Here is the problem i\'m stuck on. I want to pass the javascript variables to the rails controller.
        Ajax code in jQuery:
$("#submit_button").submit(function(event) {
  /* stop form from submitting normally */
   event.preventDefault();
  /* get values from elements on the page: */
   var mdate = $('#mdate').val();
   var phone = $('#phone').val();
  /* Send the data using post and put the results in a div */
    $.ajax({
      url: "/BookCreate/?mdate="+mdate+"&phone="+phone,
      type: "post",
      data: values,
      success: function(){
        alert('Saved Successfully');
      },
      error:function(){
       alert('Error');
      }
    });
});
Routes : ( As I am assuming your controller name is book )
match '/BookCreate', to: 'book#create'
For this you have to add jquery file to your code or this link
<script src="http://code.jquery.com/jquery-1.10.1.min.js"></script>
<script src="http://code.jquery.com/jquery-migrate-1.2.1.min.js"></script>

You can use something like
$.post("/bookings?booking[phone]=" + phone + "&booking[book_date]=" + mdate)
It will go to BookingsController#create action with params hash:
{ booking: { phone: "entered from prompt", book_date: "26 December 2013" } }
                                                                        Technically you cant pass variables between two languages.
You can pass those values to rails controller by appending in url
<script>
var mdate = "26 December 2013";
var phone = prompt('Enter your phone!');
if (phone) {
    //Passing mdate and phone variables to rails controller(book_date & phone)
    window.open("localhost:3000//controller/create?mdate="+mdate+"&phone="+phone,"_self")
}
else
{
    alert("Cancelled");
}
</script>
In your controller
def create
    data = params[:date]
    phone = params[:phone]
    @booking = Booking.new(book_param)
    if @booking.save
        redirect_to root_url
    else
        flash[:notice_booking_failed] = true
        redirect_to root_url
    end
end
NOTE: Make sure you configure your config/route.rb accordingly
More Info http://guides.rubyonrails.org/routing.html