一、ECMAScript6引入了一套新的关键字用来实现class。
    1、这些新的关键字包括class、constructor、static、extends和super

    1. "use strict";
    3. class Polygon {
    4.   constructor(height, width) {
    5.     this.height = height;
    6.     this.width = width;
    7.   }
    8. }
    10. class Square extends Polygon {
    11.   constructor(sideLength) {
    12.     super(sideLength, sideLength);
    13.   }
    14.   get area() {
    15.     return this.height * this.width;
    16.   }
    17.   set sideLength(newLength) {
    18.     this.height = newLength;
    19.     this.width = newLength;
    20.   }
    21. }
    23. var square = new Square(2);

    待整理：见见：https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Classes