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function User(name, birthday) {
	this.name = name;
	this.birthday = birthday;

	// age is calculated from the current date and birthday
	Object.defineProperty(this, "age", {
    	get() {
    		let todayYear = new Date().getFullYear();
    		return todayYear - this.birthday.getFullYear();
    	}
	});
}

let john = new User("John", new Date(1992, 6, 1));

console.log( john.birthday ); // birthday is available
console.log( john.age );      // ...as well as the age