[Nest.js] 3 - Controller

Contents
1. Controller?
2. Route?
3. Request
4. Response
5. Create Controller

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1. Controller?

- Controllers are responsible for handling incoming requests and returning responses to the client.

https://docs.nestjs.com/controllers#controllers

- A Controller's purpose is to receive specific requests for the application.

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2. Routing?

- The Routing mechanism controls which controller receives which requests.

- Each Controller has more than one route, and different routes can perform different actions.

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3. Request

- @Controller() decorator, which is required to define a basic controller.
   We put a path prefix in a @Controller('path') decorator allows to group a set of related routes,
   , and minimize repetitive code. 

- CRUD Example with HTTP Requests and Handlers

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

@Controller('users')                                // requests starting with /users 
export class CatsController {
  constructor(private readonly usersService: UsersService) {}

  @Post()                                           // POST /users
  create(@Body() createUserDto: CreateUserDto) {
    return this.usersService.create(createUserDto);
  }

  @Get()                                            // GET /users
  findAll() {
    return this.usersService.findAll();
  }

  @Get(':id')                                       // GET /users/id
  findOne(@Param('id') id: string) {
    return this.usersService.findOne(+id);
  }

  @Patch(':id')                                     // PATCH /users/id
  update(@Param('id') id: string, @Body() updateUserDto: UpdateUserDto) {
    return this.usersService.update(+id, updateUserDto);
  }

  @Delete(':id')                                    // DELETE /users/id
  remove(@Param('id') id: string) {
    return this.usersService.remove(+id);
  }
}

- DTO(Data Transfer Object) : created by TypeScript Interface or by Class
   #RECOMMEND USING CLASS
   -> Classes are part of JS ES6 Standard, and therefore they are preserved as real entities in the complied JS.
   -> TS Interfaces are removed duing the transpilation(Translate + Compilation).
   -> Nest can't refer to them at runtime.
   -> This is important because features such as Pipes enable additional possibilities
        when they have access to the metatype of the variable at runtime.

- @Body() xxxDTO
   -> it would create a xxxDTO with user's input data by matching xxxDTO's properties and names. 

- @Param() params
   -> params.id

- @Param('id) id: string
   -> id

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4. Response

- A Request Handler returns a Javascript Object or Array, it will automatically be serialized to JSON.

- Primitive type(e.g., string, number, boolean) will be sent just value without attempting to serialize it.

- Simple : just return the value, and Nest takes care of the rest

- 200 Status code is default, 201 Status code is for POST success.

- @HttpCode(...) decorator at handler-level can change the default status code.

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5. Create Controller using CLI

1. CLI 

$ nest g controller boards // "boards" is the name of controller, 
                           // create BoardsController

or

$ nest g controller boards --no-spec // --no-spec optio : exclude unit-test file

2. Look for a "boards" folder

3. Created "boards.controller.ts" in the folder

4. Imported and Registerd the controller in its module "boards.module.ts" automatically