惰性加载函数

1.常规

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

缺点：每次调用都会执行条件判断，可避免

2.改进，只在加载时做判断

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

缺点：虽避免了判断，但如果从未使用过addEvent函数，那么第一次浏览器嗅探成为多余

3.惰性载入，重写函数

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