Why `combineLatest` returns OperatorFunction<{}, number>

With RxJS 5.5 it's most likely because you're using combineLatest from rxjs/operators while you want to use it as an Observable "creation method" which means you need to use rxjs/observable/combineLatest.

I mean you need to use this:

import { combineLatest } from 'rxjs/observable/combineLatest';

... instead of this:

import { combineLatest } from 'rxjs/operators';

Edit: Since you mentioned you're using RxJS 6 you can import the static variant from rxjs.

import { combineLatest } from 'rxjs';

If you're looking for combineLatest operator you have to import it like this:

import { combineLatest } from 'rxjs/operators';