Testing async API calls using Jest’s mocking features

Jest is a great JavaScript testing framework by Facebook. It’s often used for testing React components, but it’s also a pretty good general purpose testing framework. In this tutorial I’ll give a quick and simple demo of it’s mocking capabilities for testing async functions.

This article was also published on my own personal blog.

Purpose of mocking

Mocking async function (like API calls) instead of testing with a real API is useful for a couple of reasons:

It’s faster: you don’t have to wait until the API response comes in and you don’t have to deal with rate limits.

It makes your tests ‘pure’, i.e. whether they fail or pass depends only on your code, and not on the data that the API returns.

It’s easier in the long run: no need to first login or set some state before you can start testing a certain endpoint.

Setting up Jest

Configuring Jest isn’t all that difficult, but to get started quickly I’m using the official starter kit by Facebook, create-react-app. This comes with a working Jest configuration out of the box!

Install the create-react-app and create the app:

yarn global add create-react-app



create-react-app mocking-with-jest

cd mocking-with-jest/

yarn start

This should open a browser window with a spinning React logo.

Now start the Jest testing environment:

yarn test

You should see something like this:

Congratulations, you’ve now got Jest up and running and are ready to start writing some tests!

Adding the API

We’ll be using rest.js for the making the API requests. Rest.js works well in the browser and Node.js.

yarn add rest

Create a new folder api in the folder src and add the file github.js with the following code:

Then also add request.js with the following code:

You’re now ready to make API requests using: github.getUser(‘vnglst’). Let’s test this quickly in the browser. Replace the original contents in the file App.js with the following code:

Use yarn start to open up a browser and see the result. You should see a simple list with my profile data. It works!

Writing tests

We’re now ready to write some tests. Let’s first try to unit test the function getUser. Create a folder __tests__ and in this folder a file github.test.js with the following code:

Start Jest using yarn test and you should see the following error message:

What’s happening here? It turns out that using our getUser function in a Node environment (which Jest is using to run the tests) makes our request fail. So even though our function works in the browser, it fails in our tests! Would could try and fix this, by adding a User-Agent header when we’re running this in a Node environment, but mocking would now be a better solution. Let’s set this up!

Creating a mock

Create a folder __mocks__ and in this folder a file request.js (this will be the mocked version of request.js in the parent folder):

The mocked function expects a userId.json (i.e. vnglst.json) file in a folder __mockData__. I’ve used my own Github username and data as an example here, but you can add as many example data as you need for your tests:

To let Jest mock the request module, you have to add the following line to github.test.js file:

jest.mock(‘../request’)

Now the simple unit test should pass and you’re ready to write more complicated tests!

I hope you enjoyed this tutorial and feel free to ask me any questions. You can find me on Twitter as @vnglst. The source code of this tutorial can be found here: https://github.com/vnglst/mocking-with-jest

This tutorial is based upon the async example by the creators of Jest (and their example is probably better 😂). Be sure to also check out their other examples.

BONUS: testing using async/await

If you’re using the create-react-app you can also use async/await to write your tests. This definitely makes your tests easier to write and more readable: