Class

• 有三種 Access Modifiers 可以用：public、private、protected

  protected: 類似 private，但在子類別裡面可以被調用

• 繼承用關鍵字 extends

• readonly: 不能再被修改

• 基本寫法

  class Animal {
    public readonly name: string
    constructor(name: string) {
      this.name = name
    }
  
    public SayHi() {
      console.log(`${this.name} say hi`)
    }
  }
  
  const animal = new Animal('test')
  console.log(animal)
  
  // readonly 不能被修改，會報錯
  animal.name = 'test2'

• static 靜態屬性、方法

  不需要 instance，可以直接用 class 名稱呼叫

  class Animal {
    public name: string;
    public static order = 1;
  
    constructor(name: string) {
      this.name = name;
    }
  
    static SayHi() {
      console.log("Hi!");
    }
  }
  
  Animal.SayHi();
  console.log(Animal.order);

• Abstract Class

  • 作為基底

  • 方法可以實現或寫成 abstract

  • 寫成 abstract 的好處是可以強迫 extends 的 class 都要實現方法

    abstract class Animal {
      // 子類別才可調用
      protected name: string
    
      constructor(name: string) {
        this.name = name
      }
    
      abstract sayHi(): void
    }
    
    class Dog extends Animal {
      // 如果建構子一樣，可以不寫
      constructor(name: string) {
        super(name)
      }
    
      // need to implement
      sayHi(): void {
        console.log(`${this.name} say hi!`)
      }
    }
    
    const dog: Dog = new Dog('Doggy')
    dog.sayHi()

• Parameter Properties（精簡的寫法）

  把修飾詞寫在建構子裡面

  class Student {
    constructor(public name: string, private age: number) {}
  }
  
  const student = new Student('Sam', 27)
  
  console.log(student.name)

  等同此寫法

  class Student {
    name: string;
    age: number;
    constructor(name: string, age: number) {
      this.name = name;
      this.age = age;
    }
  }

• Strict Class Initialization

  屬性要在建構子直接給值或有初始值

  class C {
    foo: number;
    bar = "hello";
    baz: boolean;
    //  ~~~
    //  Error! Property 'baz' has no initializer and is not definitely assigned in the
    //         constructor.
    constructor() {
      this.foo = 42;
    }
  }

  因為在 _initialize() 會間接給 foo 值，所以在 foo 屬性加上 ! (definite assignment assertion)

  class C {
    foo!: number;
    // ^
    // Notice this '!' modifier.
    // This is the "definite assignment assertion"
    constructor() {
      this._initialize(); // 間接初始化
    }
  
    private _initialize() {
      this.foo = 0;
    }
  }

  Ref: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-7.html