JQuery Ajax POST in Codeigniter

$(document).ready(function(){   

    $("#send").click(function()
    {       
     $.ajax({
         type: "POST",
         url: base_url + "chat/post_action", 
         data: {textbox: $("#textbox").val()},
         dataType: "text",  
         cache:false,
         success: 
              function(data){
                alert(data);  //as a debugging message.
              }
          });// you have missed this bracket
     return false;
 });
 });

In codeigniter there is no need to sennd "data" in ajax post method.. here is the example .

   searchThis = 'This text will be search';
    $.ajax({
      type: "POST",
      url: "<?php echo site_url();?>/software/search/"+searchThis,
      dataType: "html",
      success:function(data){
        alert(data);
      },

    });

Note : in url , software is the controller name , search is the function name and searchThis is the variable that i m sending.

Here is the controller.

    public function search(){
    $search = $this->uri->segment(3);
      echo '<p>'.$search.'</p>';
    }

I hope you can get idea for your work .