语法记录

keyof 与 in

export type ReducersMapObject<S = any, A extends Action = Action> = {
  [K in keyof S]: Reducer<S[K], A>
}

Never(nothing) 与 unknown(all)

T | never ⇒ T
T & unknown ⇒ T

三目结合 nerver 做校验

type StateFromReducersMapObject<M> = M extends ReducersMapObject
  ? { [P in keyof M]: M[P] extends Reducer<infer S, any> ? S : never }
  : never

infer: 推测三方的类型

type PromiseReturnType<T> = T extends Promise<infer Return> ? Return : T
type t = PromiseReturnType<Promise<string>> // string 

一些好用的

type A = typeof B;
type A = ReturnType<typeof Fn>;
type A = {
  [K in keyof S]: B;
}