customer_spec.js

import Customer from 'Customer';
import API from 'API';
import Spies from 'Spies';

beforeEach(() => {  
  Spies.start();
});

afterEach(() => {
  Spies.stop();
});

describe('find', () => {
  test('retrieves an existing customer', () => {
    Customer.find(1);
    expect(Spies.API.get).toHaveBeenCalledWith('https://api.test/customers', { id: 1 });
  });
});

describe('save', () => {
  test('saves a new customer', () => {
    Customer.save({ name: 'Kyle' });
    expect(Spies.API.post).toHaveBeenCalledWith('https://api.test/customers', { name: 'Kyle' });
  });
});

spies.js

import Customer from 'Customer';
import API from 'API';

const createSpy = (object, methods) => {
  const spy = {};

  spy.start = () => {
    methods.forEach((method) => {
      spy[method] = jest.spyOn(object, method);
    });
  }

  spy.stop = () => {
    methods.forEach((method) => {
      spy[method].mockRestore();
    });
  }

  return spy;
}

const _spies = new Map();

// Configure the objects/methods you want to spy on.
_spies.set('Customer', createSpy(Customer, ['find', 'save']));
_spies.set('API', createSpy(API, ['get', 'post', 'put']));
_spies.set('Console', createSpy(global.console, ['error', 'warn']));

const Spies = {
  start: () => _spies.forEach((spy, name) => spy.start()),
  stop: () => _spies.forEach((spy, name) => spy.stop()),
};

_spies.forEach((spy, name) => {
  Spies[name] = spy;
});

export default Spies;