Check if object is an Observable

javascript rxjs rxjs5
user5047085 picture user5047085 · Jan 3, 2017 · Viewed 16.9k times · Source

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?

Answer

cartant picture cartant · Jan 3, 2017

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.

Related questions

Chaining Observables in RxJS

I'm learning RxJS and Angular 2. Let's say I have a promise chain with multiple async function calls which depend on the previous one's result which looks like: var promiseChain = new Promise((resolve, reject) => { setTimeout(() => { resolve(1); }, 1000); }).then((result) => { …

Read More
Observable Finally on Subscribe

According to this artcle, onComplete and onError function of the subscribe are mutually exclusive. Meaning either onError or onComplete events will fire up in my subscribe. I have a logic block which needs to be executed whether I receive an …

Read More
How to Stop observable.timer in Angular2

I am implementing the following functions in Angular2's Component: export class MypageEditComponent { ngOnInit() { this.timer = Observable.timer(100, 100); this.timer.subscribe(t => { this.setFormData(); } private setFormData() { this.editUserAcountType = this.authStore.registerInfo.account_type; this.editAddress = this.authStore.registerInfo.email; …

Read More