cross-browser event listeners

addEventListener.js

// observable is a DOM node, document, window, etc.
// eventName is the string event name, say "load" or "click"
// handler is the callback function.
function addEventListener(observable, eventName, handler) {
	if ( observable.addEventListener ) {
		observable.addEventListener(eventName, handler, false);
	} else if ( observable.attachEvent ) {
		observable.attachEvent("on" + eventName, handler);
	} else {
		// error handling?
	}
}