Code that adheres to OCP

adheres-to-ocp.js

class Circle {
  constructor (radius) {
    this.radius = radius
  }

  area () {
    return 2 * Math.PI * Math.pow(shape.radius, 2)
  }
}

class Square {
  constructor (length) {
    this.length = length
  }

  area () {
    return Math.pow(shape.length, 2) 
  }
}

class AreaCalculator {
  constructor (shapes) {
    this.shapes = shapes
  }

  sum () {
    return this.shapes
      .map((shape) => {
        if (typeof shape.area === 'function') {
          return shape.area()
        } else {
          throw new Error('The area you supplied does not conform to the area interface')
        }
      })
      .reduce((totalArea, currentArea) => {
        return totalArea + currentArea
      }, 0)
  }
}