code: function defaultJsExporter(variables, options, root) {
  	const pathname = options.destination || root.source && root.source.input && root.source.input.file && root.source.input.file + '.js' || 'custom-variables.js';
  	const contents = Object.keys(variables).reduce(
  		(buffer, key) => `${ buffer }export const ${ key } = ${ JSON.stringify(variables[key]).replace(/(^|{|,)"(.+?)":/g, '$1$2:') };\n`,
  		''
  	);
  
  	return new Promise((resolve, reject) => {
  		fs.writeFile(
  			pathname,
  			contents,
  			(error) => error ? reject(error) : resolve()
  		);
  	});
  }
docs: Default export functions