🔰inheritance

JS ⟩ value ⟩ object ⟩ class ⟩ inheritance

• access properties on object literal / class's prototype object. (super.prop / super[expr] are valid in any method definition in object literal / class)

• invoke superclass constructor. (super(...args) is valid in class constructors)

// in any method definition
super.prop        // parent's property
super[expr]       // (same)

// ⭐️ subclass
class B extends A { 

    // ⭐️ subclass constructor
    // (if omitted, a default constructor is created automatically)
    constructor(...args) {
        // ⭐️ must call super() before referencing `this`
        super(...args)    // ⭐️ superclass constructor
        // intialize `this` ...
    }
    
    // override method
    method() {
        super.method()        // ⭐️ call parent's method (optional)
        // ...
    }
}

🔴 主題

• derived class init

• override constructor

• override methods

• override class fields

• super - invoke/access superclass/parent's constructor/method/properties.

📗 參考

• JS.info ⟩

  • Class inheritance

  • Prototypal inheritance

  • Class extends Object? - 說明 class A 與 class A extends Object 有何不同 ⭐️

⭐️ 重點

if a class extends another class and has no constructor(), then the following “default” constructor() is generated:

class B extends A {
  constructor(...args) { super(...args) }  // ⭐️ auto-generated
}

differences between class A and class A extends Object

                       class A          class A extends Object
---------------------------------------------------------------
must call super()         No                    Yes
in constructor
---------------------------------------------------------------
A.__proto__         Function.prototype         Object
---------------------------------------------------------------

👉 See: JS.info ⟩ Class extends Object?

👥 相關

• use prototype.