How to call class in JavaScript

<p id="test" class="aaa bbb">测试关于class的调用</p>

.aaa{
 font-size:20px;
 color:red; 
}
.bbb{
 font-size:50px;
 color:green;
}

var test=document.getElementById('test');
test.className='aaa';
test.className='aaa bbb';//因为aaa和bbb的样式相同，所以，bbb的样式会覆盖aaa的样式

function hasClass(element,className){   //判断是否存在类
    return element.className.match(new RegExp('(\\s|^)'+className+'(\s|$)'));
 
}
//添加一个Class
function addClass(element,className){
  if(!hasClass(element,className)){
     element.className+=' '+className;
  }  
}  
//删除一个Class
function removeClass(element,className){
 if(hasClass(element,className)){
    element.className=element.className.replace
   (new RegExp('(\\s|^)'+className+'(\s|$)'),' ');
 }
}