Mock node-fetch with JEST

Recently I tried my hands on Jest - the popular JavaScript Testing Library. In my application we were using node-fetch to make API calls. I used Jest to mock and test this API calls. Here in this blog I will explain on how to mock node-fetch with Jest. 

Following is my function to call the GET API. 

fetch_get.js

const fetch = require('node-fetch');

module.exports = async () => {

    return await fetch('http://YOUR_GET_URL').then(res => res.json());

};

Now to test this create a test file with name test.fetch_get.js

Step 1: First import the function.

const fetch_get = require('../fetch_get');

Step 2 : Mock the node-fetch with Jest

const fetch = require('node-fetch');

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

Step 3: Create Mock response

const mockedRes = {

  "success": true,

  "data": [

    {

        id: 1,

        text: 'Test Data 1'

    },

    {

        id: 1,

        text: 'Test Data 2'

    }

  ]

};

Step 4: Add Test 

describe("Test get fetch", () => {

    let data;

    it('It should return the data', () => {

      const response = Promise.resolve({

            ok: true,

            status: 200,

            json: () => {

                return mockedRes;

            },

        });

        fetch.mockImplementation(()=> response)

        data = await fetch_get();

        expect(data).toEqual(mockedRes);

    });

});

In above code we have created a rest and mocked the response to return our response when we call .json() method for response. Then we are simply comparing the response. This is a very simple test. Here in stead of comparing data, you may have other tests like checking length of data or checking specific data etc.

Step 5: Run Test

Run the test with npm test or npm run test and you will see the following result.

Test get fetch

    ✓ It should return the data (17 ms)

Test Suites: 1 passed, 1 total

Tests:       1 passed, 1 total

Snapshots:   0 total

Time:        0.313 s, estimated 1 s

Hope this helps you.