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override class fields

JS ⟩ types ⟩ class ⟩ inheritance ⟩ override class fields

⭐️ class fields initialization：

The class fields are initialized:

base class: before constructor()
derived class: immediately after super()

👉 參閱： derived class init 看完整的「初始化」過程。

💈範例：replit

// base class
class A {
    name = 'A';
    constructor() { log(this.name); }
}

// derived class
class B extends A {

    name = 'B';            // 🔸 overridden class fields

    constructor(){
        // ❗ `this` doesn't exist before `super()`
        super();
        log(this.name);
    }
}

new A();        // "A"
new B();        // "A"❗, "B"

class A {
    name = 'A';
    constructor() { 
        // ⭐️ overridden class fields not fully initailized at this point
        log(this.name); 
    }
}

class B extends A {
    name = 'B';        // ⭐️ overridden class field
}

new A();     // "A"
new B();     // "A"❗

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Last updated 3 years ago