dependency-injection

name	dependency-injection
description	Dependency injection patterns — IoC containers, service registration, lifetime scoping, and testing.
layer	utility
category	architecture
triggers	["dependency injection","IoC","inversion of control","DI container","service provider"]
inputs	["DI architecture decisions","Service registration and lifetime scoping","IoC container selection and configuration","Testing with dependency injection"]
outputs	["DI container setup and configuration","Service registration patterns","Lifetime management strategies","Testable architectures with DI"]
linksTo	["nestjs","typescript-patterns","testing-patterns"]
linkedFrom	[]
preferredNextSkills	["nestjs","typescript-patterns","testing-patterns"]
fallbackSkills	[]
riskLevel	low
memoryReadPolicy	selective
memoryWritePolicy	none
sideEffects	[]

Dependency Injection Patterns

Purpose

Provide expert guidance on dependency injection (DI) and inversion of control (IoC) patterns including container configuration, service lifetimes, composition roots, and how DI enables testability. Covers both framework-based DI (NestJS, Angular) and standalone containers (tsyringe, inversify, awilix).

Core Concepts

Dependency Injection inverts the control of dependency creation. Instead of a class creating its own dependencies, they are provided (injected) from the outside.

// WITHOUT DI — tightly coupled, hard to test
class OrderService {
  private db = new PostgresDatabase(); // hardcoded dependency
  private mailer = new SmtpMailer();   // hardcoded dependency

  async createOrder(data: CreateOrderDto) {
    const order = await this.db.insert('orders', data);
    await this.mailer.send(data.email, 'Order confirmed');
    return order;
  }
}

// WITH DI — loosely coupled, easy to test
class OrderService {
  constructor(
    private readonly db: Database,     // interface, not implementation
    private  : ,   
  ) {}

   () {
     order =  ..(, data);
     ..(data., );
     order;
  }
}