storybook/docs/snippets/common/isomorphic-fetch-mock.js.mdx

```js
// __mocks__/isomorphic-fetch.js

// Your fetch implementation to be added to ./storybook/main.js.
// In your webpackFinal configuration object.

let nextJson;
export default async function fetch() {
  if (nextJson) {
    return {
      json: () => nextJson,
    };
  }
  nextJson = null;
}

// The decorator to be used in ./storybook/preview to apply the mock to all stories

export function decorator(story, { parameters }) {
  if (parameters && parameters.fetch) {
    nextJson = parameters.fetch.json;
  }
  return story();  
}
```