The MS blog post, and actual proposal covers this fairly well imho: Typescript's type checking is slow and doesn't need to happen at runtime. Additionally, it

The MS blog post, and actual proposal covers this fairly well imho: Typescript's type checking is slow and doesn't need to happen at runtime. Additionally, it moves failures to users rather than developers and makes it incredibly difficult to change type semantics later on. https://devblogs.microsoft.com/typescript/a-proposal-for-type-syntax-in-javascript/ https://devblogs.microsoft.com/typescript/a-proposal-for-typ... https://github.com/giltayar/proposal-types-as-comments/ https://github.com/giltayar/proposal-types-as-comments/