Design Patterns - lesson 5 | HackIndia Skills

Design Patterns - lesson 5 | HackIndia Skills | HackIndia

Example

Refactor a logger

Before: every service creates its own logger. Later you want one format and one file target, so changes spread everywhere.

After: one Logger instance is exposed through a controlled static method. The constructor stays private.

class Logger {
  private static instance: Logger | null = null;

  private constructor() {}

  static getInstance(): Logger {
    if (Logger.instance === null) {
      Logger.instance = new Logger();
    }
    return Logger.instance;
  }

  info(message: string) {
    console.log(`[INFO] ${new Date().toISOString()} ${message}`);
  }
}

// usage
const logger = Logger.getInstance();
logger.info('Payment request created');

Use singleton with caution

One idea

When it fits

Common misuse

Refactor a logger

Keep it small

Safe refactor path

Testing trap

Do this now

Quick check