getElementsByClassName

?

function getElementsByClassName(str,root,tag){

    if(root){

        root=typeof root =='string' ? document.getElementById(root) :root; 

    }else{

        root=document.body;

    }

    tag=tag || *;

    var els=root.getElementsByTagName(tag),arr=[];

    for(var i=0,n=els.length;i<n;i++){

        for(var j=0,k=els[i].className.split(" "),l=k.length;j<l;j++)    {

            if(k[j]==str){

                arr.push(els[i]);

                break;

            }

        }

    }

    return arr;

}

　　

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