27

For my tests I am trying to mock an event stream with Observable.of() but when I try 

const actions$ = Observable.of({});
...
// in the function that is tested
actions$
  .filter(action => action.type === 'LOAD_REQUEST') 
  .first()
  .subscribe(() => { ... do something });

I get the following error

EmptyError: no elements in sequence in xxx.js

This only occurs when I use .first().

How can I mock the event stream so the tests don't fail?

select
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2 Answers2

34

.first() will emit exactly one item or throw an error (if no defaultValue parameter was provided), so calling it on an empty observable will cause an error. This is the expected behavior based on the documentation.

If you would like to get at most one item from the observable, use .take(1).

Tamas Hegedus
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15

The documentation for first() says:

Delivers an EmptyError to the Observer's error callback if the Observable completes before any next notification was sent.

So the error happens because your test data don't pass the filter() operator and emit complete immediately.

martin
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  • Thanks for this prefect answer, it was really hard to decide who to give the "accept answer" to. While you explained me why I saw the error and what I did wrong Tamas gave me the solution for fixing it. – select Nov 29 '16 at 21:48