dynamic script loader

loader.js

var myLoader = function(filename, fun) {
  var head   = document.getElementsByTagName('head')[0],
      script = document.createElement('script');

  script.type = 'text/javascript';
  script.src  = filename;
  if(typeof fun === 'function'){ script.onload = fun; }
  head.appendChild(script);
};


// Usage
myLoader('a1.js', function(){
  myLoader('a2.js', function() {
    console.log('Hello');
  });
});