How to create RESTful API in Nest JS? Step by step guidelines!

Creating a RESTful API in NestJS involves several steps. Here’s a step-by-step theoretical guide:

Step 1: Setup a New NestJS Project

Install Nest CLI:

   npm install -g @nestjs/cli

Create a New Project:

   nest new project-name

Navigate to the Project Directory:

   cd project-name

Step 2: Generate a Module, Controller, and Service

Generate a Module:

   nest g module user

Generate a Controller:

   nest g controller user

Generate a Service:

   nest g service user

Step 3: Define the User Entity

Create a User Entity: Create the src/user/user.entity.ts file:

   export class User {
     id: number;
     name: string;
     email: string;
   }

Step 4: Implement the User Service

Implement Service Logic: Open src/user/user.service.ts and implement the service methods:

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

   @Injectable()
   export class UserService {
     private users: User[] = [];
     private idCounter = 1;

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

     findOne(id: number): User {
       return this.users.find(user => user.id === id);
     }

     create(user: User): User {
       user.id = this.idCounter++;
       this.users.push(user);
       return user;
     }

     update(id: number, updatedUser: Partial<User>): User {
       const user = this.findOne(id);
       if (user) {
         Object.assign(user, updatedUser);
       }
       return user;
     }

     delete(id: number): void {
       this.users = this.users.filter(user => user.id !== id);
     }
   }

Step 5: Implement the User Controller

Implement Controller Logic: Open src/user/user.controller.ts and define the routes:

   import { Controller, Get, Post, Put, Delete, Param, Body } from '@nestjs/common';
   import { UserService } from './user.service';
   import { User } from './user.entity';

   @Controller('users')
   export class UserController {
     constructor(private readonly userService: UserService) {}

     @Get()
     findAll(): User[] {
       return this.userService.findAll();
     }

     @Get(':id')
     findOne(@Param('id') id: string): User {
       return this.userService.findOne(+id);
     }

     @Post()
     create(@Body() user: User): User {
       return this.userService.create(user);
     }

     @Put(':id')
     update(@Param('id') id: string, @Body() updatedUser: Partial<User>): User {
       return this.userService.update(+id, updatedUser);
     }

     @Delete(':id')
     delete(@Param('id') id: string): void {
       this.userService.delete(+id);
     }
   }

Step 6: Update the User Module

Update the User Module: Open src/user/user.module.ts and update it to include the controller and service:

   import { Module } from '@nestjs/common';
   import { UserController } from './user.controller';
   import { UserService } from './user.service';

   @Module({
     controllers: [UserController],
     providers: [UserService],
   })
   export class UserModule {}

Step 7: Integrate the User Module into the App Module

Update the App Module: Open src/app.module.ts and update it to include the User module:

   import { Module } from '@nestjs/common';
   import { UserModule } from './user/user.module';

   @Module({
     imports: [UserModule],
   })
   export class AppModule {}

Step 8: Run the Application

Start the NestJS Application:

   npm run start:dev

Step 9: Test the RESTful API

Test the API Endpoints: Use a tool like Postman or curl to test the RESTful API endpoints:
- GET /users: Retrieve all users.
- GET /users/:id: Retrieve a user by ID.
- POST /users: Create a new user.
- PUT /users/:id: Update a user by ID.
- DELETE /users/:id: Delete a user by ID.

This guide provides a foundational approach to creating a RESTful API in NestJS. You can further expand and customize it based on your application's requirements.

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