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70. What is Observer Pattern?

Observer Pattern is a software design pattern that allows an object, called the subject, to notify a list of observers automatically when a specific event occurs. The observers can register with the subject and receive notifications when the subject changes its state. This pattern is extensively used in event-driven systems where there are many listeners to a specific event.

In the Observer pattern, there are two main components: Subject and Observers. The Subject maintains a list of observers and provides an interface for adding and removing observers. When a change occurs in the Subject, it notifies all the observers by calling a method on each observer.

The Observer Pattern is widely used in Front-end development to update the UI with new data from the backend. For instance, when the user enters a new message, the observer pattern sends the new message to the observer and updates the UI message list accordingly.

In summary, the Observer Pattern offers a flexible way of communication between components, making it simpler to build applications with loose coupling and better scalability.