Getting Started with Nest.js

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If you have ever worked on a Node.js application before, either built a REST API or an enterprise application, you must have realised how tedious and daunting it was to maintain, especially whenever the application start to scale. The more you add new features to the application, the larger the codebase.

Creating a proper structure for such application can result into serious headache if not properly managed, especially as a result of application specific configurations. This is why Nest.js was created.

Nest.js was built mainly to eliminate disorganized codebases and give Node.js application a moderate and reasonable structure out of the box. Heavily inspired by Angular, Nest.js was built with TypeScript and uses Express.js under hood. This rightly makes it compatible with the majority of express middleware.

In this post, I will introduce and take you through the process of getting started with Nest.js. You will learn about several ways to install the framework on your machine and why you need to consider using it for your next project. In the process of doing this, you will create a very simple RESTful API that enables users to fetch, create and delete books in a bookstore.

This is a very simple application but yet broad enough to give you comprehensive insight on how to craft an application with Nest.js.

Prerequisites

Familiarity with TypeScript and a reasonable knowledge of JavaScript will help you get the best out of this tutorial. Experienced with building applications with Angular will be a plus but not a requirement as the article will give you a proper guide on how to easily begin.

You need to install Node and npm. It is advisable to also install nodemon globally on your machine.

Overview of Nest.js and why use it

Nest.js is a server-side Node.js framework for building efficient, reliable and scalable applications. Built by Kamil and backed by quite a number of reputable organizations and individuals.

Nest.js was introduced to solve the architectural problem of Node.js by giving backend applications a modular structure for organising code into separate modules.

Fully built with TypeScript, it comes with the benefits of code type checking and dependency injection which helps to facilitate the process of development of applications. If you are conversant with the structure of Angular applications, you are going to feel so comfortable with the key concepts of Nest.js and getting started with it will be quite an easy task. Anyways, this post will provide you with the required details needed to start building applications with Nest.js.

In addition, the following list shows some of the benefits of Nest.js as explained here by Kamil:

• it surrounds your route handler body with try..catch blocks

• it makes every route handler async

• it creates a global express router

• it creates a separated router for each controller

• it binds error-handling middleware

• it binds body-parser middleware (both json and extended urlencoded)

Now that you have been briefed about this awesome framework, let’s take a look at the building blocks of Nest.js.

Building blocks of Nest.js

The following are the building blocks used when building Nest.js applications:

Controllers

Typical to most web frameworks, controllers in Nest.js are responsible for handling any incoming requests and returning responses to the client side of the application. For example, if you make an API call to a particular endpoint, say /home, the controller will receive this request and based on the available resources, it will returned the appropriate response.

Nest.js was structured in a way that the routing mechanism is able to control which controller will be responsible for handling a particular request.

Defining a basic controller in Nest.js is as good as creating a TypeScript file and including a decorator @Controller() just like the code snippet below:

  // users.controller.ts 

import { Controller, Get } from '@nestjs/common';

@Controller('users')
export class UsersController {
 @Get()
 findAll() { 
   return 'This will return all the users';
 }
}

The prefix of users within the Controller decorator will prompt the UsersController to handle any /users GET request within an application and return the appropriate response as specified. Other HTTP request handled by the controller includes POST , PUT, DELETE as we will see later in the tutorial.

Once a controller is created, it needs to be added to the module definition before Nest.js can easily recognise it. This could be the root ApplicationModule or any other module created within the application. More about this in the module section of this post.

Providers

As mentioned earlier, Nest.js was heavily inspired by Angular and similar to an Angular application, one can easily create a provider and inject it into controllers or other providers too as well. These providers are also called services and based on the philosophy of Nest.js, it was designed to abstract any form of complexity and logic to a class called service.

A service provider in Nest.js is just a normal JavaScript class with a special @Injectable() decorator at the top.

For example, you can simply create a service to fetch users as shown below:

    // users.service.ts

import { Injectable } from '@nestjs/common';
import { User } from './interfaces/user.interface';

@Injectable()
export class UsersService {
  private readonly users: User[] = [];

  create(user: User) { 
    this.users.push(user);   }

  findAll(): User[] {
    return this.users;
  }
}

The provider created above is a class with two methods create() and findAll(), which can be used to create and return all users respectively. And to easily help with type checking an interface was used to specify the type of elements that should be received by the methods.

Modules

Modules are more like the most important basic building block in Nest.js. They are TypeScript files decorated with @Module decorator. This attached decorator provides metadata that Nest makes use of to organize the application structure. With modules you can easily group related files into one.

Each Nest.js application must have at least one module, usually referred to as the root module. This root module is the top-level module and usually enough for a small application but it is advisable to break a large application into multiple modules as it helps to maintain the structure of the application.

