**Course Title:** Mastering NestJS: Building Scalable Server-Side Applications **Section Title:** Dependency Injection and Service Providers **Topic:** Implement a service to handle business logic for a RESTful API and inject it into your controllers.(Lab topic) **Objective:** By the end of this topic, you will be able to create a service to handle business logic for a RESTful API and inject it into your controllers using NestJS. **Prerequisites:** * You have a basic understanding of NestJS and its architecture. * You have completed the previous topics in the Dependency Injection and Service Providers section. **Step 1: Create a new service** In this step, we will create a new service to handle business logic for our RESTful API. We will use the NestJS CLI to generate a new service. ```bash nest generate service user ``` This will create a new file called `user.service.ts` in the `src/services` directory. **Step 2: Define the service** In this step, we will define the business logic for our service. For example, let's say we want to create a service that handles user data. ```typescript // src/services/user.service.ts import { Injectable } from '@nestjs/common'; @Injectable() export class UserService { private users: any[] = []; createUser(name: string, email: string): void { this.users.push({ name, email }); } getUsers(): any[] { return this.users; } } ``` In this example, we have defined a `UserService` that has two methods: `createUser` and `getUsers`. The `createUser` method creates a new user and adds it to the `users` array, while the `getUsers` method returns the entire `users` array. **Step 3: Inject the service into a controller** In this step, we will inject the `UserService` into a controller. We will use the `@Inject` decorator to inject the service into the controller. ```typescript // src/controllers/user.controller.ts import { Controller, Get, Post, Body } from '@nestjs/common'; import { UserService } from './user.service'; @Controller('users') export class UserController { constructor(private readonly userService: UserService) {} @Post() createUser(@Body() createUserDto: any): void { this.userService.createUser(createUserDto.name, createUserDto.email); } @Get() getUsers(): any[] { return this.userService.getUsers(); } } ``` In this example, we have injected the `UserService` into the `UserController` using the `@Inject` decorator. We have also defined two methods: `createUser` and `getUsers`. The `createUser` method calls the `createUser` method on the `UserService` to create a new user, while the `getUsers` method calls the `getUsers` method on the `UserService` to return the entire `users` array. **Conclusion:** In this topic, we have learned how to create a service to handle business logic for a RESTful API and inject it into a controller using NestJS. We have also learned how to define the business logic for the service and inject it into the controller using the `@Inject` decorator. **Exercise:** Create a new service to handle business logic for a RESTful API and inject it into a controller using NestJS. **Additional Resources:** * NestJS documentation: <https://docs.nestjs.com/> * NestJS CLI documentation: <https://docs.nestjs.com/cli> **Leave a comment or ask for help:** If you have any questions or need help with this topic, please leave a comment below.