TypeScript Decorators: Complete Implementation Guide

function Component(config: { selector: string }) {
  return function<T extends { new (...args: any[]): {} }>(constructor: T) {
    return class extends constructor {
      selector = config.selector;
    };
  };
}

@Component({ selector: 'app-root' })
export class AppComponent {
  constructor() {
    console.log(this.selector); // 'app-root'
  }
}

function Log(target: any, propertyKey: string, descriptor: PropertyDescriptor) {
  const originalMethod = descriptor.value;
  
  descriptor.value = function(...args: any[]) {
    console.log(`Calling ${propertyKey} with`, args);
    const result = originalMethod.apply(this, args);
    console.log(`Result:`, result);
    return result;
  };
}

class Calculator {
  @Log
  add(a: number, b: number) {
    return a + b;
  }
}

function MinLength(min: number) {
  return function(target: any, propertyKey: string) {
    let value: string;
    
    const getter = () => value;
    const setter = (newValue: string) => {
      if (newValue.length < min) {
        throw new Error(`${propertyKey} must be at least ${min} characters`);
      }
      value = newValue;
    };
    
    Object.defineProperty(target, propertyKey, {
      get: getter,
      set: setter,
      enumerable: true,
      configurable: true
    });
  };
}

class User {
  @MinLength(3)
  username!: string;
}