Redux combineReducer源码TS数据类型约束

export default function combineReducers<M>(
  reducers: M
): M[keyof M] extends Reducer<any, any, any> | undefined
  ? Reducer<
      StateFromReducersMapObject<M>,
      ActionFromReducersMapObject<M>,
      Partial<PreloadedStateShapeFromReducersMapObject<M>>
    >
  : never

export type Action<T extends string = string> = {
  type: T
}

export interface UnknownAction extends Action {
  // Allows any extra properties to be defined in an action.
  [extraProps: string]: unknown
}

export type Reducer<
  S = any,
  A extends Action = UnknownAction,
  PreloadedState = S
> = (state: S | PreloadedState | undefined, action: A) => S

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

export type ActionFromReducersMapObject<M> = ActionFromReducer<
  ReducerFromReducersMapObject<M>
>

export type ReducerFromReducersMapObject<M> = M[keyof M] extends
  | Reducer<any, any, any>
  | undefined
  ? M[keyof M]
  : never

export type PreloadedStateShapeFromReducersMapObject<M> = M[keyof M] extends
  | Reducer<any, any, any>
  | undefined
  ? {
      [P in keyof M]: M[P] extends (
        inputState: infer InputState,
        action: UnknownAction
      ) => any
        ? InputState
        : never
    }
  : never

参考文献：

1. TypeScript -- 函数
2. redux/src/types/reducers.ts
3. redux/src/combineReducers.ts
4. Index signature is missing in type (only on interfaces, not on type alias)