The observer pattern in JavaScript

The observer pattern allows an object (the observable or subject) to maintain a list of other objects that depend on it (observers). When a state update occurs in the subject, such as an entity object being created or updated, it notifies the observers.

Implementation

A sample use case for the observer pattern is an in-memory queue. The Queue instance will have the subscribe, unsubscribe, and notify methods.

subscribe will add an additional “handler” function, unsubscribe will remove a particular “handler” function if it has been registered, and finally, notify will call each handler with a “message” payload. This is the “notification of the observers” piece, where the observable or subject ensures that each registered observer is notified.

subscribe and unsubscribe turn “observer” functionality on and off, respectively. subscribe has to be used to become an “...