--- title: 'Environment variables' --- You can use environment variables in Storybook to change its behaviour in different “modes”. If you supply an environment variable prefixed with `STORYBOOK_`, it will be available in `process.env`: ```shell STORYBOOK_THEME=red STORYBOOK_DATA_KEY=12345 npm run storybook ``` Then we can access these environment variables anywhere inside our preview JavaScript code like below: You can also access these variables in your custom ``/`` using the substitution `%STORYBOOK_X%`, for example: `%STORYBOOK_THEME%` will become `red`.

If using the environment variables as attributes or values in JavaScript, you may need to add quotes, as the value will be inserted directly. e.g. ``

You can also pass these environment variables when you are [building your Storybook](../workflows/publish-storybook.md) with `build-storybook`. Then they'll be hard coded to the static version of your Storybook.