ocp.js

/**
 * Open/Closed Principle (OCP): Software entities should be open 
 * for extension but closed for modification.
 */
class AreaCalculator { // BAD CODE
    calculate(shape) {
        if (shape instanceof Square) {
            return shape.width * shape.width;
        }else if(shape instanceof Circle) {
            return Math.PI * shape.radius * shape.radius;
        }else if(shape instanceof Rectangle) {
            return shape.width * shape.height;
        }else if(shape instanceof Triangle) {
            return shape.base * shape.height / 2;
        }else if(shape instanceof Ellipse) {
            return Math.PI * shape.radiusX * shape.radiusY;
        }
    }
}

// Good Practice
class Shape {
    area() {
        throw new Error('Area method should be implemented');
    }
}

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

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

class Rectangle extends Shape {
    constructor(width, height) {
        super();
        this.width = width;
        this.height = height;
    }

    area() {
        return this.width * this.height;
    }
}

class Ellipse extends Shape {
    constructor(radiusX, radiusY) {
        super();
        this.radiusX = radiusX;
        this.radiusY = radiusY;
    }

    area() {
        return Math.PI * this.radiusX * this.radiusY;
    }
}
const square = new Square(4);
const rectangle = new Rectangle(4, 5);
const ellipse = new Ellipse(4, 5);
console.log(rectangle.area()); // 20
console.log(square.area()); // 16
ellipse.area(); // 62.83185307179586