Vatsalya-singhi

// Users Service (Provides Business Logic)
import { Injectable } from '@nestjs/common';

// injectables include wide range of configurations such as when and how to initialise the provider
@Injectable()
export class UsersService {
  private users = ['Alice', 'Bob', 'Charlie'];

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

Vatsalya-singhi

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

@Module({
  controllers: [UsersController], // Registers the controller
  providers: [UsersService], // Registers the service
  exports: [UsersService], // Allows other modules to use UsersService
  imports: []
})

Vatsalya-singhi

import { Injectable, NestMiddleware } from '@nestjs/common';
import { Request, Response, NextFunction } from 'express';

@Injectable()
export class LoggerMiddleware implements NestMiddleware {
  use(req: Request, res: Response, next: NextFunction) {
    console.log(`[${new Date().toISOString()}] ${req.method} ${req.url}`);
    next(); // Pass control to the next handler
  }
}

Vatsalya-singhi

import { MiddlewareConsumer, Module, NestModule } from '@nestjs/common';
import { UsersController } from './users.controller';
import { UsersService } from './users.service';
import { LoggerMiddleware } from './logger.middleware';

@Module({
  controllers: [UsersController],
  providers: [UsersService],
})
export class UsersModule implements NestModule {

Vatsalya-singhi

import { ExceptionFilter, Catch, ArgumentsHost, HttpException } from '@nestjs/common';
import { Response } from 'express';

@Catch(HttpException) // Catches only HTTP-related exceptions
export class HttpExceptionFilter implements ExceptionFilter {
  catch(exception: HttpException, host: ArgumentsHost) {
    const ctx = host.switchToHttp();
    const response = ctx.getResponse<Response>();
    const status = exception.getStatus();
    const message = exception.getResponse();

Vatsalya-singhi

// main.ts
import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';
import { HttpExceptionFilter } from './http-exception.filter';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  app.useGlobalFilters(new HttpExceptionFilter()); // Apply globally
  await app.listen(3000);
}

Vatsalya-singhi

import { Controller, Get, UseFilters, HttpException, HttpStatus } from '@nestjs/common';
import { HttpExceptionFilter } from './http-exception.filter';

@Controller('users')
@UseFilters(HttpExceptionFilter) // Apply to all methods in this controller
export class UsersController {
  @Get()
  findAll() {
    throw new HttpException('Users not found', HttpStatus.NOT_FOUND);
  }

Vatsalya-singhi

@Get()
@UseFilters(HttpExceptionFilter) // Apply to only this method
findAll() {
  throw new HttpException('Users not found', HttpStatus.NOT_FOUND);
}

Vatsalya-singhi

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

@Controller('users')
export class UsersController {
  @Get(':id')
  findOne(@Param('id', ParseIntPipe) id: number) {
    return { message: `Fetching user with ID ${id}` };
  }
}

Vatsalya-singhi

// convert strings to uppercase
import { PipeTransform, Injectable, ArgumentMetadata } from '@nestjs/common';

@Injectable()
export class UppercasePipe implements PipeTransform {
  transform(value: any, metadata: ArgumentMetadata) {
    return typeof value === 'string' ? value.toUpperCase() : value;
  }
}