storybook/docs/_snippets/storybook-preview-global-loader.md

export default {
  loaders: [
    async () => ({
      currentUser: await (await fetch('https://jsonplaceholder.typicode.com/users/1')).json(),
    }),
  ],
};

// Replace your-framework with the framework you are using (e.g., react, vue3)
import { Preview } from '@storybook/your-framework';

const preview: Preview = {
  loaders: [
    async () => ({
      currentUser: await (await fetch('https://jsonplaceholder.typicode.com/users/1')).json(),
    }),
  ],
};

export default preview;