Passing javascript variables to rails controller

Here is the problem i'm stuck on. I want to pass the javascript variables to the rails controller.

<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)
}
else
{
    alert("Cancelled");
}
</script>

My controller

def new
        @booking = Booking.new
end
def create
    @booking = Booking.new(book_param)
    if @booking.save
        redirect_to root_url
    else
        flash[:notice_booking_failed] = true
        redirect_to root_url
    end
end

private
def book_param
    params.require(booking).permit(:id, :book_date, :phone)
end

Thank you in advance!