---
title: 'Test coverage'
sidebar:
  order: 5
  title: Test coverage
---

<YouTubeCallout id="wEa6W8uUGSA" title="These tests use NO CODE | component testing in Storybook" />

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.

<Video src="../_assets/writing-tests/component-test-coverage-whitebg.mp4" />

## Code instrumentation with the coverage addon

Storybook provides an official [test coverage addon](https://storybook.js.org/addons/@storybook/addon-coverage). Powered by [Istanbul](https://istanbul.js.org/), 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](https://playwright.dev/)), the coverage addon automatically instruments your code and generates code coverage data. For an optimal experience, we recommend using the [test runner](./test-runner.md) alongside the coverage addon to run your tests.

Run the following command to install the addon.

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<CodeSnippets path="storybook-coverage-addon-install.md" />

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

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<CodeSnippets path="storybook-coverage-addon-registration.md" />

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Start your Storybook with:

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<CodeSnippets path="storybook-run-dev.md" />

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

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<CodeSnippets path="test-runner-coverage.md" />

{/* prettier-ignore-end */}

![Coverage test output](../_assets/writing-tests/test-runner-coverage-result.png)

### Configure

By default, the [`@storybook/addon-coverage`](https://storybook.js.org/addons/@storybook/addon-coverage) offers zero-config support for Storybook and instruments your code via [`istanbul-lib-instrument`](https://www.npmjs.com/package/istanbul-lib-instrument) for [Webpack](https://webpack.js.org/), or [`vite-plugin-istanbul`](https://github.com/iFaxity/vite-plugin-istanbul) for [Vite](https://vitejs.dev/). 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.

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<CodeSnippets path="storybook-coverage-addon-config-options.md" />

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| Vite options           | Description                                                                                                                                                                                                                                        | Type                        |
| ---------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | --------------------------- |
| `checkProd`            | Configures the plugin to skip instrumentation in production environments<br />`options: { istanbul: { checkProd: true,}}`                                                                                                                           | `boolean`                   |
| `cwd`                  | Configures the working directory for the coverage tests.<br />Defaults to `process.cwd()`<br />`options: { istanbul: { cwd: process.cwd(),}}`                                                                                                        | `string`                    |
| `cypress`              | Replaces the `VITE_COVERAGE` environment variable with `CYPRESS_COVERAGE`.<br />Requires Cypress's [code coverage](https://docs.cypress.io/guides/tooling/code-coverage)<br />`options: { istanbul: { cypress: true,}}`                              | `boolean`                   |
| `exclude`              | Overrides the [default exclude list](https://github.com/storybookjs/addon-coverage/blob/main/src/constants.ts) with the provided list of files or directories to exclude from coverage<br />`options: { istanbul: { exclude: ['**/stories/**'],}}`  | `Array<String>` or `string` |
| `extension`            | Extends the [default extension list](https://github.com/storybookjs/addon-coverage/blob/main/src/constants.ts) with the provided list of file extensions to include in coverage<br />`options: { istanbul: { extension: ['.js', '.cjs', '.mjs'],}}` | `Array<String>` or `string` |
| `forceBuildInstrument` | Configures the plugin to add instrumentation in build mode <br />`options: { istanbul: { forceBuildInstrument: true,}}`                                                                                                                             | `boolean`                   |
| `include`              | Select the files to collect coverage<br />`options: { istanbul: { include: ['**/stories/**'],}}`                                                                                                                                                    | `Array<String>` or `string` |
| `nycrcPath`            | Defines the relative path for the existing nyc [configuration file](https://github.com/istanbuljs/nyc?tab=readme-ov-file#configuration-files)<br />`options: { istanbul: { nycrcPath: '../nyc.config.js',}}`                                        | `string`                    |
| `requireEnv`           | Overrides the `VITE_COVERAGE` environment variable's value by granting access to the `env` variables<br />`options: { istanbul: { requireEnv: true,}}`                                                                                              | `boolean`                   |

