extend.js

"use strict";

class Rectangle { 
  constructor(length,width) {
    this.length = length;
    this.width = width;
  }
  
  area() {
    console.log(this.width * this.length);
  }
}

class Square extends Rectangle {
  constructor(length){
    //super();
    this.length= length;
  }
  area() {
    console.log(this.length * this.length );
  }
}

var r = new Rectangle(10,44); 
 r.area();
 
 
var s = new Square(10);
 s.area();