If you have an application that manages a lot of data or functionality about users , we can group both the controller, services and other related files into a single module, say UsersModule for example:

import { Module } from '@nestjs/common';
import { UsersController } from './users.controller.ts';
import { UsersService } from './users.service.ts';

@Module({
  controllers: [UsersController],
  providers: [UsersService]
})

export class UsersModule {}

From the preceding file, we are exported a UsersModule that contains both the UsersController and UsersService. With this in place, we can then proceed to import and use the UsersModule within the root module of the application as shown in the following code snippet:

...
import { UsersModule } from './users/users.module';

@Module({
  ...
})

export class AppModule { }

Other important concepts

DTO

Data transfer object is an object that defines how data will be sent over the network.

Interfaces

TypeScript interfaces are used for type-checking and defining the types of data that can be passed to a controller or a Nest service.

Dependency injection

Dependency injection is a design pattern used to increase efficiency and modularity of applications. It is often used by the biggest frameworks to keep code clean and easier to use. Nest.js also makes use of it to basically create coupled components.

With this pattern, it is very easy to manage dependencies between building blocks like controllers, providers and modules. The only thing required is to define the dependency for example a UsersService() in the constructor of a controller as shown here:

...
@Controller('users')
export class UsersController {
constructor(private readonly usersService: UsersService){}
 ...
}

With some of these concepts briefly covered, you can now proceed to the next section, where you will put all the knowledge gained so far in this post into use as you will learn how to seamlessly build a RESTful API using Nest.js.

What you will build with Nest.js

As stated earlier in this post, you will create a sample application that will help you get a good grasp on some of the core concepts of Nest.js.

This application will be specifically for a bookstore. At the end of the post you would have created a micro-service that will enable users to create and add a new book with few descriptions to an existing list of books. This could be from a database, but to ensure simplicity in this post, we won’t really be connecting our application to a database yet. But instead, we will make use of a mock data of books and once a new book is created, we will push and add it to the list.

Installing Nest.js

In order to easily scaffold a new Nest.js application, you will need to globally installed Nest CLI. It is a command line interface tool specifically created to amongst other things, help to craft a new Nest.js app in no time and provide access to ( built in generators ) several commands to generate different files and produce a well-structured application.

Apart from using the CLI tool, you can also install a new Nest.js application by cloning the starter project from GitHub using Git, but for the purpose of this tutorial run the following command to install the Nest CLI:

npm i -g @nestjs/cli

This will give you access to the nest command for project installation and other project specific commands.

Next, run the command below to install a new project named bookstore-nest within your development folder:

nest new bookstore-nest

You will be asked few questions during the installation, just follow the prompt and respond accordingly. Next, once the installation is complete, change directory into the newly created project and start the application with:

// change directory
cd bookstore-nest

// start the application
npm run start

or better still, run the command below in order to use Nodemon for the project:

// start the application using nodemon
npm run start:dev

Navigate to http://localhost:3000 from your favorite browser, you will see the Hello World! message as shown here:

Generate a module

First you will start by generating a module for the bookstore. To do this, you will leverage the inbuilt file generator using Nest CLI. Run the following command to scaffold a new module for the application:

nest generate module books

The command above will create a new folder named books within the src folder. Also within the books folder you will find a books.module.ts file.

// ./src/books/books/module.ts

import { Module } from '@nestjs/common';
@Module({})
export class BooksModule {}

This was generated by the command and the module has also been added to the app.module.ts which happens to be the root module of the application.

Create routes

Next, you will create routes for the endpoints. As mentioned earlier, routes are in controllers, so you need to create controllers that will handle individual endpoints. Again, use Nest CLI to generate your controllers, run the following command:

nest generate controller books

This will create a controller inside the books folder. Since we won’t really be connecting to the database for now, create a sample mock data for the bookstore. Under the src folder, create a subfolder named mocks and within the newly created folder, create a new TypeScript file named books.mock.ts and paste the following code in it:

// ./src/mocks/books.mock.ts
export const BOOKS = [
    { id: 1, title: 'First book', description: "This is the description for the first book", author: 'Olususi Oluyemi' },
    { id: 2, title: 'Second book', description: "This is the description for the second book", author: 'John Barry' },
    { id: 3, title: 'Third book', description: "This is the description for the third book", author: 'Clement Wilfred' },
    { id: 4, title: 'Fourth book', description: "This is the description for the fourth book", author: 'Christian nwamba' },
    { id: 5, title: 'Fifth book', description: "This is the description for the fifth book", author: 'Chris anderson' },
    { id: 6, title: 'Sixth book', description: "This is the description for the sixth book", author: 'Olususi Oluyemi' },
];

Setting up service

Next, you will create a service to hold all the logic for the bookstore. Run the following command to generate a service:

nest generate service books

This command will create a new file named books.service.ts within ./src/books folder.

Get books

Next, open the newly created file and paste the following:

//  ./src/books/books.service.ts

  import { Injectable, HttpException } from '@nestjs/common';
  import { BOOKS } from '../mocks/books.mock';

  @Injectable()
  export class BooksService {
      books = BOOKS;

      getBooks(): Promise<any> {
          return new Promise(resolve => {
              resolve(this.books);
          });
      }
      getBook(bookID): Promise<any> {
          let id = Number(bookID);
          return new Promise(resolve => {
              const book = this.books.find(book => book.id === id);
              if (!book) {
                  throw new HttpException('Book does not exist!', 404);
              }
              resolve(book);
          });
      }
  }

First, you imported the requires modules from Nest.js and also BOOKS from the mock data you created earlier.

