@roots/bud-api/lib/methods/externals/index.js

bud.js core module

import isFunction from '@roots/bud-support/isFunction';
export const externals = async function (externals) {
    const current = this.hooks.filter(`build.externals`, {});
    return this.hooks.on(`build.externals`, isFunction(externals)
        ? externals(current)
        : { ...current, ...externals });
};