有关枚举类型与非枚举类型的区分

感觉解释得挺好的，例子让人看着很明白
附上链接：https://blog.csdn.net/web_zhouliang/article/details/78710034
https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Object/keys
https://blog.csdn.net/IT_HLM/article/details/79555118?utm_medium=distribute.pc_relevant_t0.none-task-blog-
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1.control&dist_request_id=1330144.6567.16180236839763441&depth_1-utm_source=distribute.pc_relevant_t0.none-
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