nestjs-dependency-injection

# NestJS Dependency Injection

Master NestJS dependency injection for building modular, testable
Node.js applications with proper service architecture, provider
patterns, and module organization.

## Table of Contents

- [Provider Patterns](#provider-patterns)
- [Module System](#module-system)
- [Injection Scopes](#injection-scopes)
- [Advanced Patterns](#advanced-patterns)
- [Best Practices](#best-practices)
- [Common Pitfalls](#common-pitfalls)
- [Resources](#resources)

## Provider Patterns

### Class Providers (Standard Pattern)

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

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

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

  findById(id: string): User | undefined {
    return this.users.find(user => user.id === id);
  }

  create(user: User): User {
    this.users.push(user);
    return user;
  }
}

// Module registration
@Module({
  providers: [UserService],
  exports: [UserService],
})
export class UserModule {}
```

### Value Providers

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

// Simple value provider
const DATABASE_CONNECTION = {
  provide: 'DATABASE_CONNECTION',
  useValue: {
    host: 'localhost',
    port: 5432,
    database: 'mydb',
  },
};

// Configuration value provider
const APP_CONFIG = {
  provide: 'APP_CONFIG',
  useValue: {
    apiUrl: process.env.API_URL,
    timeout: 5000,
    retries: 3,
  },
};

@Module({
  providers: [DATABASE_CONNECTION, APP_CONFIG],
  exports: [DATABASE_CONNECTION, APP_CONFIG],
})
export class ConfigModule {}

// Usage in service
@Injectable()
export class ApiService {
  constructor(
    @Inject('APP_CONFIG') private config: AppConfig,
  ) {}

  async fetchData(): Promise<any> {
    const response = await fetch(this.config.apiUrl, {
      timeout: this.config.timeout,
    });
    return response.json();
  }
}
```

### Factory Providers

```typescript
import { Injectable, Module } from '@nestjs/common';

// Simple factory p