block16.ts

type IActionType<X> = X & { __actionType: string; };
    
    const _devSet: { [key: string]: any } = {};
    // __actionType is used as the sentinal value in the `isAction` type guard 
    // functions. We need this so we can simply use the action creator 
    // function as the action type itself.
    // typePrefix allows different types of actions (middleware) to avoid any collisions
    function makeAction<Z extends {}>(type: string, typePrefix = '') {
      // Helpful check against copy-pasting duplicate type keys when creating 
      // new actions.
      if (__DEV__) {
        if (_devSet[type]) {
          throw new Error(
            'Attempted creating an action with an existing type key. ' + 
            'This is almost cetainly an error.',
          );
        }
        _devSet[type] = type;
      }
      return <X extends (...args: any[]) => Z>(fn: X) => {
        const returnFn: IActionType<X> = (
          (...args: any[]) => ({ ...(fn as any)(...args), type })
        ) as any;
        returnFn.__actionType = typePrefix + type;
        return returnFn;
      };
    }
    
    // This version forces action to have a `payload` key. You may need to
    // adjust it to your needs, or remove the `extends` suffix entirely.
    export function isAction<T extends { payload: any }>(
      action: Action,
      actionCreator: IActionType<(...args: any[]) => T>,
    ): action is T & Action {
      return action.type === actionCreator.__actionType;
    }