How to Call webservices using Jquery+Ajax

To enable CORS on IIS add the following to the web.config file of your server application:

<configuration>
 <system.webServer>
   <httpProtocol>
     <customHeaders>
       <add name="Access-Control-Allow-Origin" value="*" />
     </customHeaders>
   </httpProtocol>
 </system.webServer>
</configuration>

Just for explanation, when you are calling some service which is hosted on different domain (other than your application), browser sends preflight request (request with http OPTION verb instead of POST or GET) and it waits for the answer. The answer, which comes from the server, means whether the server can respond to the client request or not, according to the CORS policy. You can use JSONP only if the service can responds with JSONP, in that case the response is wrapped something like this:

jsonp_callback(<YOUR JSON>)

where jsonp_callback is global function which is used to get the JSON and to send it to the ajax callback function.

To find more about how to enable CORS visit this site http://enable-cors.org/

This is because your ASMX service is not configured to serve requests from other domains for security reasons. All you need is to add this in web.config of your web service.

<system.webServer>
    <httpProtocol>
        <customHeaders>
            <add name="Access-Control-Allow-Origin" value="*" />
        </customHeaders>
    </httpProtocol>
</system.webServer>

Please note that this will allow all websites to access your webservice. If you want only a specific site to access your web service, change the custom header like this

<add name="Access-Control-Allow-Origin" value="http://example-site.com" />

This will give you a detailed idea on CORS: http://enable-cors.org/

P.S: Please refrain from using success and error callbacks while using $.ajax as they are deprecated.

Deprecation Notice: The jqXHR.success(), jqXHR.error(), and jqXHR.complete() callbacks are removed as of jQuery 3.0. You can use jqXHR.done(), jqXHR.fail(), and jqXHR.always() instead.

Update: Try setting these too.

In web.config

<location path="service1.asmx">
    <system.web>
        <webServices>
            <protocols>
                <clear />
                <add name="HttpGet" />
                <add name="HttpPost" />
            </protocols>
        </webServices>
    </system.web>
</location>

Decorate your method like this

[ScriptMethod(UseHttpPost = true)]
public string GetJsonData()
{
    return "Hello World";
}