Singleton Pattern

Ensure a class has only one instance and provide a global access point.

What is it?

The Singleton pattern ensures there is only one instance of a class throughout the app, and provides a global point of access to that instance.

Interactive Demo: Singleton vs Regular Class

Click the buttons below to see the difference between Singleton and regular class instantiation:

Code Example

class ToastManager {
  private static instance: ToastManager;
  private constructor() {} // Private constructor prevents direct instantiation
  
  static getInstance(): ToastManager {
    if (!ToastManager.instance) {
      ToastManager.instance = new ToastManager();
    }
    return ToastManager.instance; // Always returns the same instance
  }
  
  show(message: string) {
    toast.success(message);
  }
}

// Usage - both variables point to the same instance
const toast1 = ToastManager.getInstance();
const toast2 = ToastManager.getInstance();
console.log(toast1 === toast2); // true

Common Uses

• Global toast manager
• Application-wide theme or settings service
• Redux store or centralized event bus
• Database connection pools
• Logging services

When to Use

• Need exactly one instance of a class
• Shared global state or utilities
• Expensive object creation that should be done once
• Coordinating actions across the system

Caution

• Overuse can lead to tightly coupled code
• Harder to test if used improperly
• Can create hidden dependencies
• Violates Single Responsibility Principle if it does too much