rollup/test/function/non-js-extensions/_config.js

var path = require( 'path' );

module.exports = {
	description: 'non .js extensions are preserved',
	options: {
		plugins: [{
			transform: function ( code, id ) {
				if ( path.extname( id ) === '.json' ) {
					return 'export default ' + code;
				}
			}
		}]
	}
};