function required(target: any, propertyKey: string) { let value: any; const getter = function() { if (value === undefined) { throw new Error(`Property ${propertyKey} is required`); } return value; }; const setter = function(newVal: any) { value = newVal; }; Object.defineProperty(target, propertyKey, { get: getter, set: setter, enumerable: true, configurable: true }); } class User { @required username: string; @required email: string; age?: number; constructor(username: string, email: string) { this.username = username; this.email = email; } } const user1 = new User('johndoe', 'john@example.com'); console.log('User created');