RxJS first() for Observable.of() - no elements in sequence

Question

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?

Answer 1

.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).

Answer 2

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.