typescript - 未强制执行字符串文字类型参数

Question

对于 Redux(虽然这个问题与 Redux 没有任何关系)我想知道 reducer 使用的 Action 的名称，但我想确保 reducer 中使用的名称和 Action 是一致的。所以我写了这段代码:

interface TypedAction<N> {
  type: N;
}

type TypedReducer<S, A extends TypedAction<any>> = (state: S, action: A) => S;

function addReducer<S, R extends TypedReducer<S, A>, A extends TypedAction<N>, N>(initialState: S, actionName: N, reducer: R): {} {
  // doesn't really matter what is returned here
  // the point is I need the actionName during run time
  // but want it to correspond with the reducer's action's name at compile time
  return {
    [actionName.toString()]: reducer
  };
}

但是当我尝试一个例子时:

interface MyAction extends TypedAction<'MyAction'> {
  foo: string;
}

const myActionReducer: TypedReducer<number, MyAction> = (state: number, action: MyAction) => state+1;

addReducer(1, "foo", myActionReducer); // should give a compile error, because "foo" and is not of type "MyAction"

为什么 Typescript 不强制 "foo" 应该是 "MyAction"？

Answer 1

interface TypedAction<T extends string> {
  type: T;
}

type TypedReducer<S, A extends TypedAction<any>> = (state: S, action: A) => S;

interface MyAction extends TypedAction<"MyAction"> {
  foo: number;
}

type ActionTypeAndReducer<S, A extends TypedAction<any>> = {
  [type: string]: TypedReducer<S, A>
};

function pair<ActionType extends string,
              A extends TypedAction<ActionType>,
              S>(type: ActionType, reducer: TypedReducer<S, A>): ActionTypeAndReducer<S, A> {
  return {
    [type as string]: reducer
  };
}

const myReducer: TypedReducer<any, MyAction> = (state: any, action: MyAction) => {};
pair("MyAction2", myReducer);

这将产生预期的行为。

error TS2345: 
   Argument of type 'TypedReducer<any, MyAction>' is not assignable to parameter
   of type 'TypedReducer<any, TypedAction<"MyAction2">>'.

   Types of parameters 'action' and 'action' are incompatible.                                                                                                                           
   Type 'TypedAction<"MyAction2">' is not assignable to type 'MyAction'.                                                                                                             
   Property 'foo' is missing in type 'TypedAction<"MyAction2">'.                                                                                                                 

我认为结合了 action 和 reducer 的函数可以检查这一点，所以我构建了 pair 函数。 类型没问题，但编译器提示说 type argument 必须是 string 或 number 因为它是返回对象中的键。所以我让 ActionType 扩展字符串，然后剩下的就没问题了。