separate business layer from infrastructure layer before service.spec.js

service.spec.js

import Service from './service';

test('teams with disqualified players cannot check-in', async () => {
  const db = {
    collection(collection) {
      switch (collection) {
        case 'playerDisqualifications': {
          return {
            find({teamID}) {
              expect(teamID).toBe('some team id');
              return {
                async toArray() {
                  return [
                    {
                      teamID: 'some team id',
                      reason: 'was a big meanie'
                    }
                  ]
                }
              };
            }
          };
        }
        case 'checkIns': {
          return {
            updateOne() {
              throw new Error('unexpected updateOne to checkIns collection')
            }
          };
        }
      }
    }
  };

  const service = Service(db);
  await expect(service.checkIn('some team id')).rejects.toThrow();
});