How to set or display an asp.net text box control value using Java Script?

   I have a text box control on my web form that is not visible to the application user.
   When an application user interacts with other controls on my web form, then I want to
   be able to set the value of my text box control using Java Script. The problem I am having
   is that I am not able to set the value of the text box control in my Java Script file.
   Nor am I able to display the value with and alert statement. Does anyone know how to do either
   of those. (Set the value or display the value of a text box control?)
  

   function InstigateControls()
        {
  TextBox1.Text="2"; // This won't work
         alert(TextBox1.Text); // This won't work
         alert(TextBox1.Value); // This won&# ...