Promise<T> should be Promise<T1,T2>

[ORESoftware]

Sorry if this is a dupe issue, but I am having trouble determining how to solve some typing issues surrounding promises.

If we have a method:

 foo(bar) : Promise<boolean | string> {
   if(bar){
      return Promise.resolve(true);   // boolean type
    }
    return Promise.reject('a string');  // string type
 }

I am really really confused, why the typings for Promises don't take two parameters, should it not be:

 foo(bar) : Promise<boolean, string> {    // use , instead of |
   if(bar){
      return Promise.resolve(true);   
    }
    return Promise.reject('a string');
 }

Currently, I am trying to solve this SO issue, and yet again the problem of specifying the different resolution types is coming up. I guess I just want to know why it is this way.

https://stackoverflow.com/questions/48954365/use-generics-to-specify-type-of-return-value/48958224

[jack-williams]

Do you intend the second type (call it E) to be the input type to the catch or onrejected continuations? The input to these functions can also come from a throw, not just Promise.reject, and as TypeScript doesn't do effect typing it wont track this.

I think rather than making promises (pun intended) the checker can't keep, they just treat the error type as any.

[DanielRosenwasser]

Looks like a duplicate of #5413 (#3834 is also possibly related?)

[ORESoftware]

@jack-williams yeah Promise<boolean,string> means the resolved type is always a boolean, and the rejected type is always a string, nothing too crazy... lol...is it too much to ask? idk

at the moment, mixing resolved type with rejected type is making me a little crazy upstairs

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.