TypeScript 中的类型级扩展是什么？

Question

While reading the TypeScript handbook, I came across the first sentence at the top of the page on enums :

Enums are one of the few features TypeScript has which is not a type-level extension of JavaScript.

What does this mean? What is a type-level extension?

Answer 1

TypeScript was designed from the ground up to be a superset of Javascript, with the intention to have no runtime impact.

The design goals document should give you a pretty good idea of what design principles TypeScript is based on.

Pretty much everything you write in TypeScript which has to do with types (another way to put it is, everything that wouldn't be valid JS syntax) is only used by tsc at compile time, but disappears at runtime.

enums are an exception to that, as they actually result in some JS code being produced and used at runtime .