2th of 11 Questions.
Observer Pattern
The Observer pattern defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
The Observer pattern is a behavioral design pattern where an object (the subject) maintains a list of dependents (observers) and notifies them of any state changes. This is one of the most widely used patterns in frontend development, forming the basis of event handling, reactive programming, and state management libraries like Redux and RxJS. In JavaScript, the EventTarget interface, addEventListener, and CustomEvent are native implementations of this pattern.
Vanilla JavaScript Implementation
React Implementation
Angular Implementation