6. Static Method

static method
  - 프로토타입에 정의된 것이 아닌, constructor에 정의된 method
       - 따라서, static method는 instance가 사용 불가함, inherit되지 않음
  - ex) Array.from은 Array.from(...) 은 사용할 수 있지만,
                           [1,2,3,4,5].from(...) 은 사용 불가

  - ex2) Number.parseInt(12.3)은 되지만, 12.parseInt(...)은 사용 불가

  - 사용법
       - class Person{
             constructor(age){
                this.age = age;
              }
             
              printAge(){ // instance method : prototype에 추가돼 instance가 사용 가능
                console.log(this.age)
              }

              static hey(){
                  console.log('hi!'); // Person.hey() 시 사용
               }
            }