Angular: Custom headers are ignored by $http and $resource. Why?

Question

I'm trying to access a REST service I don't control. First problem is that the service doesn't include a Access-Control-Allow-Origin header, which is a problem that, if I understand correctly, immediately limits me to JSONP.

Also, by default, this service sends XML rather than JSON, though it's capable of sending JSON. I think it should respond to my Accept header, the people responsible for the service say it looks at my Content-Type. That would mean I'd need to do a POST rather than a GET (though get makes more sense when I'm just getting some static data, right?).

Stubborn as I am, I'm trying my Accept header first. Since Angular only accepts JSON, I'd expect it to use the Accept: application/json header by default, but it doesn't, and it ignores my attempts to set it manually:

app.config(['$httpProvider', function($httpProvider){
    console.log($httpProvider.defaults.headers.common);
    delete $httpProvider.defaults.headers.common['X-Requested-With'];
    $httpProvider.defaults.headers.post['Accept'] = 'application/json, text/javascript';
    $httpProvider.defaults.headers.post['Content-Type'] = 'application/json; charset=utf-8';
    $httpProvider.defaults.headers.post['Access-Control-Max-Age'] = '1728000';
    $httpProvider.defaults.headers.common['Access-Control-Max-Age'] = '1728000';
    $httpProvider.defaults.headers.common['Accept'] = 'application/json, text/javascript';
    $httpProvider.defaults.headers.common['Content-Type'] = 'application/json; charset=utf-8';
    $httpProvider.defaults.useXDomain = true;
}]);

I do this again in the actual resource:

return $resource('http://foo.com/getStuff', {}, {
    fetch: {
        method:'JSONP',
        params: params,
        headers: {
            'Accept':'application/json, text/javascript',
            'Content-Type':'application/json; charset=utf-8'
        },
        isArray:false,
        callback: 'JSON_CALLBACK'
    }
});

But still, the request headers contain Accept: */*.

My question is: WHY? Why does Angular ignore my headers? And how do I get it to use the proper headers anyway?

And also: is there a way to use JSONP in a POST?

Edit: Originally I used Angular 1.0.7, but I just tried it with 1.2.3 and got the same results. Headers are ignored, yet everybody claims that this is the way to do it.

I also tried doing it directly with $http, rather than with $resource, with the same results.

Edit 2: Here's a JSFiddle. It's anonymized and doesn't use my real server, but using Firebug/developer tools, you can verify that it sends Accept: */* on both calls, despite my many attempts to set application/json headers. And that is my real problem here. On my real server, I'm getting an XML result because of that, despite my real server's ability to send JSON.

(Whether the real server supports jsonp is less relevant at the moment. This dummy server clearly doesn't, but that's okay. I just care about the headers.)

Edit 3: I've tried both solutions suggested below:

$http.defaults.headers.common['Accept'] = 'application/json, text/javascript';

$http.defaults.transformRequest.push(function (data, headersGetter) {
    headersGetter().Accept = "application/json, text/javascript";
    return data;
});

I've tried both statements separately. In the controller, and then in the service just before the http call itself. Still doesn't work.

Can someone give me a JsFiddle where this is shown to work?

Edit 4: I notice that when I use GET rather than JSONP, the Accept header is correct. But then the response is rejected because it doesn't have the correct header.

What kind of headers should a JSONP call have? Because there's a lot more headers in the JSONP call, but nothing that identifies it as JSONP. Does the server have to have explicit JSONP support for this to work? I suddenly realize I don't know nearly enough about jsonp.

Answer 1

I think your answer is here. According to the wiki, A JSONP call is executed through injection of a <script> tag to load the script from the host server, which responds by calling your callback, passing the data. A <script> tag generates a regular browser request (not an XmlHttpRequest), and the browser will send its own Accept header (it also sends its own User-Agent header, for example).

I would hope there is an easier client-side way to do this, but I think the only way may be the one suggested in the referenced post:

So, if you want to be able to set request headers for cross domain calls you will have to setup a server side script on your domain that will delegate the call to the remote domain (and set the respective headers) and then send the AJAX request to your script.

EDIT: here is a (rejected) jQuery bug report about this same problem.

Some more background info:

In angular, callbacks are managed automagically, so if your say this:

$http({
    method: "JSONP",
    url: "http://headers.jsontest.com?callback=JSON_CALLBACK",
}).success(function(data) {
    console.log('Return value:');
    console.log(data);
}).error(function(data) {
    console.log('Error!');
    console.log(data);
})

a <script> tag will be created that looks more or less like this:

<script type="application/javascript"
        src="http://headers.jsontest.com/?callback=angular.callbacks._1">
</script>

The content of the response to http://headers.jsontest.com/?callback=angular.callbacks._1 will be:

angular.callbacks._1({key1: "value1", key2: "value2"});

angular.callbacks._1 will contain your success function, and it will be called with the data.