Example

1. Client Side

2. Codebehind file

protected void btnSubmit_Click(object sender, EventArgs e)
{
  const string script = "";
  ClientScript.RegisterClientScriptBlock(GetType(),"ClientScript", script);
}

Task: You need to uncheck few items in checkbox list with multiple selection if someone selected specific item.

Example: You have four items in check box list Item 1, Item 2, Item 3 and Item 4. If someone select Item 4 it will uncheck the selected Item 1, Item 2, Item 3 and backward in case someone selected Item 1 or Item 2 or Item 3 it will uncheck Item 4.

Solution: You need to add the code right before body tag ends and change the listBoxId to the ID of your control. The function fires automatically for both Firefox and Internet Explorer browsers. 

 
(function() {

       

         if (document.getElementById('ControlId') != null) {
             var ie = (document.all) ? true : false;
             var ff = ((document.getElementById) && (!ie)) ? true : false;

             if (ie) {
                 var selectListie =
                 document.getElementById('ControlId');
                 selectListie.onchange = function() { getSelected() };
             }

             if (ff) {
                 var selectListff =
                 document.getElementById('ControlId');
                 selectListff.setAttribute('onchange', 'getSelected();');
             }
         }


         })();

         function getSelected() {             
             var selectListff = document.getElementById('ControlId');
             var selIndex = selectListff.selectedIndex;
                   if (selIndex != -1) {                       
                       var chkLista = document.getElementById("ControlId");
                       for (var i = 0; i < chkLista.length; i++) {
                           if (chkLista[0].selected || chkLista[1].selected || chkLista[2].selected) {
                               chkLista[3].selected = false
                               
                               return;
                           }
                       }
                   }

Task: You need to validate a asp.net checkbox list with javascript.

Solution:

You need to create a asp.net custom validator and on client validation function (ClientValidationFunction="CheckExperince") call the JavaScript function below.

function CheckExperince(source, args) {
            var checkList = document.getElementById('');
            var checkListBox = checkList .getElementsByTagName("input");
            for (var i = 0; i < checkListBox.length; i++) {
                if (checkListBox[i].checked) {
                    args.IsValid = true;
                    return;
                }
            }
            args.IsValid = false;
        }

var TypeIndex = document.getElementById('').selectedIndex
var TypeValue = document.getElementById('').options[TypeIndex].value
if(TypeValue != ""){
    //Do Something
}