Promise can be resolved with undefined but not reflected in type

[OliverJAsh]

TS 2.0.3

    new Promise<string>(resolve => {
        true ? resolve('foo') :  resolve()
    })

This returns a Promise<string>, however it is possible to call resolve with no value, meaning the return type is actually Promise<string | undefined> at runtime.

I'm not sure whether the return type should in fact be inferred as Promise<string | undefined> or whether the empty resolve() call should throw a compile error.

[kitsonk]

Dupe of #3356

[OliverJAsh]

#3356 appears to be marked as a dupe of #4260.

What's the status of this now that we have strict null checking (since #3356)?

[MicahZoltu]

Pinging on the status of this. At the moment, with strict null checking the Promise definition file is simply incorrect.

constructor(callback: (resolve : (value?: T | Thenable<T>) => void, reject: (error?: any) => void) => void);

Notice how resolve is a function that takes in an optional T or Thenable<T>. This isn't correct. It should take in a required T or Thenable<T>. It is a programming error to resolve a promise with the wrong type. If you want a promise of an optional that is fine, but your T should be T|undefined in this case.

There are a number of copies of this issue floating around and most of them are labeled as a dupe of #4260, however on cursory glance of that issue I think perhaps the reality is that #4260 is a a prerequisite before fixing this issue. Once 4260 is fixed then the definition for Promise can be fixed to work correctly with strict null checking.

[agalazis]

yes I don't get the point of asuming that void is always allowed

   new Promise<string>(resolve => {
        true ? resolve('foo') :  resolve()
    })

could be :

   new Promise<string|void>(resolve => {
        true ? resolve('foo') :  resolve()
    })

that's all.
I stumbled on this because I am using a third party promise library that uses typings that do have the same assumption as all these open issues but in my case, I don't want to allow void.
I also expect the compiler to complain about just calling resolve() without arguments instead of assuming it's allowed if I didn't explicitly allow void values.