02. Routing & Controllers

1. The Vanilla Mechanics

Routing is the mapping of an incoming HTTP request to a specific function based on the URL and Method. In raw Node.js or basic Express, this is often a chain of if/else statements or a simple router object.

Concept: Path Parsing

• Pathname: Extracting /users from /users?id=1.
• Regex Matching: Handling dynamic routes like /users/:id.

The “Raw” Implementation (Example)

const url = require('url');

const server = http.createServer((req, res) => {
  const parsedUrl = url.parse(req.url, true);
  const path = parsedUrl.pathname;

  if (req.method === 'GET' && path === '/users') {
    res.end('List of users');
  } else if (req.method === 'POST' && path === '/users') {
    res.end('Create user');
  } else if (req.method === 'GET' && path.startsWith('/users/')) {
    const id = path.split('/')[2];
    res.end(`User with ID ${id}`);
  }
});

Challenges:

• Dynamic Routes: Extracting parameters from strings requires custom splitting logic.
• Tightly Coupled Logic: Routing and business logic are mixed in the same function.
• Parameter Extraction: It’s difficult to extract query params and body payloads cleanly.

2. The NestJS Abstraction

NestJS uses Decorators to handle routing declaratively.

Key Advantages:

• @Controller(): Scopes a set of routes to a base path.
• Request Decorators: @Get(), @Post(), @Param(), @Query(), @Body() simplify data extraction.
• Metadata-driven: NestJS registers these routes automatically at bootstrap time.

The NestJS Implementation:

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

@Controller('users')
export class UsersController {
  @Get() // GET /users
  findAll() {
    return 'List of users';
  }

  @Get(':id') // GET /users/123
  findOne(@Param('id') id: string) {
    return `User with ID ${id}`;
  }

  @Post() // POST /users
  create(@Body() dto: any) {
    return 'User created';
  }
}

3. Engineering Labs

• Lab 2.1: Implement a manual router in a raw Node.js script that handles at least three nested routes with path parameters.
• Lab 2.2: Re-implement the same in NestJS and use @Param() to extract values directly into your method arguments.