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Javascript – Observer Pattern

A design pattern that is frequently used in software engineering, especially in event-driven architectures, is the observer pattern. The Subject and the Observer are the two key actors in this pattern. The Subject is in charge of keeping track of its observers and informing them of any status changes. The Observer, on the other hand, is an object that is eager to receive alerts from the Subject and responds to those alerts.

Benefits of Javascript Observer Pattern:

  1. Loose Coupling: The Observer Pattern enables objects to be loosely connected, which allows them to communicate without being overly aware of one another’s implementation specifics. As a result, the code is more adaptable, maintainable, and testable.
  2. Separation of concerns: The Observer Pattern differentiates between an observer’s and a subject’s concerns. The state is maintained by the Subject, and responses to changes in that state are provided by the Observers. Since neither the Subject nor the Observers are affected, it is simpler to alter or extend one of their behaviors without impacting the other.
  3. Extensibility: Without changing the Subject or any current Observers, the Observer Pattern makes it simple to add new Observers to the system. This makes it simple to add new features as needed to the system.
  4. Reusability: When several objects need to be alerted to changes to a shared data source, the Observer Pattern can be applied in a wide range of circumstances. It can therefore be used in a variety of applications and is a highly reusable pattern.

Observer Pattern Example:

// Define the Subject
class Subject {
constructor() {
this.observers = []; // Initialize empty list of observers
subscribe(observer) {
this.observers.push(observer); // Add an observer to the list
unsubscribe(observer) {
this.observers = this.observers.filter(obs => obs !== observer); // Remove the observer from the list
notify(data) {
this.observers.forEach(observer => observer.update(data)); // Notify all observers of the change
// Define the Observer
class Observer {
update(data) {
console.log('Data changed:', data); // Take action in response to change
// Usage
const subject = new Subject();
const observer1 = new Observer();
const observer2 = new Observer();
subject.notify('hello world'); // Output: "Data changed: hello world"
subject.notify('goodbye'); // Output: "Data changed: goodbye"

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