applying-design-patterns
Applies GoF and DDD design patterns in TypeScript: Factory, Repository, Strategy, Adapter, Decorator, Facade, Observer, Command, Specification. Use when picking the right structure for a feature or refactoring duplicated logic.
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- 2026年5月3日 03:46
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- Applies GoF and DDD design patterns in TypeScript: Factory, Repository, Strategy, Adapter, Decorator, Facade, Observer, Command, Specification. Use when picking the right structure for a feature or refactoring duplicated logic.
# Applying Design Patterns
## When to use
- Picking the right structure for a new feature
- Refactoring duplicated logic
- Need pluggable algorithms or interchangeable implementations
## Core rules
1. Use patterns by name: Factory, Repository, Strategy, Adapter, Decorator, Facade, Observer, Command, Specification
2. Patterns serve the business, not the other way around
3. Prefer composition over inheritance
4. All pattern implementations must be OOP classes with explicit typing
## Reference shape (TypeScript)
### Factory Pattern
```typescript
export class UserFactory {
static createRegular(id: string, email: string): User {
return User.create(id, Email.create(email), ['user']);
}
static createAdmin(id: string, email: string): User {
return User.create(id, Email.create(email), ['user', 'admin']);
}
}
```
### Strategy Pattern
```typescript
export interface PricingStrategy { calculate(base: number): number; }
export class DiscountPricing implements PricingStrategy {
constructor(private discount: number) {}
calculate(base: number): number { return base - this.discount; }
}
```
### Repository Pattern
```typescript
export interface OrderRepository {
findById(id: string): Promise<Order | null>;
save(order: Order): Promise<Order>;
}
```
## Examples — Do
```typescript
// Command pattern
class CreateOrderCommand {
constructor(public readonly userId: string, public readonly items: OrderItem[]) {}
}
class CreateOrderHandler {
async handle(cmd: CreateOrderCommand): Promise<Result<Order, AppError>> { ... }
}
```
## Examples — Don't
```typescript
// ❌ Over-engineering: simple logic doesn't need patterns
class UserBuilder { setName(n: string) { this.name = n; return this; } }
```
## Checklist
- [ ] Pattern matches the problem (Factory for creation, Strategy for algorithms)
- [ ] All pattern classes are OOP with explicit types
- [ ] No over-engineering simple logic
See [reference/pattern-examples.md](./reference/pattern-examples.md) for full patterns.
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