minte9
LearnRemember   JAVASCRIPT





Extend

Inheritance is a way for one class to extend another class. 
 
/**
 * Inheritance, one class extends another class
 */

 class Animal  {
    constructor(name) {
        this.speed = 0;
        this.name = name;
    }

    run(speed) {
        this.speed = speed;
        console.log(`${this.name} runs, speed ${this.speed}`);
    }

    stop() {
        this.speed = 0;
        console.log(`${this.name} stops`); // backsticks to embed vars
    }
}

class Rabbit extends Animal {
    hide() {
        super.stop(); // Look Here
        console.log(`${this.name} hides!`);
    }
}

let rabbit = new Rabbit("A");

rabbit.run(100);    // A runs, speed 100
rabbit.stop();      // A stops
rabbit.run(50);     // A runs, speed 50
rabbit.hide();      // A stops
                    // A hides!

Super

Constructors in inheriting classes must call super() before using this. 
 
/**
 * If a class extends another class and has no constructor,
 * then a default constructor is generated
 *      constructor(...args) { super(...args); }
 * 
 * We get an error when trying to add a custom constructor 
 * without calling super() first
 */

class A {
    constructor(name) {
        this.speed = 0;
        this.name = name;
    }
}

class X extends A {
    constructor(name) {
        super(name); // Look Here
        this.name = name.toUpperCase();
    }
}

class Y extends A {
    constructor(name) { // no super()
        this.name = name.toUpperCase();
    }
}

let a = new X("aaa");
console.log(a.name); // AAA

try {
    let b = new Y("bbb"); // doesnt' work
    console.log(b.name);
} catch (err) {
    console.log(err.name);      // ReferenceError
    console.log(err.message);   // Must call super in constructor
}




References: