a minimum fake implementation of fetch on Node environment

fakeFetch.js

import assign from 'object-assign';
 
export default function fakeFetch(res) {
  global.fetch = (url, params) => {
    return Promise.resolve(assign({}, res, {__arg: {url, params}}));
  };
}

test.js

import fakeFetch from './fakeFetch';

describe('api', () => {
  it('should be result is ok', done => {
    fakeFetch({
      status: 200,
      json: () => ({res: 'ok'})
    });
    fetch('/api', {user: 1}).then(res => {
      assert(res.status === 200);
      assert(res.json().res === 'ok');
      assert(res.__arg.url === '/api');
      assert(res.__arg.params.user === 1);
      done();
    });
  });
});