Factory Method Pattern: Decouple Object Creation from Usage

Code that calls new ConcreteClass() directly is tightly coupled to that class. Swapping implementations or adding new variants requires editing every call site.

Define a factory method in a base class or interface that subclasses override to control which class is instantiated. Call sites work against the abstract type returned by the factory.

interface Logger {
  log(msg: string): void;
}

class ConsoleLogger implements Logger {
  log(msg: string) { console.log(msg); }
}

class FileLogger implements Logger {
  log(msg: string) { /* write to file */ }
}

abstract class Application {
  // Factory method
  protected abstract createLogger(): Logger;

  run() {
    const logger = this.createLogger();
    logger.log('Application started');
  }
}

class ProductionApp extends Application {
  protected createLogger(): Logger { return new FileLogger(); }
}

class DevelopmentApp extends Application {
  protected createLogger(): Logger { return new ConsoleLogger(); }
}

The factory method moves the decision of which concrete class to instantiate to the subclass, following the Open/Closed Principle. New logger types can be added without modifying the Application base class.