https://blog.wongcw.com/2018/10/01/typescript-3-1-1-%e5%8f%91%e5%b8%83%ef%bc%8c%e5%be%ae%e8%bd%af%e6%8e%a8%e5%87%ba%e7%9a%84-javascript-%e8%b6%85%e9%9b%86/