KeyboardLayoutMap:keys() 方法
Limited availability
This feature is not Baseline because it does not work in some of the most widely-used browsers.
KeyboardLayoutMap 接口的 keys() 方法返回一个新的迭代器对象,该对象包含 KeyboardLayoutMap 对象中每个索引的键。
该方法与 Map.prototype.keys() 相似。