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我从 MDN 得到了这个示例代码(稍微修改以打印结果):
"use strict";
class myDate extends Date {
constructor() {
super();
}
getFormattedDate() {
var months = ['Jan','Feb','Mar','Apr','May','Jun','Jul','Aug','Sep','Oct','Nov','Dec'];
return this.getDate() + "-" + months[this.getMonth()] + "-" + this.getFullYear();
}
}
var testDate = new myDate();
document.write(testDate.getFormattedDate());它适用于 Google Chrome 浏览器和 node5，但是当我使用 babeljs 将它转译为 ES5(ES2015 预设)时它不起作用:
"use strict";
var _createClass = function () { function defineProperties(target, props) { for (var i = 0; i < props.length; i++) { var descriptor = props[i]; descriptor.enumerable = descriptor.enumerable || false; descriptor.configurable = true; if ("value" in descriptor) descriptor.writable = true; Object.defineProperty(target, descriptor.key, descriptor); } } return function (Constructor, protoProps, staticProps) { if (protoProps) defineProperties(Constructor.prototype, protoProps); if (staticProps) defineProperties(Constructor, staticProps); return Constructor; }; }();
function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }
function _possibleConstructorReturn(self, call) { if (!self) { throw new ReferenceError("this hasn't been initialised - super() hasn't been called"); } return call && (typeof call === "object" || typeof call === "function") ? call : self; }
function _inherits(subClass, superClass) { if (typeof superClass !== "function" && superClass !== null) { throw new TypeError("Super expression must either be null or a function, not " + typeof superClass); } subClass.prototype = Object.create(superClass && superClass.prototype, { constructor: { value: subClass, enumerable: false, writable: true, configurable: true } }); if (superClass) Object.setPrototypeOf ? Object.setPrototypeOf(subClass, superClass) : subClass.__proto__ = superClass; }
var myDate = function (_Date) {
_inherits(myDate, _Date);
function myDate() {
_classCallCheck(this, myDate);
return _possibleConstructorReturn(this, Object.getPrototypeOf(myDate).call(this));
}
_createClass(myDate, [{
key: 'getFormattedDate',
value: function getFormattedDate() {
var months = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'];
return this.getDate() + "-" + months[this.getMonth()] + "-" + this.getFullYear();
}
}]);
return myDate;
}(Date);
var testDate = new myDate();
document.write(testDate.getFormattedDate());为什么？
我在 babel docs 中找到了答案:
Partial support
Built-in subclassability should be evaluated on a case-by-case basis as classes such as HTMLElement can be subclassed while many such as Date, Array and Error cannot be due to ES5 engine limitations.
如果 babel 在看到 native 对象扩展(它不支持)时抛出错误，而不是假装一切顺利并产生错误代码，我将不胜感激:/