// 定义类
 class Point() {

  constructor(x, y) {
   this.x = x;
   this.y = y;
  }

  toString() {
   return '(' + this.x + ', ' + this.y + ')';
  }
 }

 var point = new Point(2, 3);
 point.toString(); //(2, 3)

Class ColorPoint extends Point {
 constructor(x, y, color) {
  super(x, y); //等同于super.constructor(x, y)
  this.color = color;
 }

 toString() {
  return this.color + '' + super();
 }
}

 //profile.js
 export var firstName = "Pandora";
 export var lastName = "G.Dragon";
 export var year = 1973;

 //export还有下面这种写法，两者是等价的
 var firstName = "Pandora";
 var lastName = "G.Dragon";
 var year = 1973;
 export({firstName, lastName, year});

 import {firstName, lastName, year} from './profile';

 function setHeader(element) {
  element.textContent = firstName + '' + lastName;
 }

 import {someMethod, another as newName} from './exporter';

 // circle.js

 // 方法-1: 返回圆的面积
 export function area(radius) {
  return Math.PI * radius * radius; 
 }
 // 方法-2： 返回圆的周长
 export function circumference(radius) {
  return 2 * Mathi.PI * radius;
 }

 // mian.js
 import {area, circumference} from 'circle';

 console.log("圆面积： " + area(4));
 console.log("圆周长： " + circumference(14));

 // main.js
 module circle from 'circle';

 console.log("圆面积： " + circle.area(4));
 console.log("圆周长： " + circle.circumference(14));

 // export-default.js
 export default function foo() { // foo()就是这个模块的默认方法
  console.log('foo');
 }

 // import-default.js
 import customName from './export-default';
 customName(); //'foo'

 export default 42;

 //circleplus.js
 export * from 'circle'; // "export *"表示输出circle模块的所有属性和方法
 export var e = 2.71828;
 export default function(x) {
  return Math.exp( x );
 }

 export {area as circleArea } from 'circle';

 //main.js
 module math from 'circleplus';
 import exp from "circleplus"; // "import exp"表示将circleplus模块的默认方法加载为exp方法。
 console.log( exp(math.pi) );