[ECMAScript] 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 = /*#__PURE__*/function () {
            function A() {
                this.a = 1;
                this.b = 2;
            }

            var _proto = A.prototype;

            _proto.print = function print() {
                console.log('print');
            };

            return A;
        }();

        var B = /*#__PURE__*/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);