1.常規(guī)

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

缺點：每次調(diào)用都會執(zhí)行條件判斷，可避免

2.改進，只在加載時做判斷

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);
    }
  }
})();

缺點：雖避免了判斷，但如果從未使用過addEvent函數(shù)，那么第一次瀏覽器嗅探成為多余

3.惰性載入，重寫函數(shù)

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);
};