Factory Method

Defines a method for object creation in a base class, allowing subclasses to override which concrete class is instantiated. Decouples creator from product.

When to Use

• The exact type of object to create is not known until runtime
• Subclasses should control the type of objects they create
• You want to provide a library/framework extension point for object creation
• Constructor logic is complex and varies by context

How to Apply

1. Define a common Product interface/abstract class
2. Create concrete product classes that implement Product
3. Define a Creator abstract class with an abstract createProduct(): Product factory method
4. Add default behaviour in Creator that uses createProduct()
5. Create concrete Creator subclasses that override createProduct() to return specific products

Example

interface Button { render(): void; onClick(f: () => void): void; }

class WindowsButton implements Button {
  render() { console.log('Render Windows button'); }
  onClick(f: () => void) { f(); }
}
class WebButton implements Button {
  render() { console.log('<button>'); }
  onClick(f: () => void) { document.addEventListener('click', f); }
}

abstract class Dialog {
  abstract createButton(): Button;       // factory method
  render() { const btn = this.createButton(); btn.render(); }
}

class WindowsDialog extends Dialog { createButton() { return new WindowsButton(); } }
class WebDialog extends Dialog     { createButton() { return new WebButton(); } }

References

• refactoring.guru/design-patterns/factory-method