load script async #js

async_load_script.js

function loadScript( url, callback ) {
  // create script element
  var script = document.createElement('script');
  script.async = true;
  script.src = url;
  
  // attach it to DOM
  var entry = document.getElementsByTagName('script')[0];
  entry.parentNode.insertBefore(script, entry);
  
  // execute callback after script is loaded
  script.onload = script.onreadystatechange = function() {
    var readyState = script.readyState;
    
    if ( !readyState || /complete|loaded/.test( script.readyState ) ) {
      if ( typeof callback === "function" ) callback();
      
      // detach event handler to avoid memory leaks in IE
      script.onload = null;
      script.onreadystatechange = null;
    }
  };
}