$.post() doesn't send data as json but as x-www-form-urlencoded instead

If you want to send the data as json then use the $.ajax function

You can specify type post and dataType json.

$.ajax({
  url: "mydomain.com/url",
  type: "POST",
  dataType: "xml/html/script/json", // expected format for response
  contentType: "application/json", // send as JSON
  data: $.param( $("Element or Expression") ),

  complete: function() {
    //called when complete
  },

  success: function() {
    //called when successful
 },

  error: function() {
    //called when there is an error
  },
});

Taken from ajax documentation

http://api.jquery.com/jQuery.ajax/

contentTypeString
Default: 'application/x-www-form-urlencoded; charset=UTF-8'

Because $.post() is for sending form-like requests. $.ajax is for sending whatever you want to. See contentType in $.ajax page for more information.

Quote:

When sending data to the server, use this content-type. Default is "application/x-www-form-urlencoded", which is fine for most cases. If you explicitly pass in a content-type to $.ajax() then it'll always be sent to the server (even if no data is sent). Data will always be transmitted to the server using UTF-8 charset; you must decode this appropriately on the server side.

this also works for me

$.ajax({
  url: "mydomain.com/url",
  type: "POST",
  dataType: "xml/html/script/json", // expected format for response
  contentType: "application/json", // send as JSON
  data: JSON.stringify(data),

  complete: function() {
    //called when complete
  },

  success: function() {
    //called when successful
 },

  error: function() {
    //called when there is an error
  },
});