how to validate a dropdownlist using javascript?

it's bit easy. see the following function

function validate()
{
   if(document.getElementById("ddlList").value == "")
   {
      alert("Please select value"); // prompt user
      document.getElementById("ddlList").focus(); //set focus back to control
      return false;
   }
}

Assuming your select element looks like this

<select id="ddlView">
<option value="0">Select</option>
<option value="1">test1</option>
<option value="2">test2</option>
<option value="3">test3</option>
</select>

function Validate()
{
var e = document.getElementById("ddlView");
var strUser = e.options[e.selectedIndex].value;
//if you need text to be compared then use
var strUser1 = e.options[e.selectedIndex].text;
if(strUser==0) //for text use if(strUser1=="Select")
{
alert("Please select a user");
}
}

function validateForm()
{
var listBoxSelection=document.forms["myForm"]["select"].value;
if(listBoxSelection==0)
  {
  alert("Please select a designation");
  return false;
  }
}

using jQuery you can do it in this way.

 function validateRadio() {
    var flag = false;
  $('#<%=RadioButtonList1.ClientID%> input').each(function(){
     if($(this).is(":checked"))
     flag = true;
   });
  return flag;
 }

function validateDropList() {
    if ($('#<%=DropDownList1.ClientID%>').val() == "") {
        return false;
    }
    else
        return true;
}
function submitForm() {
    if (!validateRadio()) {
        alert("Please do mark option.");
        return false; //do not submit form
    }
    else if(!validateDropList()){
     alert("Please do select a country.");
     return false; //do not submit form
    }
    else
        return true;

Using java script you can do it in this way.

function validateRadio() {
            var flag = false;
            var list = document.getElementById("<%=RadioButtonList1.ClientID%>"); //Client ID of the radiolist
            var inputs = list.getElementsByTagName("input");
            for (var i = 0; i < inputs.length; i++) {
                if (inputs[i].checked) {
                    flag = true;
                    break;
                }
            }         
            return flag;
    }

    function validateDropList() {

        if (document.getElementById("<%=DropDownList1.ClientID%>").value == "") {              
                return false;
            }
            else
                return true;
        }
        function submitForm() {
            if (!validateRadio()) {
                alert("Please do mark option.");
                return false; //do not submit form
            }
            else if(!validateDropList()){
             alert("Please do select a country.");
             return false; //do not submit form
            }               
            else
                return true;
        }

my asp form elements are.

<asp:radiobuttonlist id="RadioButtonList1" runat="server" xmlns:asp="#unknown">
        <asp:listitem value="a">A</asp:listitem>
        <asp:listitem value="b">B</asp:listitem>
        <asp:listitem value="c">C</asp:listitem>
        <asp:listitem value="d">D</asp:listitem>
    </asp:radiobuttonlist>
        <br />
        <br />
        <div>
        <asp:dropdownlist id="DropDownList1" runat="server" xmlns:asp="#unknown">
            <asp:listitem value="">select</asp:listitem>
            <asp:listitem value="ind">India</asp:listitem>
            <asp:listitem value="pak">PAK</asp:listitem>
            <asp:listitem value="usa">US</asp:listitem>
            <asp:listitem value="uae">UAE</asp:listitem>
        </asp:dropdownlist>
        <br />
        <br />
            <asp:button id="submitForm" runat="server" onclientclick="submitForm()" text="Submit" xmlns:asp="#unknown" />
 </div>

function validateListBox(){

var listBoxSelection=document.getElementById("dropDownDays").value;

if(listBoxSelection==0){

alert("Please select a day");

return false;

}else{

alert("Ok your selection is valid");

return true;

}

return true;

}

Try this :

<script type="text/javascript" language="javascript">
   function Validation(){
        var obj;
        obj = document.getElementById("<%=ddlID.ClientID%>");
        if (!obj_Subject.value) {
          alert("Your message here");
         return false;
     }
   }

Then you can call this function where you want.
for example, you can call it from a button click like this :

btnID.Attributes.Add("onClick", "return Validation()");

try this:
using Javascript validation for Drop down list:

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
    <script type="text/javascript">
        function displaySelected() {
            var x = document.getElementById('<%=ddlFruits.ClientID%>');
            var txt;
            if (x.value == 'Select') {
                txt = "Please select any fruit";
            }
            else {
                txt = "Selected option: " + x.value;
            }
            alert(txt);
        }
    </script>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:dropdownlist id="ddlFruits" runat="server" onchange="javascript:displaySelected();" xmlns:asp="#unknown">
            <asp:listitem text="Select" selected="True" />
            <asp:listitem text="Apple" />
            <asp:listitem text="Orange" />
        </asp:dropdownlist>
    </div>
    </form>
</body>
</html>

you can also validate using compare validator like below :

<asp:dropdownlist id="DropDownList1" runat="server" xmlns:asp="#unknown">          
        <asp:listitem value="-1">Select</asp:listitem>
        <asp:listitem>Yasser</asp:listitem>
        <asp:listitem>Zaid</asp:listitem>
    </asp:dropdownlist>
    <asp:comparevalidator id="CompareValidator1" runat="server" controltovalidate="DropDownList1" valuetocompare="-1" xmlns:asp="#unknown">
    ErrorMessage="Field is required" Operator="NotEqual" ></asp:comparevalidator>

 On page source code
<script language="javascript" type="text/javascript">
        function validate() {

            var summary = "";
            
            summary += validatelocation();
            
            if (summary != "") {
                alert(summary);
                return false;
            }
            else {
                return true;
            }

        }
function validatelocation() {
            var uid;
            var temp = document.getElementById("<%=ddllocation.ClientID %>");
            uid = temp.value;
            if (uid == "") {
                return ("Please enter location" + "\n");
            }
            else {
                return "";
            }
        }



On aspx.cs page

 protected void Page_Load(object sender, EventArgs e)
        {
          ImageButton1.Attributes.Add("onclick", "javascript:return validate()");
        }

Try this

$('#PageContent_ddllist').focus(function () {
if (this.value == '--Select--') {
$('.fld1-error').html('This field is required');
document.getElementById('PageContent_ddllist').focus();
$('#PageContent_ddllist').css('border-color', 'red');
return false;
} else {
$('.fld1-error').html('');
$('#PageContent_ddllist').css('border-color', 'green');

}
});