src/index.js

const TITLE_KEY = 'title';

const fixObjectDefaultExport = (path, t, title, ifTitleFound) => {
    if (path.node.declaration.properties) {
        const titleProperty = path.node.declaration.properties.find(node =>
            node.key && node.key.name === TITLE_KEY
        );
        if (titleProperty) {
            switch (ifTitleFound) {
                case 'skip':
                    return;
                default:
                    throw new Error(
                        `Default export object has a '${TITLE_KEY}' property; the title should, however, be generated. Please remove '${TITLE_KEY}'.`
                    );
            }
        }
        path.get('declaration').pushContainer(
            'properties', 
            t.objectProperty(t.identifier(TITLE_KEY), t.stringLiteral(title))
        );
    } else {
        throw new Error('Default export object does not have properties.');
    }
}

const replaceDefaultExportPathByNamedDefaultExportPath = (defaultExportPath, t, renameDefaultExportsTo) => {
    defaultExportPath.replaceWith(
        t.exportNamedDeclaration(
            t.variableDeclaration(
                'const',
                [
                    t.variableDeclarator(
                        t.identifier(renameDefaultExportsTo),
                        defaultExportPath.node.declaration
                    )
                ],
            ),
            []
        )
    );
}

const insertDefaultExport = (programPath, t, title) => {
    programPath.pushContainer(
        'body',
        t.exportDefaultDeclaration(t.objectExpression([
            t.objectProperty(t.identifier(TITLE_KEY), t.stringLiteral(title))
        ]))
    );
}

const plugin = babel => {
    const { types: t } = babel;

    return ({
        name: 'Storybook CSF title generation',
        visitor: {
            ExportDefaultDeclaration: (path, state) => {
                if (state.handled) {
                    return;
                }
                state.defaultExportPath = path;
            },
            ExportNamedDeclaration: (path, state) => {
                if (state.handled) {
                    return;
                }
                if (
                    state.opts.renameDefaultExportsTo &&
                    path.node.declaration && 
                    path.node.declaration.type === 'VariableDeclaration' && 
                    path.node.declaration.declarations && 
                    path.node.declaration.declarations[0] &&
                    path.node.declaration.declarations[0].type === 'VariableDeclarator' &&
                    path.node.declaration.declarations[0].id &&
                    path.node.declaration.declarations[0].id.name === state.opts.renameDefaultExportsTo
                ) {
                    state.namedDefaultExportPath = path;
                }
            },
            Program: {
                exit: (path, state) => {
                    if (state.handled) {
                        return;
                    }

                    state.handled = true;

                    const title = state.opts.toTitle(state);
                    const renameDefaultExportsTo = state.opts.renameDefaultExportsTo;
                    const programPath = path;

                    if (state.defaultExportPath) {
                        if (state.defaultExportPath.node.declaration.type === 'ObjectExpression') {
                            fixObjectDefaultExport(state.defaultExportPath, t, title, state.opts.ifTitleFound);
                        } else {
                            if (renameDefaultExportsTo) {
                                if (!state.namedDefaultExportPath) {
                                    replaceDefaultExportPathByNamedDefaultExportPath(state.defaultExportPath, t, renameDefaultExportsTo);
                                    insertDefaultExport(programPath, t, title);
                                } else {
                                    throw new Error(
                                        `Default export can't be changed to '${renameDefaultExportsTo}', as a '${renameDefaultExportsTo}' export already exists. Please rename '${renameDefaultExportsTo}'.`
                                    )
                                }
                            } else {
                                throw new Error(`Non-object default export found. Please change to named export.`)
                            }
                        }
                    } else {
                        insertDefaultExport(programPath, t, title);
                    }
                }
            }
        }
    });
};

module.exports = plugin;