visitor.js

class Shape {
  constructor() {
    this.type = this.constructor.name
  }
}
class Square extends Shape {
  constructor(side) {
    super()
    this.side = side
  }
}
class Circle extends Shape {
  constructor(radius) {
    super()
    this.radius = radius
  }
}
class Rectangle extends Shape {
  constructor(width, height) {
    super()
    this.width = width
    this.height = height
  }
}

const shapes = [new Circle(4.0), new Square(5.0), new Rectangle(6.0, 7.0)]

const matcher = function(pattern) {
  return shape => pattern[shape.type](shape)
}

const area = matcher({
  Square: square => square.side * square.side,
  Circle: circle => circle.radius * circle.radius * Math.PI,
  Rectangle: rect => rect.height * rect.width
})
const totalArea = shapes.reduce((acc, shape) => acc + area(shape), 0)
console.log(`Total area: ${totalArea}`)

const perimeter = matcher({
  Square: square => 4 * square.side,
  Circle: circle => 2 * Math.PI * circle.radius,
  Rectangle: rect => 2 * rect.height + 2 * rect.width
})
const totalPerimeter = shapes.reduce((acc, shape) => acc + perimeter(shape), 0)
console.log(`Total perimeter: ${totalPerimeter}`)