Jest with fetch-mock generating error: TypeError: Cannot read property 'prototype' of undefined when using on nodejs

Jest uses the mock at __mocks__/node-fetch.js any time node-fetch is required during the test.

The problem is that the first thing fetch-mock does is require node-fetch.

This means that Request is not defined when it is set on the config here, so calling prototype on the undefined Request causes an error here.

Someone smarter than me might know how to force Jest to require the actual node-fetch when fetch-mock requires node-fetch in the mock for node-fetch, but from what I can see it doesn't look like it is possible.

Looks like you will have to delete the mock at __mocks__/node-fetch.js and pass fetch to your code, something like this:

myclass.js

class MyClass {
  static query(fetch, sessionId, query) {
    const url = 'https://api.foobar.com';

    const body = {
      sessionId,
      query
    };

    return fetch(url, {
      method: 'post',
      body: JSON.stringify(body)
    })
      .then(res => res.json());
  }
}

module.exports = MyClass;

...then create the sandbox in your test and pass it to your code, something like this:

myclass.test.js

const fetch = require('fetch-mock').sandbox();
const MyClass = require('./myclass');

describe('MyClass', () => {
  describe('.query', () => {
    it('returns the response', () => {
      fetch.mock('*', {'result': {'fulfillment': {'speech': 'The answer'}}});

      expect.assertions(1);

      return MyClass.query(fetch, '123', 'the question').then((data) => {
        expect(data.result.fulfillment.speech).toBe('The answer');  // SUCCESS
      });
    });
  });
});

I've now found a reliable way to combine fetch-mock and jest http://www.wheresrhys.co.uk/fetch-mock/#usageusage-with-jest