Saving JSON object to WCF REST SERVICES in Javascript

I am creating a chrome extension , in that i want to send a JSON object to WFC REST Services(.svc) in javascript. My javascript code below

var myRequest = new XMLHttpRequest();

    myRequest.onreadystatechange=function() {
        if (myRequest.readyState == 4 && myRequest.status == 200) {
            console.log('Sent to server: ' + dataString + '');
            window.localStorage.removeItem(dataString);
        }
        else if (myRequest.readyState == 4 && myRequest.status != 200) {

            console.log('Server request could not be completed');
            saveDataLocally(dataString);
        }
    }

    myRequest.open("POST","http://localhost:8221/Service1.svc/SaveData",true);
        myRequest.setRequestHeader("Content-Type","application/json");
        myRequest.send(dataString);

dataString is a JSON object
dataString = "{"firstName":"suresh","lastName":"kumar"}"

Im getting Error
POST http://localhost:8221/Service1.svc/SaveData 415 (Unsupported Media Type)

Service1.svc.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.Serialization;
using System.ServiceModel;
using System.ServiceModel.Web;
using System.Text;
using OfflinetoOnlineService.ServiceReference1;

namespace OfflinetoOnlineService
{
    // NOTE: You can use the "Rename" command on the "Refactor" menu to change the class name "Service1" in code, svc and config file together.
    public class Service1 : IService1
    {
           public object SaveData(Offline dataString)
        {
            string strmessage = string.Empty;
            if (dataString != null)
            {
                strmessage = dataString.dataString + "datastring got it";
               
            }
            return strmessage;
        }

      
    }
}

IService1.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.Serialization;
using System.ServiceModel;
using System.ServiceModel.Web;
using System.Text;

namespace OfflinetoOnlineService
{
    // NOTE: You can use the "Rename" command on the "Refactor" menu to change the interface name "IService1" in both code and config file together.
    [ServiceContract]
    public interface IService1
    {

        [OperationContract]
        [WebInvoke(Method = "POST", UriTemplate = "/SaveData", RequestFormat = WebMessageFormat.Json,
            ResponseFormat = WebMessageFormat.Json)]
          object SaveData(Offline dataString);

     
    }

    [DataContract]
    public class Offline
    {
        object data = string.Empty;
        [DataMember]
        public object dataString
        {
            get { return data; }
            set { data = value; }
        }
    }

   
}

Web.config

<?xml version="1.0"?>
<configuration>

  <system.web>
    <compilation debug="true" targetFramework="4.0" />
  </system.web>
  <system.serviceModel>
    <bindings>
      <basicHttpBinding>
        <binding name="BasicHttpBinding_IService1" closeTimeout="00:01:00"
          openTimeout="00:01:00" receiveTimeout="00:10:00" sendTimeout="00:01:00"
          allowCookies="false" bypassProxyOnLocal="false" hostNameComparisonMode="StrongWildcard"
          maxBufferSize="65536" maxBufferPoolSize="524288" maxReceivedMessageSize="65536"
          messageEncoding="Text" textEncoding="utf-8" transferMode="Buffered"
          useDefaultWebProxy="true">
          <readerQuotas maxDepth="32" maxStringContentLength="8192" maxArrayLength="16384"
            maxBytesPerRead="4096" maxNameTableCharCount="16384" />
          <security mode="None">
            <transport clientCredentialType="None" proxyCredentialType="None"
              realm="" />
            <message clientCredentialType="UserName" algorithmSuite="Default" />
          </security>
        </binding>
      </basicHttpBinding>
    </bindings>
    <client>
      <endpoint address="http://localhost:8221/Service1.svc" binding="basicHttpBinding"
        bindingConfiguration="BasicHttpBinding_IService1" contract="ServiceReference1.IService1"
        name="BasicHttpBinding_IService1" />
    </client>
    <services>
      <service name="OfflinetoOnlineService.Service1" behaviorConfiguration="myServiceBehavior">
        <!-- Service Endpoints -->
        <endpoint address="" binding="basicHttpBinding" contract="OfflinetoOnlineService.IService1">
          <identity>
            <dns value="localhost"/>
          </identity>
        </endpoint>
        <endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange"/>
      </service>
    </services>
    <behaviors>
      <serviceBehaviors>
        <behavior name="myServiceBehavior" >
          <serviceMetadata httpGetEnabled="true"/>
          <serviceDebug includeExceptionDetailInFaults="false"/>
        </behavior>
        <behavior>
          <!-- To avoid disclosing metadata information, set the value below to false and remove the metadata endpoint above before deployment -->
          <serviceMetadata httpGetEnabled="true"/>
          <!-- To receive exception details in faults for debugging purposes, set the value below to true.  Set to false before deployment to avoid disclosing exception information -->
          <serviceDebug includeExceptionDetailInFaults="false"/>
        </behavior>
      </serviceBehaviors>
      <!--<endpointBehaviors>
        <behavior name="webHttp">
          <webHttp/>
        </behavior>
      </endpointBehaviors>-->
    </behaviors>
    <serviceHostingEnvironment multipleSiteBindingsEnabled="true" />
     </system.serviceModel>
 <system.webServer>
    <modules runAllManagedModulesForAllRequests="true"/>
  </system.webServer>
  
</configuration>

Please Guide me how to Save that JSON object to my Web service(.svc)