Promise catch error parameter should not be of type any.

[tinganho]

Is there any chance, you could change the any type on error parameter on the catch method of a Promise, to a type argument instead? The type argument is provided in the Promise<R, E> type annotation, instead of just Promise<R> .

 /**
 * Sugar for promise.then(undefined, onRejected)
 *
 * @param onRejected called when/if "promise" rejects
 */
 catch<U>(onRejected?: (error: any) => U | Thenable<U>): Promise<U>;

I need to type annotate many catch method calls right now with:

.catch(error: HTTPResponse<ErrorResponse> | Error)

[DanielRosenwasser]

Ron Buckton (@rbuckton) can you weigh in here?

[ivogabe]

I think it's not possible to type that accurately, as errors in a previous listener will be passed to the catch listener. Functions are not annotated with their possible errors, so the error could be everything, so any would be the most accurate type.

somePromise.then(x => {
 throw "foo";
}).catch(error => {
 // error can also be a string here
});

[tinganho]

Ivo Gabe de Wolff (@ivogabe) maybe it could be better handled with a union of type any?

.catch(error: HTTPResponse<ErrorResponse> | any)

Promise rejection are not exceptions only. you consciously reject sometimes. And I just think it is a good idea that people can handle those conscious rejections, i.e. give it a type.

[tinganho]

But after think about it. Maybe better to wait until default type argument lands in #2175? So that default error type is any. It also don't break anyones code then 😄 .

[zpdDG4gta8XKpMCd]

anything united with any cannot be differentiated, if it has any it will stay any

try:

function isString(value: string | any) : value is string {
 return typeof value === 'string';
}
let value : string | any;
if (isString(value)) {
 value // <-- check me
}

[tinganho]

Hmm strange, typeof guards works.

let value : string | any;
if ( typeof value === 'string') {
 value // <-- check me
}

[tinganho]

Just reported it here #6301.

[rbuckton]

Ivo Gabe de Wolff (@ivogabe) is correct. The exception is typed any because we cannot guarantee the correct type of the exception at design time, and neither TypeScript nor JavaScript provide the ability to guard exception types at run time. Your best option is to use type guards to provide both a design-time and run-time check in your code.