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.

With Storybook Test

When you run component tests with the Test addon, which is powered by Vitest, it can generate a coverage report. The result is summarized in the testing module, showing the percentage of statements covered by your tested stories.

Set up

Coverage is included in the Test addon and, when enabled, will be calculated when running component tests for your project. To enable coverage, press the edit button (pencil icon) in the testing module and toggle coverage on:

Before coverage can be calculated, you may need to install a support package corresponding to your coverage provider:

# For v8
npm install --save-dev @vitest/coverage-v8
 
# For istanbul
npm install --save-dev @vitest/coverage-istanbul

Usage

Because coverage is built into the Test addon, you can use it everywhere you run your tests.

Storybook UI

When you enable coverage in the Storybook UI, the coverage report will be generated and summarized in the testing module after you run your tests. You can see the percentage of statements covered by your tested stories, as well as whether the coverage meets the watermarks.

Additionally, the full coverage report will be served at the /coverage/index.html route of your running Storybook.

CLI

Like the rest of Storybook Test, coverage is built on top of Vitest. Which means you can generate a coverage report using the Vitest CLI.

Assuming you run your tests with a package script like this:

{
  "scripts": {
    "test-storybook": "vitest --project=storybook"
  }
}

Then you can generate a coverage report with:

npm run test-storybook -- --coverage

The coverage report will be saved to the configured coverage reports directory (./coverage, by default) in your project.

npx vitest --coverage

Editor extension

Coverage is also available through Vitest's IDE integrations. You can calculate and display coverage results directly in your editor.

CI

To generate coverage reports in your CI pipeline, you can use the CLI.

For example, here's a simplified GitHub Actions workflow that runs your tests and generates a coverage report:

name: Storybook Tests
on: push
jobs:
  test:
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: '20.x'
      - name: Install dependencies
        run: yarn
      - name: Run Storybook tests
        run: yarn test-storybook --coverage

For more on testing in CI, see the Test addon docs.

Configuration

Coverage provider

You can choose which provider, v8 (default) or Istanbul, to use for coverage calculation by setting the coverage.provider option in your Vitest config:

import { defineConfig } from 'vitest/config';
 
export default defineConfig({
  // ...
  test: {
    // ...
    coverage: {
      // ...
      provider: 'istanbul', // 'v8' is the default
    },
  },
});

Watermarks

Both coverage providers support watermarks, which are threshold values for coverage. The low watermark is the minimum coverage required to pass the test, and the high watermark is the minimum coverage required to be considered good. A coverage percentage between the low and high watermarks will be considered acceptable but not ideal.

In the testing module, the coverage summary will show the percentage of statements covered by your tested stories, as well as whether the coverage meets the watermarks. Below the low watermark, the icon will be red, between the low and high watermarks, it will be orange, and above the high watermark, it will be green.

To configure the watermarks, you can adjust the Vitest config:

import { defineConfig } from 'vitest/config';
 
export default defineConfig({
  // ...
  test: {
    // ...
    coverage: {
      // ...
      watermarks: {
        // These are the default values
        statements: [50, 80],
      },
    },
  },
});

Additional configuration

You can find more configuration options for coverage in the Vitest documentation.

When calculating coverage in the Storybook UI, the following options are always ignored:

• enabled
• clean
• cleanOnRerun
• reportOnFailure
• reporter
• reportsDirectory

Troubleshooting

Excluding stories from the coverage report

Until Vitest 3.0.0 is released, the generated coverage report will include the stories files themselves and output from your built Storybook application. This is misleading and they should be excluded. To do this, you can add the following to your Vitest config:

import { coverageConfigDefaults, defineConfig } from 'vitest/config';
 
export default defineConfig({
  // ...
  test: {
    coverage: {
      // 👇 Add this
      exclude: [
         ...coverageConfigDefaults.exclude,
         // This pattern must align with the `stories` property of your `.storybook/main.ts` config
         '**/*.stories.*',
         // This pattern must align with the output directory of `storybook build`
         'storybook-static/**',
       ], 
    }
  }
})

With the coverage addon

Storybook also provides a coverage addon. It is 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

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.

npx storybook@latest add @storybook/addon-coverage

Start your Storybook with:

npm run storybook

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

npm run test-storybook -- --coverage

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.

// For Vite support add the following import
// import type { AddonOptionsVite } from '@storybook/addon-coverage';
 
import type { AddonOptionsWebpack } from '@storybook/addon-coverage';
 
// Replace your-framework with the framework and builder you are using (e.g., react-webpack5, vue3-webpack5)
import type { StorybookConfig } from '@storybook/your-framework';
 
const coverageConfig: AddonOptionsWebpack = {
  istanbul: {
    include: ['**/stories/**'],
    exclude: ['**/exampleDirectory/**'],
  },
};
 
const config: StorybookConfig = {
  stories: [],
  addons: [
    // Other Storybook addons
    {
      name: '@storybook/addon-coverage',
      options: coverageConfig,
    },
  ],
};
 
export default config;

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:

npx nyc report --reporter=lcov -t coverage/storybook --report-dir coverage/storybook

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):

export default {
  // Other configuration options
  extension: ['.js', '.cjs', '.mjs', '.ts', '.tsx', '.jsx', '.vue'],
};

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.

// Replace your-framework with the framework you are using (e.g., react-webpack5, vue3-vite)
import type { StorybookConfig } from '@storybook/your-framework';
 
const config: StorybookConfig = {
  framework: '@storybook/your-framework',
  stories: ['../src/**/*.mdx', '../src/**/*.stories.@(js|jsx|mjs|ts|tsx)'],
  addons: [
    '@storybook/addon-essentials',
    '@storybook/addon-interactions',
    '@storybook/addon-coverage',
  ],
  build: {
    test: {
      disabledAddons: ['@storybook/addon-docs', '@storybook/addon-essentials/docs'],
    },
  },
};
 
export default config;

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

• Component tests for user behavior simulation
• Visual tests for appearance
• Accessibility tests for accessibility
• Snapshot tests for rendering errors and warnings
• Test runner to automate test execution
• Test coverage for measuring code coverage
• End-to-end tests for simulating real user scenarios
• Unit tests for functionality