npmjs.com/package/config for client side JavaScript
npm install lofig --save --production
You can also use https://cdn.rawgit.com/atjonathan/lofig/master/lofig.js.
import 'lofig' from 'lofig'; // not necessary if using CDN Link
// change config folder
lofig.folder = 'config/default.json';
// check if property exists
lofig.has('secret', res => {});
// return property
lofig.get('secret', res => {});
See example folder for more details.