如何在Codeigniter上执行Ajax

I am just trying to do simple ajax on my codeigniter website.

Here is the my view code and is there some way to debug from controller?

<div class="form-group">
    <label for="name" class="col-sm-2 text-left">Nama Lengkap
        <span class="text-danger">*</span>
    </label>

    <div class="col-sm-10">
         <input type="text" class="form-control"  placeholder="" name="nama_lengkap" id="nama_lengkap" value="">
    </div>
</div>

Here is my javascript

$('#nama_lengkap').change(function(){
    var nama_lengkap = $('#nama_lengkap').val();
    $.ajax({
        type: "POST",
        url: "<?php echo site_url('formulir/update_data'); ?>", 
        data:nama_lengkap,
        dataType:"json",//return type expected as json
        success: function(states){
                     console.log("sukses update" + states);
        },
    });
});

Here is my controller formulir.php

public function update_data(){
    $nama_lengkap = $_POST["nama_lengkap"];
    return $nama_lengkap;
}

Could it be that you need to adjust this line in your javascript ajax:

data:{nama_lengkap: nama_lengkap}

On header create a javascript varaible.

<script>var BASE_URL = <?php echo base_url(); ?></script>

now you dont have to use php code every where in ajax code. Just use

url: BASE_URL+'controller/function',
data:{'nama_lengkap':nama_lengkap},

and now in controller used the proper codeigniter input type syntax like this:

$nama_lengkap = $this->input->post('nama_lengkap');