A screenshot journey
React Devtools — Inspiration
Using Webpack’s “watch mode” is a great way to develop web apps. As you edit code, the app rebuilds and automatically reloads the page.
Did you know Jest also has a watch mode?
Watch mode lets you develop iteratively using snapshots of your components in different states as a way to confidently refactor and add features.
In watch mode, Jest automatically re-runs your tests when files change. You can inspect changes and update snapshots with one keypress by pressing “u”. Cool!
Let’s walk through a scenario: Adding tests to a simple app boostrapped with create-react-app.
The App component
The first test
Adding a snapshot
Refactoring the header into its own component: updated snapshot
Adding props and condtional logic to the new Header component: snapshot fails
Adding props and condtional logic to the new Header component: updated snapshot
To avoid coupling the Header implementation to the App component, we can use Enzyme’s shallow render and Enzyme-To-JSON to snapshot only the top level render tree.
Switching to a shallow snapshot: snapshot fails
Switching to a shallow snapshot: updated snapshot
Moving beyond snapshots — .find() and Markers
As you add more snapshot tests you may discover that some become brittle, need to be updated to often, or that you want to assert about more specific results than what high-level snapshotting provides.
To make DOM tests more resilient you can use “test markers” (special classes or data tags that are only meaningful to tests). For example, the Header component can be rendered into different states but it might be nice to assert that something specific appears when logged in and something else when logged out. Add a test marker and use Enzyme DOM traversal methods like .find to assert that they appear when expected.
Shallow rendering the Header in different states
Adding test marker tags (data-test="xyz") and .find()
Zoomed in on the marker implementation
At this stage we might consider removing the snapshots altogether since we’ve identified and expected what we care about.
Testing Stateful Components
- Testing stateful components using Enzyme (component.setState)
- Mocking dependencies as an alternative to shallow rendering (jest.mockFn())
- Test harnesses for React Context and other globals
Discussions around the web — when to use snapshots
Adding Jest & Enzyme
yarn add --dev react-test-renderer jest enzyme enzyme-adapter-react-16 jest-enzyme enzyme-to-json
Follow the Enzyme setup instructions for your environment.