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

class Square extends Shape {
    constructor(sideLength) {
        super(4);
        this.sideLength = sideLength;
    }
}

let square = new Square(5);
console.log("lineCount is", 
    square.lineCount);
console.log("sideLength is",
    square.sideLength);

// *** up to ES6 ***
function OldShape(lineCount) {
    this.lineCount = lineCount;
}

function OldSquare(sideLength) {
    OldShape.apply(this, [4]);
    this.sideLength = sideLength;
}

OldSquare.prototype = 
Object.create(OldShape.prototype);
OldSquare.prototype.constructor = OldSquare;

let oldSquare = new OldSquare(5);
console.log("oldSquare:");
console.log(oldSquare.lineCount);
console.log(oldSquare.sideLength);