How to setup fetch-mock with node-fetch, supertest and typescript

Question

I am trying to add some jest tests to my node typescipt project. I would like to use supertest to call my koa router, but also use fetch-mock to mock requests that are made using node-fetch.

My solution so far is below, but the fetch in my router does not use the mocked fetch setup with fetch-mock. The unit test fails because the expected response I mocked is not returned. I have tried following the documentation for global fetch mocking but without success, and typescript makes it difficult to follow the non-typescript solutions I have found. I would like to avoid using the non-global sandbox if possible as I would have to re-write a lot of code to pass the fetch around.

server.spec.ts

import * as fetchMock from 'fetch-mock';
import * as request from 'supertest';
import server from './server';

afterEach(() => {
  server.close();
  fetchMock.reset();
});

describe('router', () => {
  test('GET: should return data', async () => {
    const expectedResponse = { test: 'TEST' };
    fetchMock.get('https://myapi.com/test', expectedResponse);

    const response = await request(server).get('/test');
    expect(response.status).toEqual(200);
    expect(response.body).toMatchObject(expectedResponse);
  });
});

server.ts

import * as Koa from 'koa';
import * as Router from 'koa-router';
import fetch from 'node-fetch';

const app = new Koa();

const router = new Router();

router.get('/test', async ctx => {
  const options = { method: 'GET' };
  try {
    const response = await fetch('https://myapi.com/test', options);
    ctx.body = await response.json();
  } catch (error) {
    error.fetchUrl = url;
    throw error;
  }
});

app.use(router.routes());

const server = app.listen(3000);

export default server;

Solution 1

You can mock node-fetch module manually by yourself. Here is the solution:

server.ts:

import Koa from 'koa';
import Router from 'koa-router';
import fetch from 'node-fetch';

const app = new Koa();

const router = new Router();

router.get('/test', async ctx => {
  const options = { method: 'GET' };
  const url = 'https://myapi.com/test';
  try {
    const response = await fetch(url, options);
    ctx.body = await response.json();
  } catch (error) {
    error.fetchUrl = url;
    throw error;
  }
});

app.use(router.routes());

function createHttpServer() {
  return app.listen(3000);
}

if (require.main === module) {
  createHttpServer();
}

export default createHttpServer;

server.spec.ts:

import request from 'supertest';
import createHttpServer from './server';
import fetch from 'node-fetch';

const { Response } = jest.requireActual('node-fetch');
const server = createHttpServer();

jest.mock('node-fetch', () => jest.fn());

afterAll(done => {
  server.close(done);
});

describe('router', () => {
  test('GET: should return data', async () => {
    const expectedResponse = { test: 'TEST' };
    (fetch as jest.MockedFunction<typeof fetch>).mockResolvedValueOnce(new Response(JSON.stringify(expectedResponse)));

    const response = await request(server).get('/test');
    expect(response.status).toEqual(200);
    expect(response.body).toEqual(expectedResponse);
  });

  test('GET: should throw error', async () => {
    const mockedFetchError = new Error('some error');
    (fetch as jest.MockedFunction<typeof fetch>).mockRejectedValueOnce(mockedFetchError);
    const response = await request(server).get('/test');
    expect(response.status).toEqual(500);
  });
});

Unit test result with coverage report:

 PASS  src/stackoverflow/56735795/server.spec.ts (8.487s)
  router
    ✓ GET: should return data (51ms)
    ✓ GET: should throw error (15ms)

  console.error node_modules/koa/lib/application.js:200
    undefined

  console.error node_modules/koa/lib/application.js:201
      Error: some error
          at Object.<anonymous> (/Users/ldu020/workspace/github.com/mrdulin/jest-codelab/src/stackoverflow/56735795/server.spec.ts:26:30)
          at step (/Users/ldu020/workspace/github.com/mrdulin/jest-codelab/src/stackoverflow/56735795/server.spec.ts:32:23)
          at Object.next (/Users/ldu020/workspace/github.com/mrdulin/jest-codelab/src/stackoverflow/56735795/server.spec.ts:13:53)
          at /Users/ldu020/workspace/github.com/mrdulin/jest-codelab/src/stackoverflow/56735795/server.spec.ts:7:71
          at new Promise (<anonymous>)
          at Object.<anonymous>.__awaiter (/Users/ldu020/workspace/github.com/mrdulin/jest-codelab/src/stackoverflow/56735795/server.spec.ts:3:12)
          at Object.<anonymous> (/Users/ldu020/workspace/github.com/mrdulin/jest-codelab/src/stackoverflow/56735795/server.spec.ts:25:35)
          at Object.asyncJestTest (/Users/ldu020/workspace/github.com/mrdulin/jest-codelab/node_modules/jest-jasmine2/build/jasmineAsyncInstall.js:102:37)
          at resolve (/Users/ldu020/workspace/github.com/mrdulin/jest-codelab/node_modules/jest-jasmine2/build/queueRunner.js:43:12)
          at new Promise (<anonymous>)
          at mapper (/Users/ldu020/workspace/github.com/mrdulin/jest-codelab/node_modules/jest-jasmine2/build/queueRunner.js:26:19)
          at promise.then (/Users/ldu020/workspace/github.com/mrdulin/jest-codelab/node_modules/jest-jasmine2/build/queueRunner.js:73:41)
          at process._tickCallback (internal/process/next_tick.js:68:7)

  console.error node_modules/koa/lib/application.js:202
    undefined

-----------|----------|----------|----------|----------|-------------------|
File       |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
-----------|----------|----------|----------|----------|-------------------|
All files  |    95.24 |       50 |      100 |    94.12 |                   |
 server.ts |    95.24 |       50 |      100 |    94.12 |                28 |
-----------|----------|----------|----------|----------|-------------------|
Test Suites: 1 passed, 1 total
Tests:       2 passed, 2 total
Snapshots:   0 total
Time:        10.36s

Here is the completed demo: https://github.com/mrdulin/jest-codelab/tree/master/src/stackoverflow/56735795

Solution 2

I tried to get node-fetch mocks working with jest and typescript using manual global mocking and a bunch of different packages as suggested in other answers. Here is the only thing that worked for me;

install 'jest-fetch-mock'

Create the tests;

// To mock fetch we need to import and enable before all other imports
import { enableFetchMocks } from 'jest-fetch-mock';
enableFetchMocks();
import ...
import ...
import ...
import fetch from 'node-fetch';

describe("I lost too many hours on this", () => {
    let spyFetch;

    beforeAll(() => {
        spyFetch = (fetch as jest.MockedFunction<typeof fetch>);
        spyFetch.mockImplementation(() => Promise.resolve({json: () => 'YMMV'}));
    });

    afterEach(() => {
        spyFetch.mockClear();
    });

    it("Call method which contains fetch", async () => {
        // Setup
        ...

        await exampleClass.methodContainingFetch('TestUrl');

        expect(spyFetch.mock.calls[0][0]).toBe('TestUrl');
    });
});

If you are having issues with typescript type-hinting, for a while I used

spyFetch = (fetch as any)

The above is missing supertest but I doubt people stumbling across this question are having issues with that part.

Comments

[Joe]

Thanks for this solution. I had to add "esModuleInterop": true to my tsconfig.json but after that it all worked.

[axel]

this line ;(fetch as jest.MockedFunction<typeof fetch>).mockResolvedValueOnce(new Response(JSON.stringify(expectedResponse))) gives me this error TypeError: _nodeFetch.default.mockResolvedValueOnce is not a function - ideas?