default value (interfaces): [] (empty array - no additional including)

Description: Methods specified on the inclusion list will be included and covered with mocking, even if the same method exists on the exclusion list (by default or added by user).

Examples:

// mock with local config
const mock = mock({
    includeMethodNames: ['then'], // "then" will be mocked even if it's by default (for interfaces) excluded
});

// global config
TsJestMocker.setConfig({
    includeMethodNames: ['then'],
});

failIfMockNotProvided

Default value (classes): true Default value (interfaces): true

Description: By default, calling method on mock will throw an error if return value was not specified by user. This behavior can now be changed by setting failIfMockNotProvided to false.

Examples:

// mock with local config
const mock = mock({
    failIfMockNotProvided: false,
});

// global config
TsJestMocker.setConfig({
    failIfMockNotProvided: false,
});

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 👆 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);

Notes

ts-jest-mocker with RxJS

When using ts-jest-mocker + RxJS for mocking interfaces, there could be issues like Method schedule is not mocked as reported under this issue.

Issue appears when you pass mock to an of() operator, as under the hood operator internally checks if, so called, Scheduler was passed as a parameter.

Example:

import { firstValueFrom, of } from 'rxjs';

const testMock = mock<YourInterface>();

const observable = of(testMock);

const value = await firstValueFrom(observable);

The above code will throw Method 'schedule' is not mocked error.

This can be simply solved by excluding schedule methods from mocking.

Example:

import { firstValueFrom, of } from 'rxjs';

const testMock = mock<YourInterface>({
    excludeMethodNames: ['schedule'],
});

const observable = of(testMock);

const value = await firstValueFrom(observable);