The default.js returns the default configuration as the default export. The configuration is a plain JSON-serializable object.
This file is always exists and normally should not be edited. It can be overridden in runtime by the patch file described below.
The config.json should be placed in web root near the index.html.
This file, if exists, is merged with the default configuration in runtime (when site is opened in the browser). It allows site administrator to change some configuration parameters without recompiling all the code.
You can also use this file with the development server. Just place a config.json to the project root (near the package.json). Bear in mind that this file will not be included in compiled code!