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Unit Testing


  • Using Jest

Unit testing is a great way to protect against errors in your code before you deploy it. While Gatsby does not include support for unit testing out of the box, it only takes a few steps to get up and running. However, there are a few features of the Gatsby build process that mean the standard Jest setup doesn’t quite work. This guide shows you how to set it up.

Setting up your environment

The most popular testing framework for React is Jest, which was created by Facebook. While Jest is a general-purpose JavaScript unit testing framework, it has lots of features that make it work particularly well with React.

Note: For this guide, you will be starting with gatsby-starter-default, but the concepts should be the same or very similar for your site.

1. Installing dependencies

First, you need to install Jest and some more required packages. Install babel-jest and babel-preset-gatsby to ensure that the babel preset(s) that are used match what are used internally for your Gatsby site.

2. Creating a configuration file for Jest

Because Gatsby handles its own Babel configuration, you will need to manually tell Jest to use babel-jest. The easiest way to do this is to add a jest.config.js. You can set up some useful defaults at the same time:

Note: If you’re using Jest 28 or above, you can skip adding the moduleNameMapper options for gatsby-page-utils, gatsby-core-utils, and gatsby-plugin-utils. The mentioned bug was fixed in Jest 28.

Go over the content of this configuration file:

  • The transform section tells Jest that all js or jsx files need to be transformed using a jest-preprocess.js file in the project root. Go ahead and create this file now. This is where you set up your Babel config. You can start with the following minimal config:

Note: If you’re using Jest 26.6.3 or below, the last line has to be changed to module.exports = require("babel-jest").createTransformer(babelOptions)

  • The next option is moduleNameMapper. This section works a bit like webpack rules and tells Jest how to handle imports. You are mainly concerned here with mocking static file imports, which Jest can’t handle. A mock is a dummy module that is used instead of the real module inside tests. It is good when you have something that you can’t or don’t want to test. You can mock anything, and here you are mocking assets rather than code. For stylesheets you need to use the package identity-obj-proxy. For all other assets, you need to use a manual mock called file-mock.js. You need to create this yourself. The convention is to create a directory called __mocks__ in the root directory for this. Note the pair of double underscores in the name.
  • The next config setting is testPathIgnorePatterns. You are telling Jest to ignore any tests in the node_modules or .cache directories.

  • The next option is very important and is different from what you’ll find in other Jest guides. The reason that you need transformIgnorePatterns is because Gatsby includes un-transpiled ES6 code. By default Jest doesn’t try to transform code inside node_modules, so you will get an error like this:

This is because gatsby-browser-entry.js isn’t being transpiled before running in Jest. You can fix this by changing the default transformIgnorePatterns to exclude the gatsby module.

  • The globals section sets __PATH_PREFIX__, which is usually set by Gatsby, and which some components need.

  • You need to set testURL to a valid URL, because some DOM APIs such as localStorage are unhappy with the default (about:blank).

Note: if you’re using Jest 23.5.0 or later, testURL will default to http://localhost so you can skip this setting.

  • There’s one more global that you need to set, but as it’s a function you can’t set it here in the JSON. The setupFiles array lets you list files that will be included before all tests are run, so it’s perfect for this.

3. Useful mocks to complete your testing environment

Mocking gatsby

Finally, it’s a good idea to mock the gatsby module itself. This may not be needed at first, but will make things a lot easier if you want to test components that use Link or GraphQL.

This mocks the graphql() function, Link component, and useStaticQuery hook.

Writing tests

A full guide to unit testing is beyond the scope of this guide, but you can start with a snapshot test to check that everything is working.

First, create the test file. You can either put these in a __tests__ directory, or put them elsewhere (usually next to the component itself), with the extension .spec.js or .test.js. The decision comes down to your own preference. In this guide, you will use the __tests__ folder convention. To test the header component, create a header.js file in src/components/__tests__/:

This is a very brief snapshot test, which uses react-test-renderer to render the component, and then generates a snapshot of it on the first run. It then compares future snapshots against this, which means you can quickly check for regressions. Visit the Jest docs to learn more about other tests that you can write.

Running tests

If you look inside package.json you will probably find that there is already a script for test, which just outputs an error message. Change this to use the jest executable that you now have available, like so:

This means you can now run tests by typing npm test. If you want you could also run with a flag that triggers watch mode to watch files and run tests when they are changed: npm test -- --watch.

Run the tests again now and it should all work! You may get a message about the snapshot being written. This is created in a __snapshots__ directory next to your tests. If you take a look at it, you will see that it is a JSON representation of the <Header /> component. You should check your snapshot files into a source control system (for example, a GitHub repo) so that any changes are tracked in history. This is particularly important to remember if you are using a continuous integration system such as Travis or CircleCI to run tests, as these will fail if the snapshot is not checked into source control.

If you make changes that mean you need to update the snapshot, you can do this by running npm test -- -u.

Using TypeScript

If you are using TypeScript, you need to install typings packages and make two changes to your config.

Update the transform in jest.config.js to run jest-preprocess on files in your project’s root directory.

Note: <rootDir> is replaced by Jest with the root directory of the project. Don’t change it.

Also update jest-preprocess.js with the following Babel preset to look like this:

Once this is changed, you can write your tests in TypeScript using the .ts or .tsx extensions.

Using tsconfig paths

If you are using tsconfig paths there is a single change to your config.

  1. Add ts-jest
  1. Update jest.config.js to import and map tsconfig.json paths
  1. Add paths to jest.config.js moduleNameMapper

Other resources

If you need to make changes to your Babel config, you can edit the config in jest-preprocess.js. You may need to enable some of the plugins used by Gatsby. See the Gatsby Babel config guide for some examples.

For more information on Jest testing, visit the Jest site.

For an example encapsulating all of these techniques and a full unit test suite with @testing-library/react, check out the using-jest example.

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