Strongly type Observable Error

RxJS version: rxjs@5.0.3

Code to reproduce: none

Expected behavior: Observable<T,E> should be used for definition where E tell type of error to be expected

Actual behavior: Observable<T> where error is typed as any

Additional information: Typescript 2.3 provide default type for generics using that we can have Observable<T,E=any> for backward compability

[felixfbecker]

I disagree. TypeScript does not have error types. In Java, thrown error types are part of the interface contract, declared with throws. This does not exist in TypeScript, only the return type is part of the interface contract. The error type could be anything if you call other functions, so you should do runtime type checking, like checking for an error code. Consequently, Promises don’t have an error type, and Observables shouldn’t either.

[felixfbecker]

Btw, the equivalent issue in TypeScript is https://github.com/Microsoft/TypeScript/issues/13219. If we ever get that issue (declarations for exceptions a function can throw), Promise would be changed to have an error type, and then Observable should be changed too. Until then, rxjs should stay consistent with how TypeScript treats synchronous exceptions and Promise rejections (including, the error type should only become unknown if promise.catch() changes any to unknown).