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Test coverage

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.

<CodeSnippets paths={[ 'common/storybook-coverage-addon-install.yarn.js.mdx', 'common/storybook-coverage-addon-install.npm.js.mdx', 'common/storybook-coverage-addon-install.pnpm.js.mdx', ]} />

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

<CodeSnippets paths={[ 'common/storybook-coverage-addon-registration.js.mdx', 'common/storybook-coverage-addon-registration.ts.mdx', ]} />

Start your Storybook with:

<CodeSnippets paths={[ 'common/storybook-run-dev.yarn.js.mdx', 'common/storybook-run-dev.npm.js.mdx', 'common/storybook-run-dev.pnpm.js.mdx', ]} />

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

<CodeSnippets paths={[ 'common/storybook-test-runner-coverage.yarn.js.mdx', 'common/storybook-test-runner-coverage.npm.js.mdx', 'common/storybook-test-runner-coverage.pnpm.js.mdx', ]} />

Coverage test output

Configure

By default, the @storybook/addon-coverage offers zero-config support for Storybook and instruments your code via babel-plugin-istanbul for Babel, 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 and examples of how to use them.

<CodeSnippets paths={[ 'common/storybook-coverage-addon-config-options.js.mdx', 'common/storybook-coverage-addon-config-options.ts.mdx', ]} />

Option Description Plugin
cwd Defines the current working directory
options: { istanbul: { cwd: process.cwd(),}}		 Babel, Vite
include Select the files to collect coverage
options: { istanbul: { include: ['**/stories/**'],}}		 Babel, Vite
exclude Select the files to exclude from coverage
options: { istanbul: { exclude: ['**/stories/**'],}}		 Babel, Vite
extension Sets additional file extensions for coverage
options: { istanbul: { extension: ['.js', '.cjs', '.mjs'],}}		 Babel, Vite
nycrcPath Defines the relative path for the existing nyc configuration file
options: { istanbul: { nycrcPath: '../nyc.config.js',}}		 Babel, Vite
excludeNodeModules Disables node_modules directory introspection
options: { istanbul: { excludeNodeModules:false,}}		 Babel
ignoreClassMethods Configures a set of method names to ignore from being collected
options: { istanbul: { ignoreClassMethods: ['example', 'myMethod'],}}		 Babel
useInlineSourceMaps Enables coverage collection on source maps
options: { istanbul: { useInlineSourceMaps: false,}}		 Babel
inputSourceMap Sets the value to store the source map.
Useful for instrumenting code programmatically
options: { istanbul: { inputSourceMap: sourceMap,}}		 Babel
onCover Hook to monitor coverage collection for all tests
options: { istanbul: { onCover: (fileName, fileCoverage) => {},}}		 Babel
requireEnv Overrides the VITE_COVERAGE environment variable's value by granting access to the env variables
options: { istanbul: { requireEnv: true,}}		 Vite
cypress Replaces the VITE_COVERAGE environment variable with CYPRESS_COVERAGE.
Requires Cypress
options: { istanbul: { cypress: true,}}		 Vite
checkProd Configures the plugin to skip instrumentation in production environments
options: { istanbul: { checkProd: true,}}		 Vite
forceBuildInstrument Configures the plugin to add instrumentation in build mode
options: { istanbul: { forceBuildInstrument: true,}}		 Vite

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:

<CodeSnippets paths={[ 'common/storybook-coverage-report-lcov.js.mdx', ]} />

Troubleshooting

Run test coverage in other frameworks

If you intend on running coverage tests in frameworks with special files like Vue 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):

<CodeSnippets paths={[ 'common/storybook-coverage-report-vue.json.mdx', 'common/storybook-coverage-report-vue.js.mdx', ]} />

The coverage addon doesn't support instrumented code

As the coverage addon is based on Babel 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