Hot and cold observables

In bacon.js, an observable only emits a value when a subscriber subscribes to it; therefore, we will call it a cold observables. In Reactive Extensions, things can become a little trickier: we have both hot and cold observables.

A hot observable is an observable that propagates the data independently, irrespective of whether we have some Observer attached to it or not. An example of a hot observable is an observable created from mouse movements.

A cold observable, on the other hand, is an observable that fires the same sequence for all the subscribers. An example of a cold observable is an observable created from an array.

This behavior is important to understand, and RxJS has special methods to replay events in a hot observable or turn a cold observable into a hot observable.