Observables are like promises which provide us with abstractions that help us deal with the asynchronous nature of applications.

Observable is like a stream which allows to pass zero or more events where the callback is called for each event.

Difference between Promise and Observable

• Promise are representation of 1 future value
• Observables are representation for infinite amount of values
• Promise fetches the value immediately after creation whereas observables will only start producing values when you subscribe to them.

Scan()

Scan will show all values emitted on source observable

Reduce()

Reduce will show only the final value emitted on source observable