惰性加载函数----实现浏览器通用的事件绑定函数

var addEvent = function(elem,type,handler){
if(window.addEventListener){
addEvent = function(elem,type,handler){
elem.addEventListener(type,handler,false);
};
}else if(window.attachEvent){
addEvent = function(elem,type,handler){
elem.attachEvent('on'+type,handler);
};
}


addEvent(elem,type,handler);
};

<ol><li><span style="font-family: Arial, Helvetica, sans-serif;">var addEvent = function(elem,type,handler){</span></li><li><span style="font-family: Arial, Helvetica, sans-serif;">if(window.addEventListener){</span></li><li><span style="font-family: Arial, Helvetica, sans-serif;">addEvent = function(elem,type,handler){</span></li><li><span style="font-family: Arial, Helvetica, sans-serif;">elem.addEventListener(type,handler,false);</span></li><li><span style="font-family: Arial, Helvetica, sans-serif;">};</span></li><li><span style="font-family: Arial, Helvetica, sans-serif;">}else if(window.attachEvent){</span></li><li><span style="font-family: Arial, Helvetica, sans-serif;">addEvent = function(elem,type,handler){</span></li><li><span style="font-family: Arial, Helvetica, sans-serif;">elem.attachEvent('on'+type,handler);</span></li><li><span style="font-family: Arial, Helvetica, sans-serif;">};</span></li><li><span style="font-family: Arial, Helvetica, sans-serif;">}</span></li><li></li><li><span style="font-family: Arial, Helvetica, sans-serif;">addEvent(elem,type,handler);</span></li><li><span style="font-family: Arial, Helvetica, sans-serif;">};</span></li></ol>

addEvent函数在第一次进入条件分支之后，在函数内部会重写这个函数，重写之后的函数就是我们期望的addEvent函数，在下一次进入addEvent函数的时候，addEvent函数里不再存在条件分支语句