async-promises | Async control flow patterns using promises | Reactive Programming library

It is strange that the typing isn't a single function signature with the union of the three possible types for the arr parameter. You might want to consider filing an issue or a pull request to the async typing library to fix it.

Even so, it would be nice if the compiler would allow you to call the function the way you have, since you know it is safe to do so. But as you see, it won't, at least as of TypeScript v2.5.

The easiest workaround: since you know for a fact what the compiler doesn't know... namely, that the existing filter function will indeed accept an arr of type T[] | IterableIterator | Dictionary, it is safe to tell the compiler not to worry by asserting arr is of type any which disables type checking: