Load script/style dependencies asynchronously using Promises.

loadDependencies.js

// Use the function like so:
_loadDependecies([
    {tag: 'script', url: 'example.com/asset.js'},
    {tag: 'script', url: 'example.com/asset2.js'},
    {tag: 'link',   url: 'example.com/asset.css'}
]).then(() => {
    // Do something.
});

// Function to return a promise catch-all.
function _loadDependecies( assets = [] ) {
  const promises = assets
    .filter(asset => 'script' === asset.tag || 'link' === asset.tag)
    .map(asset => new Promise((resolve, reject) => {
      const tag = document.createElement(asset.tag);

      // Set source attributes.
      if ( 'script' === asset.tag ) {
        tag.src  = asset.url;
        tag.type = 'text/javascript';
      } else {
        tag.href = asset.url;
        tag.rel  = 'stylesheet';
      }

      // Resolve on load.
      tag.addEventListener('load', (e) => {
        resolve()
      });

      // If bad URL, reject Promise.
      tag.addEventListener('error', () => {
        reject( `Bad dependency URL: ${asset.url}`);
      });

      // Append to head.
      document.querySelector('script').insertAdjacentElement('beforebegin', tag);
    }));

  // Return a Promise catch-all.
  return Promise.all(promises);
}