first operator does not accept default value only signature with null-value predicate

[kwonoj]

RxJS version:
latest

Code to reproduce:
Rx.Observable.range(0, 10).first(null, null, true);

Expected behavior:

Actual behavior:

error TS2345: Argument of type 'null' is not assignable to parameter of type '(value: number, index: number, source: Observable) => boolean'.

Additional information:
This seems bit weird since default signature allows optional values for predicate / resultselector (

rxjs/src/operator/first.ts

Line 76 in 69d051b

export function first<T, R>(this: Observable<T>, predicate?: (value: T, index: number, source: Observable<T>) => boolean,

) and test case exists (

rxjs/spec/operators/first-spec.ts

Line 46 in 69d051b

expectObservable(e1.first(null, null, 'a')).toBe(expected);

) - not sure why it's being caused. Maybe signature ordering causes this.

[martinsik]

I tested this with rxjs@5.2.0 and typescript@2.2.1 and it compiled without any problem. What Typescript version are you using?

[kwonoj]

2.1 / 2.2 both seems have issues.

https://travis-ci.org/kwonoj/rxjs-first-signature-demo/builds/209822410#L172

https://github.com/kwonoj/rxjs-first-signature-demo/blob/f971b3426f8c5e497b219632bb27e57c0b70ec6b/index.ts#L3

[jsayol]

null is indeed not assignable to that type, even if it's optional, when strictNullChecks is enabled. That's probably your case so you should call it like this:

Rx.Observable.range(0, 10).first(undefined, undefined, true);

[kwonoj]

Yes, I am aware this can be avoided. Still type definition need to be looked into.

[seveves]

Does something like this look valid? I could create a PR then ...

export function first<T, R>(this: Observable<T>, predicate?: (value: T, index: number, source: Observable<T>) => boolean | undefined | null,
                            resultSelector?: ((value: T, index: number) => R) | void | undefined | null,
                            defaultValue?: R): Observable<T | R> {
  return this.lift(new FirstOperator(predicate, resultSelector, defaultValue, this));
}

[martinsik]

I'm looking at this again after more than a year and I think this should be the expected behavior. Even when the method is defined as optional it shouldn't accept null in my opinion. If I want to skip any of the parameters I should use undefined.

[benlesh]

This is no longer an issue.