Is the order of observables given to combineLatest relevant?

Question

I have two observables which I want to combine with combineLatest:

const o1 = from(['a', 'b', 'c']);
const o2 = of('content from o2');
combineLatest(o1, o2)
  .subscribe(result => console.log(result));

As I understood combineLatest, when any observable emits, it will combine the latest values of all observables and emit them:

When any observable emits a value, emit the latest value from each. (https://www.learnrxjs.io/operators/combination/combinelatest.html)

However, with the code above, the output is:

["c", "content from o2"]

So only when o2 emits, combineLatest emits an value.

If I switch the order I hand over o1 and o2, it works as expected:

const o1 = from(['a', 'b', 'c']);
const o2 = of('content from o2');
combineLatest(o2, o1)
  .subscribe(result => console.log(result));

Output:

["content from o2", "a"]
["content from o2", "b"]
["content from o2", "c"]

Is this expected behavior? If so, why? This seems to contradict the definition ob combineLatest.

Answer 1

Yes, in this case order matters, but it is a timing issue. combineLatest can not influence the order in which the values of the observables are emitted.

In your first case, o1 emits all values before o2 emits its value. In your second case o2 emits its value first and then o1 emits all its values. In real world examples you have a natural delay of observable values (service calls etc..) and so normally order does not matter.