docs: add testing for wiki

wiki/testing.md

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+# Testing
+
+Testing is an essential aspect of software development, ensuring that your code behaves as expected and remains
+maintainable over time. This document guides you through two approaches to testing with InversifyJS, using a consistent
+example to demonstrate and evaluate the principles.
+
+Consider the following classes for our examples:
+
+```typescript
+import { injectable } from 'inversify';
+
+@injectable()
+export class UserDAL {
+  getUsers(): User[] | [] { return [] }
+}
+
+@injectable()
+export class UserApiService {
+  fetchUsers(): User[] | [] { return [] }
+}
+
+@injectable()
+export class UserService {
+  constructor(private userDal: UserDAL, private userApi: UserApiService) {}
+
+  getAllUsers(): User[] {
+    const fromDb = this.userDal.getUsers();
+    const fromApi = this.userApi.fetchUsers();
+
+    return [...fromDb, ...fromApi];
+  }
+}
+```
+
+## Integration Tests
+
+Integration tests ensure that different parts of your application work together harmoniously. These tests are 
+crucial for verifying the interactions between your classes and their dependencies, much like how components 
+interact in a real-world scenario.
+
+### Real Instances and Mocks
+
+In integration testing, the decision to use real instances or mocks depends on the test's focus. For instance, when
+testing UserService that interacts with a database and an API, you might want to use a mock database or a mock API
+client in some scenarios, while in others, you might prefer the real implementations to ensure full interaction.
+
+In the following example, we decide to mock the API but let the database run:
+```typescript
+import { Container } from 'inversify';
+import { UserService } from './user.service';
+import { UserDAL } from './user.dal';
+import { UserApiService } from './user.api.service';
+
+describe('Fetching Users Integration Test', () => {
+  let userService: UserService;
+  let mockUserApiService: jest.Mocked<UserApiService>;
+
+  beforeAll(() => {
+    const container = new Container();
+
+    container.bind(UserService).toSelf();
+    container.bind(UserDAL).toSelf();
+
+    mockUserApiService = { fetchUsers: jest.fn().mockReturnValue([{ id: 1, name: 'Mock User from API' }]) };
+    container.bind(UserApiService).toConstantValue(mockUserApiService);
+
+    userService = container.get(UserService);
+  });
+
+  test('should fetch users from both DAL and API', async () => {
+    const users = userService.getAllUsers();
+    expect(users).toHaveLength(3); // Assuming 2 users from DAL and 1 mock user from API
+    expect(mockUserApiService.fetchUsers).toHaveBeenCalled();
+  });
+});
+```
+
+In this example, we use a real `UserDAL` instance and a mock `UserApiService`. The test verifies that `UserService`
+correctly integrates the data from both the database and the API.
+
+## Unit Tests
+
+Unit testing is about isolating a single unit of work and validating its correctness. This isolation is crucial for
+pinpointing issues and ensuring that each component functions and behaves as expected, independently.
+Unlike integration tests, unit tests mock all class dependencies to validate the code behavior. Here, we mock both
+`UserDAL` and `UserApiService` for testing `UserService`.
+
+```typescript
+import { Container } from 'inversify';
+import { UserService } from './user.service';
+import { UserDAL } from './user.dal';
+import { UserApiService } from './user.api.service';
+
+describe('User Service Unit Spec', () => {
+  let userService: UserService;
+  let mockUserDal: jest.Mocked<UserDal>;
+  let mockUserApiService: jest.Mocked<UserApiService>;
+
+  beforeAll(() => {
+    const container = new Container();
+
+    container.bind(UserService).toSelf();
+
+    mockUserDal = { getUsers: jest.fn().mockReturnValue([{ id: 1, name: 'Mock User from DB' }]) };
+    mockUserApiService = { fetchUsers: jest.fn().mockReturnValue([{ id: 2, name: 'Mock User from API' }]) };
+
+    // Bind the mocks to the container
+    container.bind<UserDAL>(UserDAL).toConstantValue(mockUserDal as UserDAL);
+    container.bind<UserApiService>(UserApiService).toConstantValue(mockUserApiService as UserApiService);
+
+    // Get the instance of UserService
+    userService = container.get(UserService);
+  });
+
+  test('getting all users should return combined users from DAL and API', async () => {
+    const users = userService.getAllUsers();
+    expect(users).toHaveLength(2);
+    expect(mockUserApiService.fetchUsers).toHaveBeenCalled();
+  });
+});
+```
+
+## Unit Tests using Automock
+
+Automock is a stand-alone unit testing library. It improves the unit testing process by offering a virtual,
+isolated environment and automated mock generation, allowing for the creation of efficient test suites and
+an overall enhanced testing experience.
+
+#### Setting Up Automock with InversifyJS
+
+Integrating Automock with InversifyJS involves a few straightforward steps. You'll need to install
+`@automock/jest` or `@automock/sinon`, along with the InversifyJS adapter `@automock/adapters.inversify`.
+
+For Jest:
+
+```bash
+$ npm i -D @automock/jest @automock/adapters.inversify
+```
+
+For Sinon:
+
+```bash
+$ npm i -D @automock/sinon @automock/adapters.inversify
+```
+
+### A Practical Example
+
+Here's an example of using Automock in a unit test, using the same `UserService` as before:
+
+```typescript
+import { TestBed } from '@automock/jest';
+import { UserService } from './user.service';
+import { UserDAL } from './user.dal';
+import { UserApiService } from './user.api.service';
+
+describe('User Service Unit Spec', () => {
+  let userService: UserService;
+  let mockUserDal: jest.Mocked<UserDAL>;
+  let mockUserApiService: jest.Mocked<UserApiService>;
+
+  beforeAll(() => {
+    const { unit, unitRef } = TestBed.create<UserService>(UserService);
+
+    mockUserDal = unitRef.get(UserDAL);
+    mockUserApiService = testBed.get(UserApiService);
+
+    userService = unit;
+  });
+
+  test('getting all users should return combined users from DAL and API', async () => {
+    mockUserDal.getUsers.mockResolvedValue([{ id: 1, name: 'Mock User from DAL' }]);
+    mockUserApiService.fetchUsers.mockResolvedValue([{ id: 2, name: 'Mock User from API' }]);
+
+    const users = await userService.getAllUsers();
+
+    expect(users.length).toBe(2);
+    expect(users[0].name).toBe('Mock User from DAL');
+    expect(users[1].name).toBe('Mock User from API');
+
+    expect(mockUserDal.getUsers).toHaveBeenCalled();
+    expect(mockUserApiService.fetchUsers).toHaveBeenCalled();
+  });
+});
+```
+
+In this example, we use `TestBed.create(UserService)` to automatically mock the dependencies of `UserService`. The 
+test verifies that `UserService` integrates data from both the mocked `UserDAL` and `UserApiService`.
+
+For full examples and the InversifyJS guide, visit Automock documentation:
+* [https://github.com/automock/automock](https://github.com/automock/automock)
+* [https://automock.dev](https://automock.dev)