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NestJS Cheat Sheet

A comprehensive cheat sheet for NestJS, covering core concepts, modules, controllers, services, and common utilities.

Core Concepts

Modules

Definition: Modules organize application components.

Usage: Use @Module() decorator to define a module.

Example:

import { Module } from '@nestjs/common';
import { AppController } from './app.controller';
import { AppService } from './app.service';

@Module({
  imports: [],
  controllers: [AppController],
  providers: [AppService],
})
export class AppModule {}

Imports: Used to import other modules.
Controllers: Defines controllers within the module.
Providers: Defines services, repositories, and other providers within the module.

Controllers

Definition: Controllers handle incoming requests and return responses.

Usage: Use @Controller() decorator to define a controller.

Example:

import { Controller, Get } from '@nestjs/common';
import { AppService } from './app.service';

@Controller()
export class AppController {
  constructor(private readonly appService: AppService) {}

  @Get()
  getHello(): string {
    return this.appService.getHello();
  }
}

@Get(), @Post(), @Put(), @Delete(), @Patch(), @Options(), @Head(): Methods to handle specific HTTP requests.

Services

Definition: Services contain business logic and are used by controllers.

Usage: Use @Injectable() decorator to define a service.

Example:

import { Injectable } from '@nestjs/common';

@Injectable()
export class AppService {
  getHello(): string {
    return 'Hello World!';
  }
}

Dependency Injection

Providers and Injection

@Injectable(): Marks a class as a provider.
@Inject(): Injects a provider into a class.

Constructor Injection Example:

import { Injectable } from '@nestjs/common';

@Injectable()
export class CatsService {
  constructor(@Inject('DATABASE_CONNECTION') private readonly connection: Connection) {}
}

Custom Providers: Allows defining custom provider logic.

Value Provider Example:

import { Module } from '@nestjs/common';

const config = { api_key: 'yourkey' };

@Module({
  providers: [
    {
      provide: 'CONFIG',
      useValue: config,
    },
  ],
})
export class AppModule {}

Scopes

Default Scope (Singleton)

Instances are created once and shared across the application.

Request Scope

Instances are created per request. Use REQUEST from @nestjs/core.

Transient Scope

Instances are not shared. New instance on each injection.

Middleware & Interceptors

Middleware

Definition: Functions that are executed before the route handler.

Usage: Implement NestMiddleware interface.

Example:

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('Request...');
    next();
  }
}

Applying Middleware:

import { Module, NestModule, MiddlewareConsumer } from '@nestjs/common';
import { LoggerMiddleware } from './logger.middleware';

@Module({})
export class AppModule implements NestModule {
  configure(consumer: MiddlewareConsumer) {
    consumer
      .apply(LoggerMiddleware)
      .forRoutes('*');
  }
}

Interceptors

Definition: Interceptors augment request/response flow.

Usage: Implement NestInterceptor interface.

Example:

import { Injectable, NestInterceptor, ExecutionContext, CallHandler } from '@nestjs/common';
import { Observable } from 'rxjs';
import { tap } from 'rxjs/operators';

@Injectable()
export class LoggingInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    console.log('Before...');

    const now = Date.now();
    return next
      .handle()
      .pipe(
        tap(() => console.log(`After... ${Date.now() - now}ms`)),
      );
  }
}

Applying Interceptors:

import { UseInterceptors } from '@nestjs/common';

@UseInterceptors(LoggingInterceptor)
export class AppController {}

Pipes

Pipes Overview

Definition: Pipes transform or validate request data.

Types:

  • Transformation: Transform input data to the desired output.
  • Validation: Evaluate input data and throw an exception if invalid.

Built-in Pipes

ValidationPipe

Validates request body using class-validator.

ParseIntPipe

Parses a string to an integer.

ParseBoolPipe

Parses a string to a boolean.

ParseArrayPipe

Parses a string to an array.

ParseUUIDPipe

Parses a string to a UUID.

Custom Pipes

Creating a Custom Pipe:

import { PipeTransform, Injectable, ArgumentMetadata, BadRequestException } from '@nestjs/common';

@Injectable()
export class CustomValidationPipe implements PipeTransform {
  transform(value: any, metadata: ArgumentMetadata) {
    if (!this.isValid(value)) {
      throw new BadRequestException('Validation failed');
    }
    return value;
  }

  private isValid(value: any): boolean {
    // Validation Logic Here
    return true; // Replace with actual validation
  }
}

Applying a Pipe:

import { Controller, Get, Query, UsePipes } from '@nestjs/common';
import { CustomValidationPipe } from './custom-validation.pipe';

@Controller()
export class AppController {
  @Get()
  @UsePipes(new CustomValidationPipe())
  getHello(@Query('param') param: string): string {
    return `Hello ${param}`;
  }
}
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