Real-World Adapter Patterns

API Version Adapter

Adapt between API versions:

typescript
1interface ShippingServiceV2 {
2  createShipment(request: ShipmentRequestV2): ShipmentResponseV2;
3}
4
5// Adapt old V1 API to new V2 interface
6class V1toV2Adapter implements ShippingServiceV2 {
7  constructor(private v1Service: ShippingServiceV1) {}
8
9  createShipment(request: ShipmentRequestV2): ShipmentResponseV2 {
10    const v1Request = this.downgradeRequest(request);
11    const v1Response = this.v1Service.createShipment(v1Request);
12    return this.upgradeResponse(v1Response);
13  }
14
15  private downgradeRequest(v2: ShipmentRequestV2): ShipmentRequestV1 {
16    return {
17      from: v2.origin,
18      to: v2.destination,
19      weight: v2.packages[0]?.weight ?? 0
20    };
21  }
22}

Data Format Adapter

Convert between data formats:

typescript
1interface AddressNormalizer {
2  normalize(input: unknown): NormalizedAddress;
3}
4
5class XMLAddressAdapter implements AddressNormalizer {
6  normalize(xml: string): NormalizedAddress {
7    const parsed = parseXML(xml);
8    return {
9      street: parsed.querySelector('street')?.textContent ?? '',
10      city: parsed.querySelector('city')?.textContent ?? '',
11      // ...
12    };
13  }
14}
15
16class JSONAddressAdapter implements AddressNormalizer {
17  normalize(json: string): NormalizedAddress {
18    const data = JSON.parse(json);
19    return {
20      street: data.address_line_1,
21      city: data.city_name,
22      // Map different field names
23    };
24  }
25}

Event System Adapter

Adapt between event systems:

typescript
1interface EventEmitter {
2  on(event: string, handler: Function): void;
3  emit(event: string, data: unknown): void;
4}
5
6// Adapt DOM events to custom event system
7class DOMEventAdapter implements EventEmitter {
8  constructor(private element: HTMLElement) {}
9
10  on(event: string, handler: Function): void {
11    this.element.addEventListener(event, (e) => handler(e.detail));
12  }
13
14  emit(event: string, data: unknown): void {
15    this.element.dispatchEvent(new CustomEvent(event, { detail: data }));
16  }
17}

Testing with Adapters

Create mock adapters for testing:

typescript
1class MockShippingAdapter implements ShippingService {
2  public calls: Array<{ from: Address; to: Address }> = [];
3
4  getRate(from: Address, to: Address): Rate {
5    this.calls.push({ from, to });
6    return { amount: 10.00, currency: 'USD' };
7  }
8
9  createLabel(): Label {
10    return { trackingNumber: 'MOCK123', url: 'http://mock' };
11  }
12}
13
14// In tests
15const mockAdapter = new MockShippingAdapter();
16const service = new OrderService(mockAdapter);
17await service.processOrder(order);
18expect(mockAdapter.calls).toHaveLength(1);

Summary

Adapters are essential for:

• Third-party API integration
• Legacy system migration
• Data format conversion
• Testing isolation