Testing Strategy - Draft2019 Jan 06
Some notes and considerations for a testing strategy for React web apps, I've gathered over time from various BBC News Labs projects
Type of tests - high level
- Unit test - a single function or service (Jest)
- Component test - a single component - functionality (jest/Enzyme)
- Snapshot Test - a single component - regression, eg changes against previous versions (Jest)
- End to End Test - Interaction between multiple components, usually from point of view ouser (Cypress)
- Performance test - How the app performs in difference environment
- Coverage tests - how much of your application of the app is covered by tests
Visual studio code
- Code spell checker, A basic spell checker that works well with camelCase code, to make sure you are using real names for variables, and keeping typos to a minimum.
- ESLint ←
- CSS linting
- Jest ←
- Prettier - auto code formatting, can be combined with ESLint.
- Jest Snippets
- React Jest Snippets
- Bracket Pair Colorizer - “colorizing matching brackets” in VsCode
- Import Cost - is a visual code plugin, that allows you to see the size of our imports in context.
create-react-app comes with some setup for testing with jest.
→ See more and a great overview of what’s available out of the box here. ← Type of tests available
- “smoke test” verifying that a component renders without throwing
- shallow rendering
- testing some of the output
- full rendering
- testing component lifecycle
- state changes.
- Snapshot testing
Snapshot testing is a feature of Jest that automatically generates text snapshots of your components and saves them on the disk so if the UI output changes, you get notified without manually writing any assertions on the component output. Read more about snapshot testing.
Cool feature is coverage reporting, if you run
npm test -- --coverage as gives you a nice table in terminal.
npm testruns the watcher with interactive CLI. However, you can force it to run tests once and finish the process by setting an environment variable called
For continuous integration/deployment, can use
npm run test-ci by adding this to your
package.json npm scripts.
"test-ci": "CI=true react-scripts test --env=jsdom --verbose",
Tests components in isolation
Create React App doesn’t include any tools for this by default, but you can easily add Storybook for React (source) or React Styleguidist (source) to your project. These are third-party tools that let you develop components and see all their states in isolation from your app.
also see storybook docs
Note: If building React components in isolation consider checking out nwb docs**
The primary reason ESLint was created was to allow developers to create their own linting rules. ESLint is designed to have all rules completely pluggable. The default rules are written just like any plugin rules would be. They can all follow the same pattern, both for the rules themselves as well as tests. While ESLint will ship with some built-in rules to make it useful from the start, you’ll be able to dynamically load rules at any point in time.
Key selling point:
No need to discuss style in code review
Setup pre-commit and pre-push hooks, these can be used to keep the code clean over time. The hooks can check against linting rules and/or whether test are passing or not.
Husky can prevent bad
git pushand more 🐶 ❤️ woof!
Seems like to set it up, based on their README as of version
0.14 it might be enough to install it as dev dependency and add these two npm scripts
"precommit": "npm install && npm run-script lint && npm run-script test-ci",
"prepush": "npm install && npm run-script lint && npm run-script test-ci",
Note, if you use
git commit --no-verify flag it will skip the commit hooks, however this practice is highly discouraged.
This is the VS Code's extension that automatically generates PropTypes code for React components, like
ReactPropTypesin the Jetbrains's Platform
##### VS Code Search react-proptypes-generate in Marketplace and install it
There is a Jest extension for Visual code, if that is your editor of choice.
If you use Visual Studio Code, there is a Jest extension which works with Create React App out of the box. This provides a lot of IDE-like features while using a text editor: showing the status of a test run with potential fail messages inline, starting and stopping the watcher automatically, and offering one-click snapshot updates.
Simple and complete React DOM testing utilities that encourage good testing practices.
As an alternative or companion to
enzyme, you may consider using
react-testing-libraryis a library for testing React components in a way that resembles the way the components are used by end users. It is well suited for unit, integration, and end-to-end testing of React components and applications. It works more directly with DOM nodes, and therefore it's recommended to use with
jest-domfor improved assertions.
Enzyme documentation uses Chai and Sinon for assertions but you don’t have to use them because Jest provides built-in
Enzyme supports full rendering with
mount(), and you can also use it for testing state changes and component lifecycle.
The idea is that you can mock a function and test it’s receiving the right arguments, and return some sample output.
Might be similar to Jest mocking function.
Mock functions make it easy to test the links between code by erasing the actual implementation of a function, capturing calls to the function (and the parameters passed in those calls), capturing instances of constructor functions when instantiated with
new, and allowing test-time configuration of return values. There are two ways to mock functions: Either by creating a mock function to use in test code, or writing a
manual mockto override a module dependency.
End to end browser testing
Fast, easy and reliable testing for anything that runs in a browser.
Possible replacement for selenium if you were familiar with that.
Visual regression testing
automates visual regression testing of your responsive web UI by comparing DOM screenshots over time.
it installs PhantomJS and chromium as dependencies
capture screenshots of a particular web application/URL across a wide range of mobile devices and browsers
You might generally go for the latest browser, and latest device, but it's good to compile an explicit list of devices. You can also look at existing analytics to see what are the most common devices and browser combinations for your site/app.
There is also a browserstack screenshots API
Testing API end points
- What We Can Learn About Testing From The Wheel - Kent C. Dodds (Ignite Fluent 2016) - 5min video
- Write tests. Not too many. Mostly integration.
- “Just Say No to More End-to-End Tests” - Google testing blog
- Test-driven Development Using React - pluralsight
- Testing React Applications with Jest - pluralsight
- Cucumber.js BDD guides