Adapter Pattern: Bridge Incompatible Interfaces

A third-party library or legacy module exposes an interface that is incompatible with what the rest of your code expects. You cannot modify either side.

Create an adapter class that wraps the incompatible component and exposes the expected interface. The adapter translates calls and data between the two shapes.

// What our app expects
interface Analytics {
  trackEvent(name: string, data: Record<string, unknown>): void;
}

// Third-party library with different API
class LegacyTracker {
  sendEvent(category: string, action: string, label: string) {
    console.log('Legacy:', category, action, label);
  }
}

// Adapter
class LegacyTrackerAdapter implements Analytics {
  constructor(private tracker: LegacyTracker) {}

  trackEvent(name: string, data: Record<string, unknown>): void {
    const label = JSON.stringify(data);
    this.tracker.sendEvent('app', name, label);
  }
}

// Usage: app only knows Analytics interface
const analytics: Analytics = new LegacyTrackerAdapter(new LegacyTracker());
analytics.trackEvent('page_view', { path: '/home' });

The adapter isolates the incompatibility in one place. Switching to a different analytics provider means writing one new adapter, not changing every call site in the app.