为什么我的响应AJAX总是返回false，使用数据库检查器

Question

I want to check username in the form valaible or not avalaible to create. Then i use ajax for check in database without refrash page. Then i create this, but the response always return false. I try only controller with print_r, the function cek_user() can get value true and false. Please help how to my ajax return false or true. Thank before.

I using ajax and CI

here my contoller User.php

public function cek_user()
{
  $username     = $this->input->post('user_name');
  $query = $this->db->query("SELECT * FROM tb_user where username='".$username."'");

if($query->num_rows() > 0)
  {
    return false;
  }
  else
  {
    return true;
  }
}

and this my form add_user.php

<div class="form-group col-md-6">
      <div class="input-group">
          <span class="input-group-addon">@</span>
          <input id="username" name="username" type="text" class="form-control col-xs-6" placeholder="Username">
      </div>
</div>
<div id="chk_username" class="form-group col-md-6">

</div>

and this my ajax script add_user.php

$('#username').change(function(){

    var username = $('#username').val();

    if(username != "")
    {
    $.ajax(
        {
            type    : "POST",
            url     : "<?php echo base_url();?>user/cek_user",
            data    : { user_name:username },
            dataType: "text",
            success : function(response)
            {
                if (response == true) 
                {
                    $('#chk_username').html("<div class='btn btn-sm btn-block btn-success'>Username tersedia</div>");
                    console.log("true");
                }
                else if(response == false)
                {
                    $('#chk_username').html("<div class='btn btn-sm btn-block btn-danger'>Username tidak tersedia</div>");
                    console.log("false");
                }  
            }
        });
    }
    });

how to get true or false value from this ajax script ?