TypeScript Design Patterns

Patterns with TypeScript

• Builder Pattern: class QueryBuilder<T> with chained methods returning refined types — .where() returns QueryBuilder<T & { filtered: true }>. Compile-time validation of builder usage
• Strategy Pattern: interface SortStrategy<T> { sort(items: T[]): T[] } — swap algorithms at runtime. Generic type parameter ensures strategy matches data type
• Repository Pattern: interface Repository<T extends Entity> { find(id: string): Promise<T | null>; save(entity: T): Promise<T> } — abstract data access. Swap implementations (SQL, MongoDB, in-memory for tests)
• Observer/Event Emitter: class TypedEventEmitter<Events extends Record<string, any[]>> — type-safe event names and listener arguments. emit<K extends keyof Events>(event: K, ...args: Events[K])
• Dependency Injection: class Container with typed bindings — container.bind<ILogger>(TOKENS.Logger).to(ConsoleLogger). Inversify or tsyringe for production DI containers
• Result Type: type Result<T, E> = { ok: true; value: T } | { ok: false; error: E } — explicit error handling without exceptions. Forces callers to handle both success and failure