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.
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.
Please note: This guide assumes you’re using Jest 29 or above.
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:
Go over the content of this configuration file:
transformsection tells Jest that all
jsxfiles need to be transformed using a
jest-preprocess.jsfile 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:
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
The next option is very important and is different from what you’ll find in other Jest guides. The reason that you need
transformIgnorePatternsis 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.jsisn’t being transpiled before running in Jest. You can fix this by changing the default
transformIgnorePatternsto exclude the
__PATH_PREFIX__, which is usually set by Gatsby, and which some components need.
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
setupFilesarray lets you list files that will be included before all tests are run, so it’s perfect for this.
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
Slice, or GraphQL.
This guide explains how you can set up Jest with Gatsby, if you want to know how to write tests we highly recommend checking out the Testing React components guide. It explains how to install
@testing-library/react which is our recommended utility for Jest.
Visit the Jest docs to learn more about other tests that you can write.
If you look inside
package.json you will probably find that there is already a
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.
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.
<rootDir> is replaced by Jest with the root directory of the project. Don’t change it.
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
If you are using tsconfig paths there is a single change to your config.
- Add ts-jest
jest.config.jsto import and map
- Add paths to
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.