Require functions in external scripts on demand

demand.js

/**
 * demand.js
 * @author: Rob Griffiths
 * On demand javascript
 *
 * Usage:
 *	demand('alerter', 'alerter.js');
 *	...
 * 	alerter('OMG!'); // script is only inserted now and executed
 *
 */
(function () {
	var window = this;
	var document = window.document;

	function demand(functionName, filename) {
		if (!(functionName in window && filename in included)) {
			window[functionName] = function () {
				var arg = arguments;
				var script = require(filename);
				script.onload = function () {
					window[functionName].apply(this, arg);
				};
			}
		}
	}

	var included = [];

	function require(filename) {
		if (filename in included) {
			return included[filename];
		}

		var script = document.createElement('script');  
		script.setAttribute('type', 'text/javascript');  
		script.setAttribute('src', filename);
		
		included[filename] = script;

		document.getElementsByTagName('HEAD')[0].appendChild(script); 
		return included[filename];
	}

	window['demand'] = demand;
})();