Append multiple async scripts to the page.

appendScripts.js

(function(d, u) {
  const i = u.length
  let s;
  while (i--) {
    s = d.createElement('script');
    s.async = true;
    s.src = u[i];
    d.head.appendChild(s);
  }
}(document, [
  'script1.js',
  'script2.js',
  'script3.js',
  'script4.js'
]));