Singleton Pattern

Singleton is a creational design pattern that lets you ensure that a class has only one instance, while providing a global access point to this instance.

Purpose and Use Case

Use the Singleton pattern when a class in your application must have only a single instance available to all clients (for example, a database connection object, a centralized logger, or a global configuration manager).

Pattern Structure

Regardless of the programming language (TypeScript, PHP, Python, C++, etc.), the conceptual structure of the pattern relies on three pillars:

1. Private constructor: Prevents other objects from using the new operator directly.
2. Private static attribute: Stores the reference to the single created instance.
3. Public static method (getInstance): Acts as a global access point that initializes the instance on first request and returns it on subsequent calls.

How It Works

1. Request: The client calls Singleton.getInstance().
2. Verification: The class checks whether the static variable instance already holds an object.
3. Lazy initialization: If the instance does not exist (null or undefined), it privately invokes the constructor and assigns it.
4. Return: Returns the existing instance, ensuring a single shared object across the entire application.

Real-World Implementation Examples

// LoggerService.ts — Centralized logging service
class LoggerService {
  private static instance: LoggerService;
  private logs: string[] = [];
 
  private constructor() {}
 
  public static getInstance(): LoggerService {
    if (!LoggerService.instance) {
      LoggerService.instance = new LoggerService();
    }
    return LoggerService.instance;
  }
 
  public log(message: string): void {
    const timestamp = new Date().toISOString();
    const entry = `[${timestamp}] ${message}`;
    this.logs.push(entry);
    console.log(entry);
  }
 
  public getHistory(): string[] {
    return [...this.logs];
  }
}
 
// Usage anywhere in the application:
const logger1 = LoggerService.getInstance();
logger1.log("User logged in successfully");
 
const logger2 = LoggerService.getInstance();
logger2.log("HTTP 200 response received");
 
// Both references point to the exact same instance and shared log history
console.log(logger1 === logger2); // true