Check if object is an Observable

I am using RxJS 5 and have this method:

Queue.prototype.drain = function (obs, opts) {};

in the method, I would like to check if the user passed in an Observable for the first argument or if they omitted the Observable and just passed in an options object.

So basically I need to do something like this:

if(!Rx.Observable.isObservable(obs)){  //this method is fictitious
    opts = obs || {};
    obs = Rx.Observable.interval(1000);
}

I assume RxJS provides users with this kind of check but I cannot find the documentation that shows you how to do this type checking.

Anyone know how?

Solution

Since writing this answer, RxJS version 6 has been released and, in that version, an isObservable function was added to the public API. It can be imported like this:

import { isObservable } from "rxjs";

The function signature is:

export function isObservable<T>(obj: any): obj is Observable<T>

Since it is defined with a typeguard, the compiler can help you out like this:

const result: any = ...;

if (isObservable(result)) 
{
   result.pipe(...);   // compiler now knows it's an observable.
}

Internally, RxJS tests for an Observable using instanceof:

if (result instanceof Observable) {
  ...
}

So you could use:

if (!(obs instanceof Rx.Observable)) {
  opts = obs || {};
  obs = Rx.Observable.interval(1000);
}

instanceof can be used to determine whether or not an object is an Observable from the RxJS library that you happen to be using.

To determine if the object is a foreign observable, you can look for a Symbol.observable property.

If the property is present and is a function, you can obtain an RxJS Observable from the foreign observable by passing the value returned by calling the object's Symbol.observable property to Rx.Observable.from.