Also question is, are observables synchronous?

Observables can either be asynchronous or synchronous depending on the underlying implementation. An example of an asynchronous observable is one that wraps an http call. However when you are dealing with simple next-ing of values from a subject, the values will be received synchronously.

Subsequently, question is, what is the difference between promises and observables? We could also say that the main difference between a promise and an observable is that a promise emits only a single value, whereas an observable emits multiple values.

Beside above, is RxJS asynchronous?

Conclusion. Although JavaScript is single-threaded, RxJS is follows the same principles as other libraries for reactive streams. We can create asynchronous streams, have some degree of concurrency and web workers even allow for parallelism.

Is subscribe asynchronous?

As you may know, subscriptions are used to handle async method call. Thus, the code inside the subscribe() method is executed only when the async method return its result (after a http call for instance). While waiting for the async response, the program continues and execute the following code.