Next, you created two different methods named getBooks() and getBook() to retrieve the list of books from the mock data and to fetch just one book using the bookID as a parameter.

Add book

Next, add the method below to the /src/books/books.service.ts immediately after the getBook() method:

//  ./src/books/books.service.ts

import { Injectable, HttpException } from '@nestjs/common';
import { BOOKS } from '../mocks/books.mock';
@Injectable()
export class BooksService {
    books = BOOKS;
    ...
    addBook(book): Promise<any> {
        return new Promise(resolve => {
            this.books.push(book);
            resolve(this.books);
        });
    }
}

The method above will be used to push a new book to the existing list

Delete book

Finally, add the last method to delete a particular book using the bookID as a parameter:

//  ./src/books/books.service.ts

import { Injectable, HttpException } from '@nestjs/common';
import { BOOKS } from '../mocks/books.mock';
@Injectable()
export class BooksService {
    books = BOOKS;
    ...
    deleteBook(bookID): Promise<any> {
        let id = Number(bookID);
        return new Promise(resolve => {
            let index = this.books.findIndex(book => book.id === id);
            if (index === -1) {
                throw new HttpException('Book does not exist!', 404);
            }
            this.books.splice(1, index);
            resolve(this.books);
        });
    }
}

Inject service into controller

Here, you will use dependency injection design pattern to pass the BooksService into the BooksController through a constructor. Open the BooksController created earlier and paste the following code in it:

// ./src/books/books.controller.ts

import { Controller, Get, Param, Post, Body, Query, Delete } from '@nestjs/common';
import { BooksService } from './books.service';
import { CreateBookDTO } from './dto/create-book.dto';

@Controller('books')
export class BooksController {
    constructor(private booksService: BooksService) { }

    @Get()
    async getBooks() {
        const books = await this.booksService.getBooks();
        return books;
    }

    @Get(':bookID')
    async getBook(@Param('bookID') bookID) {
        const book = await this.booksService.getBook(bookID);
        return book;
    }

    @Post()
    async addBook(@Body() createBookDTO: CreateBookDTO) {
        const book = await this.booksService.addBook(createBookDTO);
        return book;
    }

    @Delete()
    async deleteBook(@Query() query) {
        const books = await this.booksService.deleteBook(query.bookID);
        return books;
    }
}

Here in this controller, first, the important modules were imported from @nestjs/common and you also import both the BooksService and CreateBookDTO respectively. CreateBookDTO is a data transfer object, a TypeScript class created for type-checking and to define the structures of what an object looks like when creating a new book. We will create this DTO in a bit.

Next, you used constructor to inject the BooksService into the controller and created four different methods which are:

• getBooks(): Used to fetch the list of all books. It has @Get() decorator attached to it. This helps to map any GET request sent to /books to this controller.

• getBook(): Used to retrieve the details of a particular book by passing the bookID as a parameter.

• addBook(): Used to create and post a new book to the existing book list. And because we are not persisting into the database, the newly added book will only be held in memory.

• deleteBook(): Used to delete a book by passing the bookID as a query parameter.

  Each of the methods has a special decorator attached to it, which makes it very easy to route each HTTP request to a specific method within the controller.

The DTO

In the previous section, you made use of a data transfer object called CreateBookDTO. To set it up, navigate to the ./src/books folder and create a new subfolder name dto. Next, within the newly created folder, create another file and call it create-book.dto.ts and paste the following in it:

// ./src/books/dto/create-book.dto.ts

export class CreateBookDTO {
    readonly id: number;
    readonly title: string;
    readonly description: string;
    readonly author: string;
}

You are almost done with the application, the next line of action is to take a look at the BooksModule and update it accordingly. You will do that in the next section.

Update the books module

Navigate back to the BooksModule created earlier and update it with the code below:

// ./src/books/books.module.ts

import { Module } from '@nestjs/common';
import { BooksController } from './books.controller';
import { BooksService } from './books.service';
@Module({
  controllers: [BooksController],
  providers: [BooksService]
})
export class BooksModule {}

Test the application

Start the application again if it is not running at the moment with:

npm run start

and use postman to test the API

Get Books

Get book using bookID

Create a new book

Delete a book

Conclusion

We have barely scratched the surface on what Nest.js has to offer the Node.js world in this post. To get more conversant with this awesome framework, first, we took a quick look at the fundamentals and basic building blocks of Nest.js and then proceeded to build a RESTful API where you also learnt about dependency injection amongst other things.

I hope this tutorial as given you enough information to try out Nest.js for your next application. Feel free to drop your thoughts in the comment section below and find the complete source code of this tutorial here on GitHub.