btjs-context.js

var self = this;

braintree.setup($("#btClientToken").val(), 'dropin', {
  container: 'dropin',
  paymentMethodNonceReceived: function (event, nonce) {
    $.validate({
      modules: 'security , date',
      onSuccess: $.proxy(function () {
        // do Ajax call and use nonce
        // this = self;
        return false;
      }, self)
    });
  }
});