| Webpack 5 options      | Description                                                                                                                                                                                                                                        | Type                        |
| ---------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | --------------------------- |
| `autoWrap`             | Provides support for top-level return statements by wrapping the program code in a function<br />`options: { istanbul: { autoWrap: true,}}`                                                                                                         | `boolean`                   |
| `compact`              | Condenses the output of the instrumented code. Useful for debugging<br />`options: { istanbul: { compact: false,}}`                                                                                                                                 | `boolean`                   |
| `coverageVariable`     | Defines the global variable name that Istanbul will use to store coverage results<br />`options: { istanbul: { coverageVariable: '__coverage__',}}`                                                                                                 | `string`                    |
| `cwd`                  | Configures the working directory for the coverage tests.<br />Defaults to `process.cwd()`<br />`options: { istanbul: { cwd: process.cwd(),}}`                                                                                                        | `string`                    |
| `debug`                | Enables the debug mode for additional logging information during the instrumentation process<br />`options: { istanbul: { debug: true,}}`                                                                                                           | `boolean`                   |
| `esModules`            | Enables support for ES Module syntax<br />`options: { istanbul: { esModules: true,}}`                                                                                                                                                               | `boolean`                   |
| `exclude`              | Overrides the [default exclude list](https://github.com/storybookjs/addon-coverage/blob/main/src/constants.ts) with the provided list of files or directories to exclude from coverage<br />`options: { istanbul: { exclude: ['**/stories/**'],}}`  | `Array<String>` or `string` |
| `extension`            | Extends the [default extension list](https://github.com/storybookjs/addon-coverage/blob/main/src/constants.ts) with the provided list of file extensions to include in coverage<br />`options: { istanbul: { extension: ['.js', '.cjs', '.mjs'],}}` | `Array<String>` or `string` |
| `include`              | Select the files to collect coverage<br />`options: { istanbul: { include: ['**/stories/**'],}}`                                                                                                                                                    | `Array<String>` or `string` |
| `nycrcPath`            | Defines the relative path for the existing nyc [configuration file](https://github.com/istanbuljs/nyc?tab=readme-ov-file#configuration-files)<br />`options: { istanbul: { nycrcPath: '../nyc.config.js',}}`                                        | `string`                    |
| `preserveComments`     | Includes comments in the instrumented code<br />`options: { istanbul: { preserveComments: true,}}`                                                                                                                                                  | `boolean`                   |
| `produceSourceMap`     | Configures Instanbul to generate a source map for the instrumented code<br />`options: { istanbul: { produceSourceMap: true,}}`                                                                                                                     | `boolean`                   |
| `sourceMapUrlCallback` | Defines a callback function invoked with the filename and the source map URL when a source map is generated<br />`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`](https://storybook.js.org/addons/@storybook/addon-coverage). However, that doesn't mean you can't use additional reporting tools (e.g., [Codecov](https://about.codecov.io/)). For instance, if you're working with [LCOV](https://wiki.documentfoundation.org/Development/Lcov), you can use the generated output (in `coverage/storybook/coverage-storybook.json`) and create your own report with:

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<CodeSnippets path="storybook-coverage-report-lcov.md" />

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***

## 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`):

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<CodeSnippets path="storybook-coverage-report-vue.md" />

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### The coverage addon doesn't support optimized builds

If you generated a production build optimized for performance with the [`--test`](../sharing/publish-storybook.md#customizing-the-build-for-performance) 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`](../writing-docs/index.md), [coverage addon](https://storybook.js.org/addons/@storybook/addon-coverage)). To resolve this issue, you'll need to adjust your Storybook configuration file (i.e., `.storybook/main.js|ts`) and include the [`disabledAddons`](../api/main-config-build.md#testdisabledaddons) option to allow the addon to run tests at the expense of a slower build.

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<CodeSnippets path="storybook-coverage-addon-optimized-config.md" />

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### The coverage addon doesn't support instrumented code

As the [coverage addon](https://storybook.js.org/addons/@storybook/addon-coverage) 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](https://github.com/yannbf/storybook-coverage-recipes) for more information.

**Learn about other UI tests**

* [Test runner](./test-runner.md) to automate test execution
* [Visual tests](./visual-testing.md) for appearance
* [Accessibility tests](./accessibility-testing.md) for accessibility
* [Interaction tests](./interaction-testing.md) for user behavior simulation
* Coverage tests for measuring code coverage
* [Snapshot tests](./snapshot-testing.md) for rendering errors and warnings
* [End-to-end tests](./stories-in-end-to-end-tests.md) for simulating real user scenarios
* [Unit tests](./stories-in-unit-tests.md) for functionality