延迟加载(lazy loading)

function addHandler(target, eventType, handler) {
if (target.addEventListener) {
addHandler = function(target, eventType, handler) {
target.addEventListener(eventType, handler, false);
 };
} else {
addHandler = function(target, eventType, handler) {
target.attachEvent("on" + eventType, handler);
 };
 }
 addHandler(target, eventType, handler);
}

function removeHandler(target, eventType, handler) {
if (target.removeEventListener) {
removeHandler = function(target, eventType, handler) {
target.removeEventListener(eventType, handler, false);
 };
} else {
removeHandler = function(target, eventType, handler) {
target.detachEvent("on" + eventType, handler);
 };
 }
 removeHandler(target, eventType, handler);
}

当一个函数在页面中不会被立刻调用时，延迟加载是最好的选择。