这种算语言特性吧,就像为数组不存在的下标赋值时,也会自动扩容一样 点语法也是通过 `[[GetOwnProperty]]` 取值的,在 `ECMA-262` 文档中有对此作出相关描述  > ECMA 文档链接 [ECMA-262 10 Ordinary and Exotic Objects > Behaviours](https://link.segmentfault.com/?enc=UfTlGpua7OayJ0GHKaO4ww%3D%3D.Vt47e8vlILzgmQQ6U18FV5RcMVniPpURrdVqRjZAfcEVuQ7YdQ853sA%2BNI%2BRzlhB9ul7gknlj5i0HqT1hCV6ezHfgNazzYuCDT2fI9dR0Ss%3D)