1. ES2015 class getter setter

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;