TYPO3 Core

I followed the instructions here: https://docs.typo3.org/typo3cms/CoreApiReference/JavaScript/Ajax/Backend/Index.html. The request succeeds, but the params array is always empty. I'm using jQuery and can't figure out what could be wrong when sending data like this:

$.ajax({
         type: 'POST',
         url: TYPO3.settings.ajaxUrls['Class::Function'],
         data: {foo: "bar"},
         success: function (result) {
                console.log(result);
          },
         error: function (error) {
          }
});

With prototype it's working. Any special format required?