Services in Nest.js are providers, meaning you can inject them into modules and classes through dependency injection. In Nest.js, a service is not just a construct but a first-class citizen, managed by the framework's built-in Inversion of Control (IoC) container. Unlike in Express.js, where middleware or simple JavaScript functions often serve the same purpose but without formal dependency management, Nest.js services offer a structured way to write business logic, making the application more maintainable and testable.

A service in Nest.js is responsible for fetching data from the database and saving data back to the database, functioning as a liaison between the controller and the database. This concept aligns with Nest.js's adherence to the Single Responsibility Principle, separating the business logic from the controller layer, a stark contrast to Express.js where the roles are often less clearly delineated. 

A service can be injected into a controller using Nest.js's built-in Dependency Injection system. You can also export the service from the current module, enabling its use in other parts of that specific module. This is another feature where Nest.js differentiates itself from frameworks like Express.js, offering native support for modularity and code reuse through its export system.

In this lesson, we'll focus on creating the song service, a specific component tailored to manage song-related data. Creating specialized services for different aspects of your application promotes better maintainability and is a cornerstone of Nest.js's modular architecture.

@Injectable()
export class SongsService {}

@Controller('songs')
export class SongsController {
  constructor(private songsService: SongsService){

  }

We can inject the service into a constructor function. Now the SongsController is dependent on SongsService

Let's create a new service by using nest-cli

---
title: Creating a Service in Nest.js 
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---

## What is Service

Services in Nest.js are providers, meaning you can inject them into modules and classes through dependency injection. In Nest.js, a service is not just a construct but a first-class citizen, managed by the framework's built-in Inversion of Control (IoC) container. Unlike in Express.js, where middleware or simple JavaScript functions often serve the same purpose but without formal dependency management, Nest.js services offer a structured way to write business logic, making the application more maintainable and testable.

![What is a Service](https://d2uusema5elisf.cloudfront.net/courses/nestjs-fundamentals/module_02/lesson_02.03/public/assets/what-is-service.png)

A service in Nest.js is responsible for fetching data from the database and saving data back to the database, functioning as a liaison between the controller and the database. This concept aligns with Nest.js's adherence to the Single Responsibility Principle, separating the business logic from the controller layer, a stark contrast to Express.js where the roles are often less clearly delineated. 

A service can be injected into a controller using Nest.js's built-in Dependency Injection system. You can also export the service from the current module, enabling its use in other parts of that specific module. This is another feature where Nest.js differentiates itself from frameworks like Express.js, offering native support for modularity and code reuse through its export system.

In this lesson, we'll focus on creating the song service, a specific component tailored to manage song-related data. Creating specialized services for different aspects of your application promotes better maintainability and is a cornerstone of Nest.js's modular architecture.

## Creating a Service

```ts
@Injectable()
export class SongsService {}
```

### Injecting Service

Haider Malik

Creating a Service in Nest.js

It means you can inject into another class.

You will have a service and the service is responsible for fetching data from

If you have to interact with database, you have to write the source code inside

You will create a method to interact with database.

And this service can be injected into controllers.

If you have multiple controllers into module, you can inject the same service

You can also export the service from a current module and you can use that

And all you need to do is to inject a particular decorator.

It means you can inject as a dependency into another class.

This is how you will inject the service into controller or into a class.

You have to inject it in the constructor.

It will be treated as a dependency for the songs controller.

Now the songs controller is dependent on songs controller is dependent on the

I'm going to use ness_cli to generate a service.

It will create a class and it will inject into our songs module.

You can see that we have the song service backfile which is testing file and we

It has registered as a provider inside the songs module.

Providers could be factories, helper function.

You will learn more about dependency injection in more details in the later

So I told you services are responsible to interact with database.

Right now we are not going to interact with database.

We will use the Postgres SQLs, Postgres database in the later videos.

For that here I'm going to create a local DB which should be array, local array

record in the disk, private did only songs array.

Let's define a new method to create a new song and it is going to return.

Make sure you get the song from the create method.

It is a method push and we need to provide a new song.

And let's create another method, find all and it is going to return a simple

This starts songs and I can return this starts songs array.

So we have created a method now it is time to inject this service into the

I want you to do it except the challenge and do it.

Now inside the create I can use the song service by using this, this dot song

I'm going to provide a dummy name, animals by Martin.

Don't worry we will create a separate song model and separate interface for the

And for the find all I'm going to use the start song service dot point all.

Let's send the API request to create a new song.

If you send all the songs request, I got all the songs.

So our service is working now in the real world application.

We will write the database code here here.

And here what we will do fetch the songs from the DB.

But we will implement this feature in later videos.

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