Append Javascript to head tag in HTML, so it not blocking rendering.

append.js

(function(){
  	  var yourscript = document.createElement('script');
		     yourscript.type = 'text/javascript';
		     yourscript.async = true;
		     yourscript.src = 'http://domain.tld/path/to.js';
		  (document.getElementsByTagName('head')[0]||document.getElementsByTagName('body')[0]).appendChild(yourscript);
		})();