Overloading functions with specialized parameters not working as expected.

[seshualluvada]

The below code has a specialized signature for Subscribe function that takes "NotificationClicked" as the event Name and NotificationClickedEventArgs type as the eventArgs parameter. The general signature takes any string as eventName and the base EventArgs as the eventArgs parameter.

My understanding is that typescript should give a compiler error where I am executing the code PublishSubscribe.Subscribe("NotificationClicked", new EventArgs()); because the specialized signature expects a NotificationClickedEventArgs type. As per my observation, typescript is compiling successfully which most likely is a bug. This works correctly on return types though.

class EventArgs{
  EventName: string;
  EventData: any;
}

class NotificationClickedEventArgs extends EventArgs{
  NotificationID: string;
}

class PublishSubscribe{
  static Subscribe:{
    (eventName: "NotificationClicked", eventArgs: NotificationClickedEventArgs):void;
    (eventName: string, eventArgs :EventArgs):void;
  } = (eventName: string, eventArgs: EventArgs) => {  }
}

PublishSubscribe.Subscribe("NotificationClicked", new EventArgs());

[DanielRosenwasser]

This is because you have that catch-all non-specialized signature. Unfortunately, you can't remove it because of an old requirement before string literal types became a thing. #6278 seeks to fix that, but I I'm not sure if we can get it into 1.8. @mhegazy @ahejlsberg?

[seshualluvada]

This behavior is working as expected with return type overloading as shown below. Even though I have the non-specialized signature that is returning the base EventArgs type, TypeScript is deducing accurately that the Subscribe method will be returning NotificationClickedEventArgs when the parameter passed is "NotificationClicked", which makes me believe that the type checking for the following parameter types is not getting stricter based on the parameter values provided for previous parameters.

class EventArgs{
  EventName: string;
  EventData: any;
}

class NotificationClickedEventArgs extends EventArgs{
  NotificationID: string;
}

class PublishSubscribe{
  static Subscribe:{
    (eventName: "NotificationClicked"):NotificationClickedEventArgs;
    (eventName: string):EventArgs;
  } = (eventName: string):EventArgs => { 
        if (eventName === "NotificationClicked")
          return new NotificationClickedEventArgs();  
        else
          return new EventArgs();
      }
}

PublishSubscribe.Subscribe("NotificationClicked").NotificationID //This is allowed
PublishSubscribe.Subscribe("SomethingElse").NotificationID //This errors as expected as NotificationID is not a property of EventArgs

[mhegazy]

With #6744 in place (you can try it in typescript@next), you can remove the string overload, and that will give you the error you expect.

[mhegazy]

also worth noting, is it will make your function more restrictive, i.e.

class PublishSubscribe{
  static Subscribe:{
    (eventName: "NotificationClicked", eventArgs: NotificationClickedEventArgs):void;
  };
}

PublishSubscribe.Subscribe("NotificationClicked", new EventArgs()); // Erro EventArg is not NotificationClickedEventArgs

PublishSubscribe.Subscribe("NotificationClicked", new NotificationClickedEventArgs()); //  OK

var eventName = "NotificationClicked";
PublishSubscribe.Subscribe(eventName, new EventArgs()); // Error, string is not "NotificationClicked"