怎样用js变化select下拉员的值

<script type="text/javascript">
    var message = "${message}";
   if(message.length>0){
     alert(message);
    }
     function showDiv(){
     var type="${linkType}";
     var selectValue = document.getElementById("selectName");
     var div = document.getElementById("back1");
     var div1 = document.getElementById("back2");
     var div2 = document.getElementById("back3");
    if(type!=null){
      if(type=="1"){
    div.style.display = "block";
    selectValue.options[0].text = "aa";
    div2.style.display = "none";
    div1.style.display = "none";
     }
    if(type=="2"){
        var optionValue = document.getElementById("option2");
       // selectValue.text = "教育部专业网站";
       // optionValue.text = "教育部专业网站";
//改变select下来源显示的值，但这样做老是不成功，希望有人帮我，当我传的option中的value值不一样时，select显示的value也跟着变化
        selectValue.options[1].text = "bb";
    div.style.display = "none";
    div2.style.display = "none";
     div1.style.display = "block";
    
    }
    if(type=="3"){
    div.style.display = "none";
    div1.style.display = "none";
    div2.style.display = "block";
    selectValue.options[2].text = "cc";
     }
    }
    }
  function showFriendLink(num){ 
   var div = document.getElementById("back1");
   var div1 = document.getElementById("back2");
   var div2 = document.getElementById("back3");
    if(num=="1"){
    div.style.display = "block";
    div2.style.display = "none";
    div1.style.display = "none";
    }
    if(num=="2"){
    div.style.display = "none";
    div2.style.display = "none";
    div1.style.display = "block";
    }
    if(num=="3"){
    div.style.display = "none";
    div1.style.display = "none";
    div2.style.display = "block";
    }
  }
</script>



<select id="selectName" name="selectName"  onchange="showFriendLink(this.options[this.selectedIndex].value)">
                                                    <option id="option1"  value="1">aa</option>
                                                    <option id="option2"  value="2">bb</option>
                                                    <option id="option3"  value="3">cc</option>
                                              </select>