RxJS: multicast’s Secret

Photo by Tim Mossholder on Unsplash

multicast has a secret. And so does publish — which wraps multicast. And it’s sometimes really useful.

The secret

The documentation for multicast and publish mentions a ConnectableObservable. A connectable observable is type of observable that waits until its connect method is called before it begins emitting notifications to subscribers. However, the multicast and publish operators don’t always return a connectable observable.

Let’s start by looking at the source code for publish:

It’s clear that publish is just a thin wrapper around multicast. It creates a subject, which is passed to multicast along with the optional selector function. The interesting bit is inside the multicast implementation, which contains the following code: