Jest + PactJS

apiClient.js

export default {
  getCustomers(port) {
    return fetch(`http://localhost:${port}`)
      .then(response => response.json())
  },
};

apiClient.test.js

/**
 * @jest-environment node
 */
import { 
  addFetchNode,
  removeNodeFetch,
  getPort, 
  createPact, 
} from './pact-utils';
import apiClient from './api-client';

describe('apiClient', () => {
  let provider;
  let port;
  
  beforeEach(async () => {
    // there is no `fetch` when using `node` environment, so I just manually add `node-fetch` to `global.fetch`
    // I'm not sure I can use Pact-JS with jsdom, so that I wouldn't have to do this??
    addNodeFetch(); 
    
    port = getPort(); // returns a port++, so that each test case has a different port
    provider = createPact(port); // returns a new Pact() with configs and the passed port
    await provider.setup();
  });
  
  afterEach(async () => {
    removeNodeFetch();
    
    await provider.verify();
    await provider.finalize();
  });
  
  it('returns a list of customers', async () => {
    provider.addInteraction({ /* interaction here */ });
    
    // I still don't have a clean way to load the port, so I'm passing it for now. 
    const customers = await apiClient.getCustomers(port); 
    
    // expectations here..
  });
});