ihg优悦会等级:请问这段javascript的意思？

麻烦具体解释一下，我JAVA学的不好。

<script>
if (!document.getElementById)
document.getElementById = function() { return null; }

function initializeMenu(menuId, actuatorId) {
var menu = document.getElementById(menuId);
var actuator = document.getElementById(actuatorId);

if (menu == null || actuator == null) return;

//if (window.opera) return; // I'm too tired

actuator.parentNode.style.backgroundImage = "url(images/+.png)";
actuator.onclick = function() {
var display = menu.style.display;
this.parentNode.style.backgroundImage =
(display == "block") ? "url(images/+.png)" : "url(images/-.png)";
menu.style.display = (display == "block") ? "none" : "block";

return false;
}
}
window.onload = function() {
initializeMenu("productsMenu", "productsActuator");
initializeMenu("compareMenu1", "compareActuator1");
initializeMenu("compareMenu2", "compareActuator2");
initializeMenu("compareMenu3", "compareActuator3");
initializeMenu("compareMenu4", "compareActuator4");
initializeMenu("compareMenu5", "compareActuator5");
initializeMenu("compareMenu6", "compareActuator6");
initializeMenu("compareMenu7", "compareActuator7");
}
</script>
这是在ASP中实现树状菜单的javascript,麻烦会JAVA的解释一下。。谢谢