[javascript] A CORS POST request works from plain JavaScript, but why not with jQuery?

I'm trying to make a Cross Origin post request, and I got it working in plain JavaScript like this:

var request = new XMLHttpRequest();
var params = "action=something";
request.open('POST', url, true);
request.onreadystatechange = function() {if (request.readyState==4) alert("It worked!");};
request.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
request.setRequestHeader("Content-length", params.length);
request.setRequestHeader("Connection", "close");
request.send(params);

But I would like to use jQuery, but I can't get it to work. This is what I'm trying:

$.ajax(url, {
    type:"POST",
    dataType:"json",
    data:{action:"something"}, 
    success:function(data, textStatus, jqXHR) {alert("success");},
    error: function(jqXHR, textStatus, errorThrown) {alert("failure");}
});

This results in Failure. If anyone knows why jQuery doesn't work, please let us all know. Thanks.

(I'm using jQuery 1.5.1, and Firefox 4.0, and my server is responding with a proper Access-Control-Allow-Origin header)

Modify your Jquery in following way:

$.ajax({
            url: someurl,
            contentType: 'application/json',
            data: JSONObject,
            headers: { 'Access-Control-Allow-Origin': '*' }, //add this line
            dataType: 'json',
            type: 'POST',                
            success: function (Data) {....}
});

Cors change the request method before it's done, from POST to OPTIONS, so, your post data will not be sent. The way that worked to handle this cors issue, is performing the request with ajax, which does not support the OPTIONS method. example code:

        $.ajax({
            type: "POST",
            crossdomain: true,
            url: "http://localhost:1415/anything",
            dataType: "json",
            data: JSON.stringify({
                anydata1: "any1",
                anydata2: "any2",
            }),
            success: function (result) {
                console.log(result)
            },
            error: function (xhr, status, err) {
                console.error(xhr, status, err);
            }
        });

with this headers on c# server:

                    if (request.HttpMethod == "OPTIONS")
                    {
                          response.AddHeader("Access-Control-Allow-Headers", "Content-Type, Accept, X-Requested-With");
                          response.AddHeader("Access-Control-Allow-Methods", "GET, POST");
                          response.AddHeader("Access-Control-Max-Age", "1728000");
                    }
                    response.AppendHeader("Access-Control-Allow-Origin", "*");

Another possibility is that setting dataType: json causes JQuery to send the Content-Type: application/json header. This is considered a non-standard header by CORS, and requires a CORS preflight request. So a few things to try:

1) Try configuring your server to send the proper preflight responses. This will be in the form of additional headers like Access-Control-Allow-Methods and Access-Control-Allow-Headers.

2) Drop the dataType: json setting. JQuery should request Content-Type: application/x-www-form-urlencoded by default, but just to be sure, you can replace dataType: json with contentType: 'application/x-www-form-urlencoded'

You are sending "params" in js: request.send(params);

but "data" in jquery". Is data defined?: data:data,

Also, you have an error in the URL:

$.ajax( {url:url,
         type:"POST",
         dataType:"json",
         data:data, 
         success:function(data, textStatus, jqXHR) {alert("success");},
         error: function(jqXHR, textStatus, errorThrown) {alert("failure");}
});

You are mixing the syntax with the one for $.post

Update: I was googling around based on monsur answer, and I found that you need to add Access-Control-Allow-Headers: Content-Type (below is the full paragraph)

http://metajack.im/2010/01/19/crossdomain-ajax-for-xmpp-http-binding-made-easy/

How CORS Works

CORS works very similarly to Flash's crossdomain.xml file. Basically, the browser will send a cross-domain request to a service, setting the HTTP header Origin to the requesting server. The service includes a few headers like Access-Control-Allow-Origin to indicate whether such a request is allowed.

For the BOSH connection managers, it is enough to specify that all origins are allowed, by setting the value of Access-Control-Allow-Origin to *. The Content-Type header must also be white-listed in the Access-Control-Allow-Headers header.

Finally, for certain types of requests, including BOSH connection manager requests, the permissions check will be pre-flighted. The browser will do an OPTIONS request and expect to get back some HTTP headers that indicate which origins are allowed, which methods are allowed, and how long this authorization will last. For example, here is what the Punjab and ejabberd patches I did return for OPTIONS:

Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: GET, POST, OPTIONS
Access-Control-Allow-Headers: Content-Type 
Access-Control-Max-Age: 86400