Example typed redux middleware

pledge-middleware.ts

export enum PledgeStatus {
  Starting = "Pledge_Starting",
  Completed = "Pledge_Completed",
  Error = "Pledge_Error"
}

export type Pledge<T> =
  | { status: PledgeStatus.Starting }
  | { status: PledgeStatus.Completed; result: T }
  | { status: PledgeStatus.Error; error: Error };

export type PledgeToPromise<AP> = {
  [P in keyof AP]: "pledge" extends P ? (AP[P] extends Pledge<infer R> ? Promise<R> : AP[P]) : AP[P];
};

interface WithPromise<R> {
  pledge: Promise<R>;
}
// tslint:disable-next-line:no-any
function hasPledge<R>(obj: any): obj is WithPromise<R> {
  return "pledge" in obj && obj.pledge && obj.pledge instanceof Promise;
}

export function pledgeMiddleware<A>(dispatch: (a: A) => void): (action: PledgeToPromise<A>) => void {
  return action => {
    if (hasPledge(action)) {
      // tslint:disable-next-line:no-any
      const anyAction: any = action;
      dispatch({ ...anyAction, pledge: { status: PledgeStatus.Starting } });
      action.pledge
        .then(r => {
          dispatch({
            ...anyAction,
            pledge: { status: PledgeStatus.Completed, result: r }
          });
        })
        .catch((e: Error) => {
          dispatch({
            ...anyAction,
            pledge: { status: PledgeStatus.Error, error: e }
          });
        });
    } else {
      // tslint:disable-next-line:no-any
      return dispatch(action as any);
    }
  };
}