Ruby on Rails

I'm trying to create a controller action for this script's 'payment' function I don't understand these 2 lines of code below (before the entire block) return res.token is gonna be seen in my controller as params what?

return paypal.request.post(SETEC_URL).then(function(res)

return res.token;

paypal.Button.render({      payment: function() {          var SETEC_URL = 'https://mystore.com/api/paypal/set-express-checkout';            return paypal.request.post(SETEC_URL).then(function(res) {              return res.token;          });      }  }, '#paypal-button-container');