Singleton Pattern

1. Introduction

Singletons helps us to work with an instances of a class or object so we have a global state in our application

2. Learning Objectives

• To be able to describe the Pattern
• To know where applied.

3. Key Concepts

• Single instance that can be share through all the application.
• When instantiating a new instance we need to double check if there is already an instance of our class.
• Shouldn’t be modifiable
• Object.freeze to freeze the object to ensure is not modifiable
• ES2015 are already module
• We don’t longer need to explicitly create singletons, because modules are singletons by default
• Tradeoffs : Global scope pollution. hard to test because having only one instance all the test rely on the modification on the previous test
• Singletons helps us to work with an instances of a class or object so we have a global state in our application

4. Examples and Practical Cases

• we can use singleton to connect to databases

let instance;

class DBConnection {
	constructor(uri) {
		if (instance) {
			throw new Error('Only one connection can exist!');
		}
		this.uri = uri;
		instance = this;
	}
	
	connect() {
		this.isConnected = true;
		console.log(`DB ${this.uri} has been connected!`);
	}
	
	disconnect() {
		this.isConnected = false;
		console.log('DB disconnected');
	}
}

const databaseConnector = Object.freeze(new DBConnection());
const connection = databaseConnector;

5. Reflections and Synthesis

• Summary:
  singletons are class or objects that are inmutable and its goals it’s to centralize a logic such as connecting to a DB. They only need to exist a single instance of our class
• Open Questions:
  Why modules are singletons by default?

6. Conclusion

A final recap and how it connects to your overall learning journey on FrontendMasters.

7. References and Additional Resources

• [[Module Pattern]] (Include links to external resources or documents that provide deeper insights into the topic)