I'm really confused by this file index.js:
import Cookies from 'js-cookie';
import createHash from 'sha.js';
import Evaporate from 'evaporate';
import SparkMD5 from 'spark-md5';
import './css/bootstrap-progress.css';
import './css/styles.css';
...
Why would you import a .css file in a .js file? I've never seen this before. Is this common? What does it allow you to do?
The repository appears to be using Parcel. See Parcel's docs on CSS:
Parcel includes support for CSS out of the box. To add a CSS file, either ... ... or import it from a JavaScript file:
import './index.css';
What it will do is, if that JavaScript module is included, when the module is imported, it will insert the CSS (from that ./css/styles.css path in the source code) onto the page. It's not a native JavaScript functionality of ES6 modules - it's something that a bundler (like Parcel, or Webpack, etc) manages. When bundling, the CSS text will be turned into a JavaScript string somewhere in the resulting bundle, and then put onto the page with something like
const style = document.createElement('style');
document.body.appendChild(style);
style.textContent = `<CONTENT OF IMPORTED CSS>`;