This is crucial to understand! Online materials can often be incorrect.

Very short but meaty writeup.

🏆 Here are 4 Laws of Observables 🏆

Law 1

• All Observables are lazy, without exception.

Law 2

• Observables are either cold, serving a single execution context per subscription in a unicast manner.

• Or hot, serving one or more subscribers with the same execution context, in a multicast manner.

Law 3

• Subject is both an Observable and an Observer

• The Observable part is multicast, but still lazily executed only when the first subscriber comes.

• The Observer part is the one created eagerly that allows us to call .next() and send data through the Subject;

• The Observer part does not imply that the execution context of the Observable is 'alive' before the first subscription is made.

Law 4

• There are stateful Subjects, like BehaviorSubject or ReplySubject, which extend the Observer part.

• Adding support for holding 1..n previous states, which the Observable part will multicast to potential subscribers.

By reading this, do you think that RxJS is a complex subject (pun intended :D)?

Angular Team announced plans to potentially make RxJS optional in the future.

Would you like to keep RxJS mandatory?
Or optional is the right way to go?