举例说明你对es6的extends的理解

es6引入的class其实本质上就是一个函数，而extends关键字则是通过原型实现继承，请看以下例子：

 class A { constructor() {     this.a = 1;     this.b = 2; }        }        class B extends A { constructor() {     this.c = 3;     this.d = 4; }        }

es6代码经过babel编译后转化为es5的兼容性代码如下：

 "use strict";        function _inheritsLoose(subClass, superClass) { subClass.prototype = Object.create(superClass.prototype); subClass.prototype.constructor = subClass; subClass.__proto__ = superClass;        }        var A = function () { function A() {     this.a = 1;     this.b = 2; } var _proto = A.prototype; _proto.print = function print() {     console.log('print'); }; return A;        }();        var B = function (_A) { _inheritsLoose(B, _A); function B() {     var _this;     _this = _A.call(this) || this;     _this.c = 3;     _this.d = 4;     return _this; } return B;        }(A);