Rxjs: take and takeLast don't work for Subject

Created on 10 Dec 2016 · 6Comments · Source: ReactiveX/rxjs

take and takeLast don't work for Subject but works for Observable.

Checked versions: 5.0.0-beta.12, 5.0.0-rc.4, 5.0.0-rc.5

Works properly for Observable:

let observable = Observable.of('first', 'second', 'third');
observable.subscribe(s => console.log(s));
observable.take(1).subscribe(s => console.log(s));

Actual result::

second
third
first

Does not work for Subject

let subject = new Subject<string>();
subject.subscribe(s => console.log(s));
subject.next('first');
subject.next('second');
subject.next('third');
subject.take(1).subscribe(s => console.log(s));

Expected result should be as in Observable example above.
Actual result:

second
third

Even if you add setTimeout for avoiding potential callback issues, result will not changes.

Source

vladbrk

Most helpful comment

Subjects are hot by default, so the three events have already passed by once you subscribe the second time.

If you change things around, you'll get the behavior you're looking for.

let subject = new Subject<string>();
subject.subscribe(s => console.log(s));
subject.take(1).subscribe(s => console.log(s));
subject.next('first');
subject.next('second');
subject.next('third');

david-driscoll on 10 Dec 2016

👍3

All 6 comments

Subjects are hot by default, so the three events have already passed by once you subscribe the second time.

If you change things around, you'll get the behavior you're looking for.

let subject = new Subject<string>();
subject.subscribe(s => console.log(s));
subject.take(1).subscribe(s => console.log(s));
subject.next('first');
subject.next('second');
subject.next('third');

david-driscoll on 10 Dec 2016

👍3

@david-driscoll has the correct answer here. Since subject try to synchronously emit values that are next-ed into them, if you subscribe after you've done all of your next-ing, it's not going to push any values through that new subscription, therefore take(1) never saw a value to take.

I'm closing this one for now. If you have more questions or feel this was in error, feel free to open another issue or comment again in here.

Thanks @vladbrk!

benlesh on 11 Dec 2016

@david-driscoll @blesh thanks for answers but what about takeLast, how to make it works. Even if I shift it before calls next, it will not works.

vladbrk on 11 Dec 2016

takeLast waits until observable completes. In above snippet, subject never completes so takeLast won't pick any values.