A simple script loader

loadscript.js

// loadScript :: String, function, predicate [, arg1 [, arg2 [, ...]]] -> undefined
// The optional arguments will get passed to the callback
var loadScript = (function() {
  var scripts = {};

  return function(url, callback, predicate) {
    var
      body = document.body,
      script = document.createElement('script'),
      loaded = false,
      optionalArgs = [].slice.call(arguments, 3);

    if (predicate || scripts[url]) {
      callback.apply(null, optionalArgs);
    } else {
      script.onload = script.onreadystatechange = function() {
        if (!loaded && (!this.readyState || this.readyState === 'loaded' || this.readyState === 'complete')) {
          loaded = true;
          script.onload = script.onreadystatechange = null;
          scripts[url] = 1;
          callback.apply(null, optionalArgs);
        }
      };

      script.src = url;
      body.appendChild(script);
    }
  };
})();