条件预加载(conditional advanceloading)

 1 var addHandler = document.body.addEventListener ?
 2 function(target, eventType, handler) {
 3 target.addEventListener(eventType, handler, false);
 4 } : function(target, eventType, handler) {
 5 target.attachEvent("on" + eventType, handler);
 6 };
 7
 8 var removeHandler = document.body.removeEventListener ?
 9 function(target, eventType, handler) {
10 target.removeEventListener(eventType, handler, false);
11 } : function(target, eventType, handler) {
12 target.detachEvent("on" + eventType, handler);
13 };

预加载适用于一个函数马上要被用到，并且在整个页面的生命周期中频繁出现的场合。