ES 6のクラスとES 5のコントラスト

class Shape {
    constructor (id, x, y) {
        this.id = id
        this.move(x, y)
    }
    move (x, y) {
        this.x = x
        this.y = y
    }
}

var Shape = function (id, x, y) {
    this.id = id;
    this.move(x, y);
};
Shape.prototype.move = function (x, y) {
    this.x = x;
    this.y = y;
};

class Rectangle extends Shape {
    constructor (id, x, y, width, height) {
        super(id, x, y)
        this.width  = width
        this.height = height
    }
}
class Circle extends Shape {
    constructor (id, x, y, radius) {
        super(id, x, y)
        this.radius = radius
    }
}

var Rectangle = function (id, x, y, width, height) {
    Shape.call(this, id, x, y);
    this.width  = width;
    this.height = height;
};
Rectangle.prototype = Object.create(Shape.prototype);
Rectangle.prototype.constructor = Rectangle;
var Circle = function (id, x, y, radius) {
    Shape.call(this, id, x, y);
    this.radius = radius;
};
Circle.prototype = Object.create(Shape.prototype);
Circle.prototype.constructor = Circle;

class Shape {
    …
    toString () {
        return `Shape(${this.id})`
    }
}
class Rectangle extends Shape {
    constructor (id, x, y, width, height) {
        super(id, x, y)
        …
    }
    toString () {
        return "Rectangle > " + super.toString()
    }
}
class Circle extends Shape {
    constructor (id, x, y, radius) {
        super(id, x, y)
        …
    }
    toString () {
        return "Circle > " + super.toString()
    }
}

var Shape = function (id, x, y) {
    …
};
Shape.prototype.toString = function (x, y) {
    return "Shape(" + this.id + ")"
};
var Rectangle = function (id, x, y, width, height) {
    Shape.call(this, id, x, y);
    …
};
Rectangle.prototype.toString = function () {
    return "Rectangle > " + Shape.prototype.toString.call(this);
};
var Circle = function (id, x, y, radius) {
    Shape.call(this, id, x, y);
    …
};
Circle.prototype.toString = function () {
    return "Circle > " + Shape.prototype.toString.call(this);
};

class Rectangle extends Shape {
    …
    static defaultRectangle () {
        return new Rectangle("default", 0, 0, 100, 100)
    }
}
class Circle extends Shape {
    …
    static defaultCircle () {
        return new Circle("default", 0, 0, 100)
    }
}
var defRectangle = Rectangle.defaultRectangle()
var defCircle    = Circle.defaultCircle()

var Rectangle = function (id, x, y, width, height) {
    …
};
Rectangle.defaultRectangle = function () {
    return new Rectangle("default", 0, 0, 100, 100);
};
var Circle = function (id, x, y, width, height) {
    …
};
Circle.defaultCircle = function () {
    return new Circle("default", 0, 0, 100);
};
var defRectangle = Rectangle.defaultRectangle();
var defCircle    = Circle.defaultCircle();

class Rectangle {
    constructor (width, height) {
        this._width  = width
        this._height = height
    }
    set width  (width)  { this._width = width               }
    get width  ()       { return this._width                }
    set height (height) { this._height = height             }
    get height ()       { return this._height               }
    get area   ()       { return this._width * this._height }
}
var r = new Rectangle(50, 20)
r.area === 1000

var Rectangle = function (width, height) {
    this._width  = width;
    this._height = height;
};
Rectangle.prototype = {
    set width  (width)  { this._width = width;               },
    get width  ()       { return this._width;                },
    set height (height) { this._height = height;             },
    get height ()       { return this._height;               },
    get area   ()       { return this._width * this._height; }
};
var r = new Rectangle(50, 20);
r.area === 1000;