Unit Testing Using Mocked Imports

isomorphic-fetch-mock.js

// A simple isomorphic-fetch mock that returns what you pass into it so we can assert that the methods in products.js
// are passing the correct values (I'd add rejection logic in here too to test those cases but this is just for demo purposes)
const fetch = (url, options) => {
  return Promise.resolve({url, options});
};

export default fetch;

products.js

// Super simple module for fetching product data
import fetch from 'isomorphic-fetch';

export const defaultOptions = {
  headers: {
    'Content-Type': 'application/json'
  }
};

const mergeOptions = (...options) => {
  return Object.assign({}, defaultOptions, ...options);
};

export const getProduct = (id, options = {}) => {
  return fetch(`http://localhost/product/${id}`, mergeOptions(options));
};

export const getProducts(options = {}) => {
  return fetch('http://localhost/products', mergeOptions(options));
};

products.test.js

// A test that mocks the isomorphic-fetch import inside products.js
import {
  getProduct,
  getProducts,
  defaultOptions
} from './products';

jest.mock('isomorphic-fetch', () => { // <-- the explicit mock of fetch
  return require('./isomorphic-fetch-mock');
});

describe.only('products.js', () => {
  it('getProduct(id, options)', (cb) => {
    const expected = {
      url: 'http://localhost/product/1',
      options: Object.assign({}, defaultOptions, {
        testy: 'westy'
      })
    };

    getProduct(1, {testy: 'westy'}).then((result) => {
      expect(result).toEqual(expected);
      cb();
    });
  });

  it('getProducts(options)', (cb) => {
    const expected = {
      url: 'http://localhost/products',
      options: Object.assign({}, defaultOptions, {
        testy: 'westy'
      })
    };

    getProducts({testy: 'westy'}).then((result) => {
      expect(result).toEqual(expected);
      cb();
    });
  });
});