storybook/__mocks__/fs.js

const fs = jest.genMockFromModule('fs');

// This is a custom function that our tests can use during setup to specify
// what the files on the "mock" filesystem should look like when any of the
// `fs` APIs are used.
let mockFiles = Object.create(null);

// eslint-disable-next-line no-underscore-dangle
function __setMockFiles(newMockFiles) {
  mockFiles = newMockFiles;
}

// A custom version of `readdirSync` that reads from the special mocked out
// file list set via __setMockFiles
const readFileSync = (filePath = '') => mockFiles[filePath];
const existsSync = filePath => !!mockFiles[filePath];

// eslint-disable-next-line no-underscore-dangle
fs.__setMockFiles = __setMockFiles;
fs.readFileSync = readFileSync;
fs.existsSync = existsSync;

module.exports = fs;