延迟加载(lazy loading)

 1 function addHandler(target, eventType, handler) {
 2 if (target.addEventListener) {
 3 addHandler = function(target, eventType, handler) {
 4 target.addEventListener(eventType, handler, false);
 5  };
 6 } else {
 7 addHandler = function(target, eventType, handler) {
 8 target.attachEvent("on" + eventType, handler);
 9  };
10  }
11  addHandler(target, eventType, handler);
12 }
13
14 function removeHandler(target, eventType, handler) {
15 if (target.removeEventListener) {
16 removeHandler = function(target, eventType, handler) {
17 target.removeEventListener(eventType, handler, false);
18  };
19 } else {
20 removeHandler = function(target, eventType, handler) {
21 target.detachEvent("on" + eventType, handler);
22  };
23  }
24  removeHandler(target, eventType, handler);
25 }

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。