純js實現(xiàn)：
html

<div class="a ">11</div>
<div class="b ">11</div>
<div class="c ">11</div>

css

.b{
  background:red;
}

js

function addClass (elements,className){
        for (var i=0;i < elements.length;i++){
            var element = elements[i];
            if(!element.className.match(new RegExp('(\\s|^)'+className+'(\\s|$)'))){
                element.className +=' '+className;
            }
        }
    }
var elements = document.getElementsByTagName("div");
addClass(elements,"b");

更新

（setAttr 不覆蓋原來屬性，如果需要覆蓋修改下代碼即可）

function getAttr(element,attrName){
                if(typeof element!='object'||typeof attrName!='string') return;
                return attrName =='class'? element.className:element.getAttribute(attrName);
 }

function setAttr (element,attrName,attrValue){
                if(typeof element !='object'|| typeof attrName!='string' || typeof attrName!='string') return;
                            var _attrValue = getAttr(element,attrName);
                            if(!_attrValue){
                                    _attrValue =  attrValue;
                            }else if(!_attrValue.match(new RegExp('(\\s|^)'+attrValue+'(\\s|$)'))){
                                   _attrValue = _attrValue + ' ' + attrValue;
                                }
                            attrName == 'class'? element.className=_attrValue:element.setAttribute(attrName,_attrValue);

                        
 }

var element = document.getElementsByTagName("div")[0];
setAttr(element,"class","b");

setAttr(element,"id","b");

setAttr(element,"id","b c");

更新

封裝為類似jquery的調(diào)用方式

function MyJquery(selector){ //這里演示，只實現(xiàn)id選擇器
    this.element = document.getElementById(selector);
}

MyJquery.prototype.getAttr = function (attrName){
                if(typeof attrName!='string') return;
                return attrName =='class'? this.element.className:this.element.getAttribute(attrName);
 };
     
MyJquery.prototype.setAttr = function (attrName,attrValue){
                if(typeof attrName!='string' || typeof attrName!='string') return;
                 var _attrValue = this.getAttr(attrName);
                  if(!_attrValue){
                                        _attrValue =  attrValue;
                                    }else if(!_attrValue.match(new RegExp('(\\s|^)'+attrValue+'(\\s|$)'))){
                                        _attrValue = _attrValue + ' ' + attrValue;
                                    }
                  attrName == 'class'? this.element.className = _attrValue:this.element.setAttribute(attrName, _attrValue);
                        
 };
         
             
 var $ = function (selector) {
            return new MyJquery(selector);
}
$("aa").setAttr("class","b");
     

$(".p1").addclass('p2')