`Promise.catch` should allow to widen promise type

[danielrentz]

Promise.catch can be used to silence rejections and to make the result optional by converting rejections to undefined (or more generally, to add any other result type to the promise chain). This is not possible anymore with better-ts-lib:

// before (TS lib)
const promise1 = Promise.resolve("a").then(() => 42, () => undefined);  // Promise<number | undefined>
const promise2 = Promise.resolve(42).catch(() => undefined);            // Promise<number | undefined>

// after (better-ts-lib)
const promise1 = Promise.resolve("a").then(() => 42, () => undefined);  // Promise<number | undefined>
const promise2 = Promise.resolve(42).catch(() => undefined);            // <=== type error

Could you add a catch<U> similar to the existing then<U>?

[danielrentz]

Ok just found #32 that discusses exactly this problem. Feel free to close if this is still the way to go here. Although I think it is inconsistent compared with the "lax" definition of "Promise.then".

[danielrentz]

BTW, in #32 there one can read "That being said, I wouldn't mind making the callback function of .catch mandatory." which did not happen as it seems.

[uhyo]

I kind of think that making the catch definition lax is fine. There are other ways to prevent unexpectedly widened Promise value types if you want.

[danielrentz]

@uhyo I would like to create a PR. Should I make the callback parameter required (as mentioned in #32)?

[uhyo]

Yes. catch() without callback isn't useful and can be prohibited I think.

[danielrentz]

@uhyo Somehow I cannot build the project, calling npm run build:lib always ends with Error: ENOENT: no such file or directory, open '.../better-typescript-lib/TypeScript/src/lib/libs.json' (tried Linux and Windows). Is there something special to do before (despite the usual npm i)? Seems I have never worked with projects using a subproject.

[uhyo]

Thank you for working on this!

I think you need to run:

git submodule init

and:

git submodule update

[danielrentz]

Thanks I will try and if I succeed, will add this to CONTRIBUTING.md