Test coverage

Watch a video tutorial on the Storybook channel

Test coverage is the practice of measuring whether existing tests fully cover your code. That means surfacing areas which aren't currently being tested, such as: conditions, logic branches, functions and variables.

Coverage tests examine the instrumented code against a set of industry-accepted best practices. They act as the last line of QA to improve the quality of your test suite.

Code instrumentation with the coverage addon

Storybook provides an official test coverage addon. Powered by Istanbul, which allows out-of-the-box code instrumentation for the most commonly used frameworks and builders in the JavaScript ecosystem.

Set up the coverage addon

Engineered to work alongside modern testing tools (e.g., Playwright), the coverage addon automatically instruments your code and generates code coverage data. For an optimal experience, we recommend using the test runner alongside the coverage addon to run your tests.

Run the following command to install the addon.

Update your Storybook configuration (in .storybook/main.js|ts) to include the coverage addon.

Start your Storybook with:

Finally, open a new terminal window and run the test-runner with:

Coverage test output

Configure

By default, the @storybook/addon-coverage offers zero-config support for Storybook and instruments your code via istanbul-lib-instrument for Webpack, or vite-plugin-istanbul for Vite. However, you can extend your Storybook configuration file (i.e., .storybook/main.js|ts) and provide additional options to the addon. Listed below are the available options divided by builder and examples of how to use them.

Vite optionsDescriptionType
checkProdConfigures the plugin to skip instrumentation in production environments
options: { istanbul: { checkProd: true,}}
boolean
cwdConfigures the working directory for the coverage tests.
Defaults to process.cwd()
options: { istanbul: { cwd: process.cwd(),}}
string
cypressReplaces the VITE_COVERAGE environment variable with CYPRESS_COVERAGE.
Requires Cypress's code coverage
options: { istanbul: { cypress: true,}}
boolean
excludeOverrides the default exclude list with the provided list of files or directories to exclude from coverage
options: { istanbul: { exclude: ['**/stories/**'],}}
Array<String> or string
extensionExtends the default extension list with the provided list of file extensions to include in coverage
options: { istanbul: { extension: ['.js', '.cjs', '.mjs'],}}
Array<String> or string
forceBuildInstrumentConfigures the plugin to add instrumentation in build mode
options: { istanbul: { forceBuildInstrument: true,}}
boolean
includeSelect the files to collect coverage
options: { istanbul: { include: ['**/stories/**'],}}
Array<String> or string
nycrcPathDefines the relative path for the existing nyc configuration file
options: { istanbul: { nycrcPath: '../nyc.config.js',}}
string
requireEnvOverrides the VITE_COVERAGE environment variable's value by granting access to the env variables
options: { istanbul: { requireEnv: true,}}
boolean
Webpack 5 optionsDescriptionType
autoWrapProvides support for top-level return statements by wrapping the program code in a function
options: { istanbul: { autoWrap: true,}}
boolean
compactCondenses the output of the instrumented code. Useful for debugging
options: { istanbul: { compact: false,}}
boolean
coverageVariableDefines the global variable name that Istanbul will use to store coverage results
options: { istanbul: { coverageVariable: '__coverage__',}}
string
cwdConfigures the working directory for the coverage tests.
Defaults to process.cwd()
options: { istanbul: { cwd: process.cwd(),}}
string
debugEnables the debug mode for additional logging information during the instrumentation process
options: { istanbul: { debug: true,}}
boolean
esModulesEnables support for ES Module syntax
options: { istanbul: { esModules: true,}}
boolean
excludeOverrides the default exclude list with the provided list of files or directories to exclude from coverage
options: { istanbul: { exclude: ['**/stories/**'],}}
Array<String> or string
extensionExtends the default extension list with the provided list of file extensions to include in coverage
options: { istanbul: { extension: ['.js', '.cjs', '.mjs'],}}
Array<String> or string
includeSelect the files to collect coverage
options: { istanbul: { include: ['**/stories/**'],}}
Array<String> or string
nycrcPathDefines the relative path for the existing nyc configuration file
options: { istanbul: { nycrcPath: '../nyc.config.js',}}
string
preserveCommentsIncludes comments in the instrumented code
options: { istanbul: { preserveComments: true,}}
boolean
produceSourceMapConfigures Instanbul to generate a source map for the instrumented code
options: { istanbul: { produceSourceMap: true,}}
boolean
sourceMapUrlCallbackDefines a callback function invoked with the filename and the source map URL when a source map is generated
options: { istanbul: { sourceMapUrlCallback: (filename, url) => {},}}
function

What about other coverage reporting tools?

Out of the box, code coverage tests work seamlessly with Storybook's test-runner and the @storybook/addon-coverage. However, that doesn't mean you can't use additional reporting tools (e.g., Codecov). For instance, if you're working with LCOV, you can use the generated output (in coverage/storybook/coverage-storybook.json) and create your own report with:


Troubleshooting

Run test coverage in other frameworks

If you intend on running coverage tests in frameworks with special files like Vue 3 or Svelte, you'll need to adjust your configuration and enable the required file extensions. For example, if you're using Vue, you'll need to add the following to your nyc configuration file (i.e., .nycrc.json or nyc.config.js):

The coverage addon doesn't support optimized builds

If you generated a production build optimized for performance with the --test flag, and you're using the coverage addon to run tests against your Storybook, you may run into a situation where the coverage addon doesn't instrument your code. This is due to how the flag works, as it removes addons that have an impact on performance (e.g., Docs, coverage addon). To resolve this issue, you'll need to adjust your Storybook configuration file (i.e., .storybook/main.js|ts) and include the disabledAddons option to allow the addon to run tests at the expense of a slower build.

The coverage addon doesn't support instrumented code

As the coverage addon is based on Webpack5 loaders and Vite plugins for code instrumentation, frameworks that don't rely upon these libraries (e.g., Angular configured with Webpack), will require additional configuration to enable code instrumentation. In that case, you can refer to the following repository for more information.

Learn about other UI tests

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