获取class兼容IE的写法

if(!document.getElementsByClassName){
    document.getElementsByClassName=function(classname){
        var allTags=document.getElementsByTagName(‘*‘);
        var element=[];
        for(var i= 0,len=allTags.length;i<len;i++){
            if(allTags[i].className==classname){
                element.push(allTags[i]);
            }
        }
        return element;
    }
}