参考 MDN 中关于 forEach 的介绍部分。  * [Array.prototype.forEach() - JavaScript | MDN](https://link.segmentfault.com/?enc=WKHDeOWovaj5EHS0Dhtz2g%3D%3D.aXFqC4Pu3Jpi5%2B3XVKPLpXGlv8mQP0rBoK%2BdGVX3RxxRiNK1H%2Bh5hUqN8oKceQN0y9IogtHW91QSjgX6IRvwIjTcOgQ8YJ5qtyjc5DxJ7tSsKO%2FxuFCJSy7Zr7lboCMz)