override constructor

✅ case 1：none has constructor

// no constructors
class A {}
class B extends A {}

let b = new B();        // ✅ OK

❌ case 2：returning from derived constructor without calling super() first

class A2 {}

class B2 extends A2 {
    constructor(){
        // ---------------------------------------------------
        // ⛔ ReferenceError: 
        //   Must call `super()` constructor in derived class 
        //   ☐ before accessing `this` or 
        //   ☑ returning from derived constructor
        // ---------------------------------------------------
    }
}

let b2 = new B2();

❌ case 3：accessing this before calling super()

class A3 {}

// (accessing `this` before calling `super()`)
class B3 extends A3 {
    constructor(){
        // ---------------------------------------------------
        // ⛔ ReferenceError: 
        //   Must call `super()` constructor in derived class 
        //   ☑ before accessing `this` or 
        //   ☑ returning from "derived constructor"
        // ---------------------------------------------------
        this.name = "Joe"
    }
}

let b3 = new B3();

✅ case 4：

class A4 {}

class B4 extends A4 {
    constructor(){
        
        // ⭐ "derived constructor" expects "parent (base) constructor"
        //    to create an empty object and assign it to `this`.
        super();

        // ✅ now it's OK to access `this`.
        this.name = "Joe";
    }
}

let b4 = new B4();
log(b4.name);            // Joe

🔸 default derived constructor

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
}

👉 JavaScript.info

🔸 derived constructor

When a regular function is executed with new, it creates an empty object and assigns it to this.
But when a derived constructor runs, it doesn’t do this. It expects the parent (base) constructor to do the job.
So a derived constructor must call super in order to execute its parent (base) constructor, otherwise the object for this won’t be created. And we’ll get an error.

👉 JavaScript.info

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Last updated 2 years ago