TypeScript Decorator Patterns

npm init -y
npm install typescript ts-node @types/node
npx tsc --init

// Core types for TypeScript Decorator Patterns

type Result<T, E = Error> = 
  | { ok: true; value: T }
  | { ok: false; error: E };

interface Config {
  readonly name: string;
  readonly version: string;
  debug: boolean;
  features: Map<string, boolean>;
}

type DeepReadonly<T> = {
  readonly [P in keyof T]: T[P] extends object
    ? DeepReadonly<T[P]>
    : T[P];
};

type EventMap = {
  'item:created': { id: string; data: unknown };
  'item:updated': { id: string; changes: Partial<unknown> };
  'item:deleted': { id: string };
};

class TypeSafeTypeScript {
  private registry = new Map<string, unknown>();
  private listeners = new Map<string, Set<Function>>();

  register<T>(key: string, value: T): this {
    this.registry.set(key, value);
    this.emit('item:created', { id: key, data: value });
    return this;
  }

  resolve<T>(key: string): Result<T> {
    const value = this.registry.get(key);
    if (value === undefined) {
      return { ok: false, error: new Error(\`Key "\${key}" not found\`) };
    }
    return { ok: true, value: value as T };
  }

  on<K extends keyof EventMap>(
    event: K,
    handler: (payload: EventMap[K]) => void
  ): () => void {
    if (!this.listeners.has(event)) {
      this.listeners.set(event, new Set());
    }
    this.listeners.get(event)!.add(handler);
    return () => this.listeners.get(event)?.delete(handler);
  }

  private emit<K extends keyof EventMap>(event: K, payload: EventMap[K]): void {
    this.listeners.get(event)?.forEach(fn => fn(payload));
  }
}

const manager = new TypeSafeTypeScript();

// Type-safe event listeners
const unsub = manager.on('item:created', (payload) => {
  console.log(\`Created: \${payload.id}\`);
});

// Register values
manager
  .register('config', { debug: true })
  .register('version', '1.0.0');

// Type-safe resolution
const result = manager.resolve<{ debug: boolean }>('config');
if (result.ok) {
  console.log('Debug mode:', result.value.debug);
}

unsub(); // Clean up listener

import { describe, it, expect } from 'vitest';

describe('TypeSafeTypeScript', () => {
  it('should register and resolve values', () => {
    const mgr = new TypeSafeTypeScript();
    mgr.register('key', 'value');
    const result = mgr.resolve<string>('key');
    expect(result.ok).toBe(true);
    if (result.ok) expect(result.value).toBe('value');
  });

  it('should return error for missing key', () => {
    const mgr = new TypeSafeTypeScript();
    const result = mgr.resolve('missing');
    expect(result.ok).toBe(false);
  });
});