fes.js/packages/fes-preset-built-in/src/plugins/features/externals.js

export default (api) => {
    api.describe({
        key: 'externals',
        config: {
            schema(joi) {
                // https://webpack.js.org/configuration/externals/#externals
                return joi.alternatives(joi.object(), joi.string(), joi.function());
            }
        }
    });
};