not working code, just the general idea

load structure.js

function preload(url, loadedCb) {
  var tagType         = 'img' || 'object',
      elem            = document.createElement(elem),
      correctCallback = 'onerror' || 'onload';
      
  if (tagType === 'img') {
    elem.width  = '1';
    elem.height = '1';
  }
  
  elem[correctCallback] = function() {
    loadedCb(url);
    document.head.removeChild(elem);
  };
  
  elem.src = url;
  
  document.head.appendChild(elem);
};

// assumes preload if you want immediate execution
function inject(url, completeCb) {
  var elem = document.createElement('script');
  elem.src = url;
  
  elem.onload = function() { // and other variations
    completeCb(url);
  };
  
  document.head.appendChild(elem);

  // ie mem leaks, etc here
}


// Use them:

preload('jquery.js', function(url) {
  inject(url, function() {
    alert('jQuery');
  });
});