Update item using CSOM

 <script language="ecmascript" type="text/ecmascript">
    function updateListItem() {
        var clientContext = new SP.ClientContext.get_current();
        var oWeb = clientContext.get_web();
        var oListColl = oWeb.get_lists();
        var oList = oListColl.getByTitle(LISTNAME);    
        var oListItem = oList.getItemById(ITEMID);
        oListItem.set_item('testcolumn', 'Item Updated');
        oListItem.update();      
        clientContext.executeQueryAsync(Function.createDelegate(this, this.onQuerySucceeded), Function.createDelegate(this, this.onQueryFailed));
    }

    function onQuerySucceeded() {
        alert('Item updated successfully.');
    }
    function onQueryFailed(sender, args) {
        alert('Request failed. ' + args.get_message() + '\n' + args.get_stackTrace());
    }
</script>
<input onclick="updateListItem()" type="button" value="Update List Item"/>

Updating a column for a particular item in a list using LINQ

SPListItem resultItem = (from SPListItem oItem in oWeb.Lists["CustomList"].Items
                         where Convert.ToString(oItem["ColumnName"]) == 'Value'
                         select oItem).First();
resultItem["Title"] = "Sample";
resultItem.Update();

Get current logged in username using REST API

$.ajax({
   url: _spPageContextInfo.webAbsoluteUrl + "/_api/SP.UserProfiles.PeopleManager/GetMyProperties?$select=DisplayName",
   type: "GET",
   headers: {"accept": "application/json;odata=verbose"},
   success: function (data) {
      userName=data.d.DisplayName;
   },
   error: function (xhr) {
      alert(xhr.status + ': ' + xhr.statusText);
   }
}); 

Get current logged in username using Client-Side Object Model

ExecuteOrDelayUntilScriptLoaded(getUserName, "sp.js");
var userName='';
function getUserName(){
var clientContext = SP.ClientContext.get_current();
var website = clientContext.get_web();
currentUser = website.get_currentUser();
clientContext.load(website);
clientContext.load(currentUser);
clientContext.executeQueryAsync(onRequestSucceeded, onRequestFailed);
}

function onRequestSucceeded() {
userName=currentUser.get_title();
alert(userName);
}

function onRequestFailed(sender, args) {
    alert('Error: ' + args.get_message());
}

Consuming Rest Service

            BasicHttpBinding basicHttpbinding = new BasicHttpBinding(BasicHttpSecurityMode.None);
            basicHttpbinding.Name = "BasicHttpBinding_temp";
            basicHttpbinding.Security.Transport.ClientCredentialType = HttpClientCredentialType.Windows;
            basicHttpbinding.Security.Message.ClientCredentialType = BasicHttpMessageCredentialType.UserName;
            EndpointAddress endpointAddress = new EndpointAddress("http://siteurl/Service.svc");

Create an object for the service referenced and pass the above two objects as its constructor parameter

Reading JSON data from a webservice

Create a class for the JSON data

public class JSONData

{

public string id{ get; set; }

public string name{ get; set; }

}

Use this to consume the webservice

string requestURL = "Webservice URL";

CredentialCache credCache = new CredentialCache();

credCache.Add(new Uri(requestURL), "NTLM", CredentialCache.DefaultNetworkCredentials);

HttpWebRequest spRequest = (HttpWebRequest)HttpWebRequest.Create(requestURL);

spRequest.Credentials = credCache;

spRequest.UserAgent = "Mozilla/4.0+(compatible;+MSIE+5.01;+Windows+NT+5.0";

spRequest.Method = "GET";

spRequest.Accept = "application/json;odata=verbose";

HttpWebResponse endpointResponse = (HttpWebResponse)spRequest.GetResponse();

if (endpointResponse.StatusCode == HttpStatusCode.OK)

{

                using (endpointResponse)

{

using (var reader = new StreamReader(endpointResponse.GetResponseStream()))

{

JavaScriptSerializer serializer = new JavaScriptSerializer();

try

{

string jSON = reader.ReadToEnd();

JSONData data = new JavaScriptSerializer().Deserialize<JSONData>(jSON);

string strId = data.id;

string strName=data.name;

}

catch (Exception ex)

{

throw ex;

}

}

}

}

Check item exists based on a column value using REST API

<script type="text/javascript">
function PreSaveAction() {
var result = false;
var value=$("select[title=MyColumn] option:selected").text();
var requestUri = _spPageContextInfo.webAbsoluteUrl + "/_api/Web/Lists/getbytitle('MyList')/Items?$select=LinkTitle,MyColumn/Title&$expand=MyColumn&$filter=MyColumn/Title eq '" + value;
$.ajax
({
url: requestUri,
type: "GET",
cache: true,
async: false,
headers:{
"ACCEPT": "application/json;odata=verbose"
        },
success: function (data) {
if($(data.d.results).length == 0)
result = true;
else
alert("Retry!!!");
},
error: function () {
}
});
return result;
}
</script>