angular2 TS2322 Type 'Promise<void>' is not assignable to type 'Promise<string>' when given a string

Question

Using Angular 2, I have a service that handles my API calls. In the service, I have a handleError function that returns a Promise<string> for the error message. I'm trying to add some functionality to the handleError that will redirect the page afterwards to the login screen if there are credential errors. When I do that, TypeScript throws a TS2322 error even though I'm returning the same Promise and then trying to do some chains off of it.

error TS2322: Type 'Promise<void>' is not assignable to type 'Promise<string>'. Type 'void' is not assignable to type 'string'.

private handleError(error: Error): Promise<string> {
    let message = "Something went wrong loading data from the API.";
    if (error.response.status === 0) {
      message = "Unable to contact server.  Please file a bug report.";
    }
    if (error.message) {
      message = "API Service Error retrieving page: " + error.message;
    }
    this.alerts.error(message);
    if (error.response.status === 403) {
      if (error.message.indexOf("Authentication credentials") !== -1) {
          return Promise.reject<string>(message)  // this line errs with TS2322
          .then(() => this.appState.refresh())
          .then(() => this.redirects.login())
          .then(() => this.alerts.warning("Redirected to login because no credentials found."));
      } else {
          this.redirects.index();
      }
    }
    return Promise.reject<string>(message);
}

Answer 1

      return Promise.reject<string>(message)  // this line errs with TS2322
      .then(() => this.appState.refresh())
      .then(() => this.redirects.login())
      .then(() => this.alerts.warning("Redirected to login because no credentials found."));

Typescript is yelling at you because appState.refresh , redirects.login , or alerts.warning returns a void value. To chain it correctly, set a variable with the chains, then return the variable:

    let result = Promise.reject<string>(message);

      result.then(() => this.appState.refresh())
            .then(() => this.redirects.login())
            .then(() => this.alerts.warning("Redirected to login because no credentials found."));

    return result;

It's worth mentioning also that none of the then s will be called because your Promise rejected.