Promise handles a single event when an async operation completes or fails.
Note: There are
Promise libraries out there that support cancellation, but ES6
Promise doesn't so far.
Observable is like a
Stream (in many languages) and allows to pass zero or more events where the callback is called for each event.
Observable is preferred over
Promise because it provides the features of
Promise and more. With
Observable it doesn't matter if you want to handle 0, 1, or multiple events. You can utilize the same API in each case.
Observable also has the advantage over
Promise to be cancelable. If the result of an HTTP request to a server or some other expensive async operation isn't needed anymore, the
Subscription of an
Observable allows to cancel the subscription, while a
Promise will eventually call the success or failed callback even when you don't need the notification or the result it provides anymore.
Observable provides operators like
reduce, ... similar to an array
There are also powerful operators like
replay(), ... that are often quite handy.