Polymer <core-xhr> - no POST params sent - post

Question

I'm desperately trying to send data via POST to a express.js application. The express.js app works fine but for whatever reason the POST data isn't sent to the server properly:
var xhrConfig = {
url: "http://localhost:3000/test",
body: {"foo": "bar"},
// body: JSON.stringify({"foo": "bar"}),
// body: 'foo=bar',
method: "POST"
};
document.createElement('core-xhr').request(xhrConfig);
My express.js console.log(req.body) output is always {}. No matter if body is sent stringified or raw or JSON. I also tried params instead of body just to make sure.
I tried the same in jQuery to exclude the possibility of having a bug in my express.js route but $.ajax({url: 'http://localhost:3000/test', data: {foo: 'bar'}, type: 'POST'}); works perfectly fine.
So what's the reason that req,body is always empty? Any ideas?
//edit:
body: "foo=bar",
headers: {'Content-Type': 'application/x-www-form-urlencoded'},
This one works now but is there a way to be able to use body: {"foo": "bar"} instead of converting it to foo=bar first?

Answer 1

As far as I know, core-xhr is a low level element, and the body object on it's conf doesn't accept a JS object but the string body you want to use.
For the use you describe, you could try core-ajax, a higher level element that can be configured with an object. Doc on core-ajax: https://www.polymer-project.org/docs/elements/core-elements.html#core-ajax.
<core-ajax
auto
url="http://localhost:3000/test"
body='{"foo": "bar"}'
handleAs="json"
method: "POST"
on-core-response="{{handleResponse}}"></core-ajax>
Hope it helps. If you need a more detailed example, don't hesitate to ask.

Answer 2

Looks like express looks at the Content-Type header to determine what type of body it has, below works for me.
this.$.xhr.request({
url: "/my/url",
method: "POST",
headers:{
"Content-Type":"application/json"
},
body: JSON.stringify(survey),
callback: function(response){
console.log(response);
}
});

Answer 3

I had this problem as well on Polymer. This problem is very frustrating because there isn't much documentation on Polymer on core-xhr.
As you have already answered the first question yourself, I'll answer the latter: yes/no. If you look at the source of <core-xhr>, it just feeds the params straight to xhr.send(params). however, there is a method inside of called toQueryString, which does what you want to do.
toQueryString: function(params) {
var r = [];
for (var n in params) {
var v = params[n];
n = encodeURIComponent(n);
r.push(v == null ? n : (n + '=' + encodeURIComponent(v)));
}
return r.join('&');
}