Class

class
  1. 선언
       class Person{
            consturctor(name, age){
                 this.name = name;
                 this.age = age;
               }
          
             speak(){
                console.log(`${this.name} : hello`)
               }
            }


   2. object 생성
         const ellie = new Person('ellie', 20);
            // ellie, 20이 this.name, this.age로 전달

   3. getter / setter
          - 사용자가 잘못된 값을 입력했을 때, 
            그것을 방지하기 위해 사용

          - class Person{
                constructor(name, age){
                    this.name = name;
                    this.age = age;
                 }
  

                get age(){
                   // getter를 추가하면 this.age를 사용하려는 순간 this.age의 값을 불러오지 않고,
                       getter를 호출함
                   return this.age;
                 }

                set age(value){ 
                   //setter를 추가하면, 아래에 적힌 this.age에 값을 할당하려는 순간,
                     값을 할당하지 않고 setter(set age(value))를 호출함 
                         - 개발자가 사전에 정한 setter의 조건을 만족하는지 확인하기 위해
                    if(value<0){
                       console.log("age cannot be lower than 0"):
                     }

                    this.age = (value<0)?0:value;
                  }

                 speak(){
                     console.log(`name : ${this.name}, age : ${this.age}`);
                   }

                  const p1 = new Person('kim', 20);
                  p1.speak();

        3-1. getter / setter 사용법
              위 코드를 사용하면 오류가 발생하는 이유
                  A. setter의 경우
                        - new Person('kim', 20)을 통해 age에 값 할당시키려고 함
                        - set age(value) 호출
                        - if문 뒤, this.age = (value<0)?0:value;를 통해 this.age에 값 할당하려고 함
                        - set age(value) 호출
                        - if문 뒤, this.age = (value<0)?0:value를 통해 this.age에 값 할당하려고 함
                        - set age(value) 호출
                        - ......

                 B. getter의 경우
                         - p1.speak()를 통해 this.age를 사용하려고 함
                         - get age() 호출
                         - 내부의 return this.age를 통해 this.age 사용하려고 함
                         - get age() 호출
                         - 내부의 return this.age를 통해 this.age 사용하려고 함
                         - get age() 호출
                         - ....

              이를 해결하기 위해서 get, set에 _를 붙여주자!
                  - get age(){
                        return this._age
                      }

                  - set age(value){
                         ...this._age = ....;
                      }

              _는 private, 또는 내부적인 속성이라는 뜻
              
              const p1 = new Person("kim", -1) 을 통해 this.age에 -1을 할당하려는 순간,
              set age(value)가 호출(이 때, value로는 -1이 들어갑니다)
              this._age에 -1이 할당되고
              p1.speak()를 통해 this.age가 사용되려는 순간, 
              get age()가 호출되어 this.age에 this._age의 값인 -1이 반환됨


   3. public / private
          - class Experiment{
                 publicfield = 2;
                 #privatefield = 0;
             }
            const experiment = new Experiment();
            console.log(experiment.publicfield)
            console.log(experiment.privatefield)

           생성자를 쓰지 않고 class 내부에 field를 만들면
           클래스의 필드에 대해 외부에서 접근이 가능하지만
           #을 붙이면, 클래스 외부에서 접근 및 수정이 불가능해짐

    4. static
          - class Article {
                 static publisher = 'Dream Coding'
                 constructor(articleNumber){
                     this.articlenNumber = articleNumber;
                  }
                  
                 static printPublisher(){
                    console.log(Article.publisher)
                   }
                }

           const article1 = new Article(1)
           const article2 = new Article(2)
           console.log(article1.publisher) // undefined
           console.log(Article.publisher) // Dream Coding

           class 내부에 static으로 선언된 field, method는 
           object에 할당되는 것이 아닌, class 원본에만 있는 것이여서
           article1.publisher이 undefined가 나옴

    5. 상속 / 다양성
           - class Shape{
                 constructor(width, height, color)
                      this.width = width;
                      this.height = height;
                      this.color = color;
                
                 draw()
                    console.log(`drawing ${this.color} color of`)
                      
                 getArea (){
                      return this.width * this.height;
                 }
               }

              위는 도형에 관한 class를 선언한 것
              이는 삼각형, 사각형 등 다양한 도형 관련 
              class를 만들 때, 미리 공통된 속성이 있는 
              class를 만들어서 보다 적게 기술하기 위함

         class Shape의 것을 쓰고 싶다면
         class Rectangle extends Shape{} 로 작성, 
         이를 상속이라고 함(class (새 클래스 이름) extends (원본 class 이름))
         
         다양성
              - 도형의 넓이를 구한다 가정했을 때,
                사각형과 삼각형의 넓이 구하는 공식이 다름
                따라서, 각 class 마다 getArea method를 재구성해줘야 함
                     - class Triangle extends Shape{
                           getArea(){
                                return this.width * this.height / 2;
                            }
                위와 같이 상속받은 class의 method를 재정의한 것을
                메서드 오버라이딩 이라고 함(이 때, 원본의 원래 method는 호출 X)
                     cf) 오버라이딩했지만, 원본의 메소드를 호출하고 싶다면 super 키워드 이용
                             - super.draw();

           

      6. instanceOf(왼쪽의 객체가 오른쪽 class에서 만들어진 것인지 boolean 값 return)
               console.log(rectangle instanceOf Rectangle)
                     // t
               console.log(triangle instanceOf Shape)
                     // Triangle은 Shape에서 상속되었으므로
                         t