Insert a script and creates a global object. Inspired by the Google Analytics tracking code, and other services that don’t care about blocking pages.

insert-async-script.js

(function(window, document, tag, src, object) {
  window[object] = window[object] || [];

  var script = document.createElement(tag);
  script.async = true;
  script.src = src;
  
  var firstScript = document.getElementsByTagName(tag)[0];
  firstScript.parentNode.insertBefore(script, firstScript);
}(window, document, 'script', '//example.com/path/to/file.js', 'myObject'));