Get The List Item Value in SharePoint 2010 Using ECMAScript

First Set the j query refference into your page.
<script type="text/javascript" src="../../Scripts/jquery-1.7.2.min.js"></script>

Get the query string  value in the edit or display page.

 <Script type="text/javascript">
   function GetQueryStringParams(sParam)
{

    var sPageURL = window.location.search.substring(1);
    var sURLVariables = sPageURL.split('&');
    for (var i = 0; i < sURLVariables.length; i++)
    {
        var sParameterName = sURLVariables[i].split('=');
        if (sParameterName[0] == sParam)
        {
            return sParameterName[1];
        }
    }
}</Script>

Finaly write the ecmca sript (Which will load on page load)

<script type="text/javascript">
  ExecuteOrDelayUntilScriptLoaded(getSetListItem, "sp.js");
  var itemId=GetQueryStringParams('id');
      var listItem;
        var list;
        var clientContext;
        function getSetListItem() {
            this.clientContext = SP.ClientContext.get_current();
            if (this.clientContext != undefined && clientContext != null) {
                var webSite = clientContext.get_web();
                this.list = webSite.get_lists().getByTitle("ENS Email");
                this.listItem = list.getItemById(itemId);
                clientContext.load(this.listItem);
                this.clientContext.executeQueryAsync(Function.createDelegate(this, this.OnLoadSuccess),
Function.createDelegate(this, this.OnLoadFailed));
            }
        }
       
         function OnLoadSuccess(sender, args) {
         var  approval=false;
            approval= this.listItem.get_item("Approval_x0020_Needed");
//where Approval_x0020_Needed is a yes/no field in the list
            if (approval==true)
            {
//to show or hide the list items
              $("nobr:contains('Approval Status')").
      parent('h3').parent('td').parent('tr').show();
       $("nobr:contains('Approval Comments')").
      parent('h3').parent('td').parent('tr').show(); 

            }
            else
            {
//to show or hide the list items
              $("nobr:contains('Approval Status')").
      parent('h3').parent('td').parent('tr').hide();
       $("nobr:contains('Approval Comments')").
      parent('h3').parent('td').parent('tr').hide(); 

            }
        }
        function OnLoadFailed(sender, args) {
            alert('Request failed. ' + args.get_message() + '\n' + args.get_stackTrace());
        }
  </script>
 


To set the value use following

    function OnLoadSuccess(sender, args) {
            var value = this.listItem.get_item("SampleOne");
            this.listItem.set_item("SampleTwo", value);
            this.listItem.update();
            this.clientContext.load(this.listItem);
            this.clientContext.executeQueryAsync(Function.createDelegate(this, this.OnLoadSuccess1),
Function.createDelegate(this, this.OnLoadFailed));
        } 
 function OnLoadSuccess1(sender, args) {
            alert(this.listItem.get_item("SampleTwo"));
        } 
        function OnLoadFailed(sender, args) {
            alert('Request failed. ' + args.get_message() + '\n' + args.get_stackTrace());
        }


you can also call the function like
<input id="btnGetSetListItem" onclick="getSetListItem()" type="button" value="Get & Set List Item" />