Given a URL for a JS or CSS file, this function will load the asset and return a Promise which will reject on error or resolve when the asset is loaded.

loadAsset.js

/**
 * Given a URL for a JS or CSS file, this function will
 * load the asset and return a Promise which will reject
 * on error or resolve when the asset is loaded.
 */
function loadAsset(url){
  return new Promise(async (resolve, reject)=>{
    var asset;
    if(url.trim().substr(-3).toLowerCase() === '.js'){
      asset = document.createElement('script');
      asset.addEventListener('load', resolve);
      asset.addEventListener('error', reject);
      document.head.appendChild(asset);
      asset.setAttribute('src', url);
    }else{
      var styles = await fetch(url)
      	.then(c=>c.text())
      	.catch(reject);
      asset = document.createElement('style');
      asset.appendChild(document.createTextNode(styles));
      document.head.appendChild(asset);
      resolve();
    }
  });
}