Inheritance in JavaScript

class Base {
  constructor(x) {
    this.x = x;
  }

  greet() {
    return `Base: ${this.x}`;
  }
}

class Derived extends Base {
  constructor(x, y) {
    super(x);       // must run before using `this`
    this.y = y;
  }

  greet() {
    return `${super.greet()} and Derived: ${this.y}`;
  }
}

class Shape {
  constructor(name) {
    this.name = name;
  }

  area() {
    return 0;
  }

  describe() {
    return `${this.name} has area ${this.area()}`;
  }
}

class Circle extends Shape {
  constructor(radius) {
    super("Circle");
    this.radius = radius;
  }

  area() {
    return Math.PI * this.radius ** 2;
  }
}

class Square extends Shape {
  constructor(side) {
    super("Square");
    this.side = side;
  }

  area() {
    return this.side * this.side;
  }
}

const shapes = [new Circle(2), new Square(3)];
shapes.forEach(s => console.log(s.describe()));

console.log(shapes[0] instanceof Shape);
console.log(shapes[0] instanceof Circle);
console.log(shapes[1] instanceof Circle);

class Broken extends Shape {
  constructor(name) {
    this.name = name; // uses `this` before super()
  }
}
try {
  new Broken("test");
} catch (e) {
  console.log(e instanceof ReferenceError);
}

Circle has area 12.566370614359172
Square has area 9
true
true
false
true