CodeIgniter, Ajax... How to use insert_id()

Excuse my English, I hope you can understand my question...

In my program, when you submit a new record to the database (MySQL) using a modal+form, it is drawn in a data table without refreshing or recharging the actual page, it's appended dynamically.

What I need to do is, in every new record, draw/append the data AND 2 buttons (1 for deleting that record and 1 for updating it) with the ID of the new data.

I read I can use insert_id() to get the last inserted id, but I don't know how to use it and how to work with the result in my Ajax call.

Any hint, please? I'm pretty new to programming.

Thanks for your time.

EDIT

I need the data I introduced in the modal/form (for that I was using .serialize()) AND the ID generated in the DB... I don't know how to use the model/controller returned ID info.

I have this in my controller:

    public function nuevo_elemento_diccionario()
{
    $this->load->helper('form');
    $this->load->library('form_validation');

    // Validation rules that I deleted for this post
    if ($this->form_validation->run() === FALSE)
    {              
        $this->load->view("header");
        $this->load->view("section-aside");
        $this->load->view("vista_nuevo_elemento_diccionario", $datos);
        $this->load->view("footer");
    }
    else
    {
        $last_id = $this->mod_diccionarios->nuevo_elemento_diccionario();
        echo json_encode(array('last_id' => $last_id));
        redirect('/diccionarios/lista_diccionarios');
    }
}

Model:

public function nuevo_elemento_diccionario()
 {
     $datos = array(
        'ELDI_Key' => $this->input->post('eldi_key'),
        'ELDI_Display' => $this->input->post('eldi_display'),
        'ELDI_SuDiccionario' => $this->input->post('eldi_sudiccionario'),
    );
    $this->db->insert('elementos_diccionario', $datos);
   /* $ultima_id = $this->db->insert_id();*/
    $last_id = $this->db->insert_id();
    return  $last_id;
  }

Javascript/Ajax (where you see var key = $('input[name="eldi_key"]').val(); and using it, is where I need to use the ID to use the new ID for delete and update clicking the generated buttons):

function nuevo_elemento() 
{
    var id =$('input[name="eldi_id"]').val();
    var display = $('input[name="eldi_display"]').val();
    var key = $('input[name="eldi_key"]').val();
    key = key.split(" ").join("_").toLowerCase();

    swal({
        title: 'Añadir elemento',
        text: "¿Quieres añadir este elemento?",
        type: 'warning',
        showCancelButton: true,
        confirmButtonColor: '#3085d6',
        cancelButtonColor: '#d33',
        confirmButtonText: 'Sí',
        cancelButtonText: 'No'
        }).then((result) => {
        if (result.value) {
            $.ajax({
                type: "POST",
                url: $("#base_url").attr("valor") + 
 "diccionarios/nuevo_elemento_diccionario",
                data: $("#formNuevoElementoDiccionario").serialize(),
                success: function(data) {
                    console.log(data);
                    $('#Modal_Add').modal('hide');

                    $("#formNuevoElementoDiccionario").trigger("reset");

                    var html = 
                    '<tr>'+
                        '<td>' + key + '</td>' +
                        '<td>'+ display +'</td>'+
                        '<td>'+
                            '<span class="btn btn-default editar_elemento" 
 id=' + key + ' value="' + key + '">' +
                                '<a href="'+$("#base_url").attr("valor") + 
 "diccionarios/elemento_diccionario/"+key+'" title="Editar">' +
                                    '<i class="material-icons">edit</i>' +
                                '</a>' +
                            '</span>' +
                        '</td>' + 
                        '<td>' +
                            '<span class="btn btn-default borrar_elemento" 
 id="'+ key +'" value="'+ key +'">'+
                                    '<i class="material- 
 icons">delete_outline</i>'+   
                            '</span>'+
                        '</td>'+
                    '</tr>'

                    $('#tabla-diccionarios').append(html);
                    swal("Añadido", "Elemento añadido correctamente", 
"success");

                    $(".borrar_elemento").off();                        
                    evento_borrar_elemento();
                },
                error: function(XMLHttpRequest, textStatus, errorThrown) {
                    console.log("Error: " + textStatus);
                    }
            });
        }
        })
}

You can use insert_id() only after inserting data to DB. If you need user last id in controller or js try this:

public function nuevo_elemento_diccionario()
{
    $datos = array(
        'ELDI_Key' => $this->input->post('eldi_key'),
        'ELDI_Display' => $this->input->post('eldi_display'),
        'ELDI_SuDiccionario' => $this->input->post('eldi_sudiccionario'),
    );
    $this->db->insert('elementos_diccionario', $datos);
    $ultima_id = $this->db->insert_id();
    return $ultima_id;
}

Then you can use json_encode() for return data to js in Controller:

public function nuevo_elemento_diccionario()
{
    $this->load->helper('form');
    $this->load->library('form_validation');

    // Validation rules that I deleted for this post
    if ($this->form_validation->run() === FALSE)
    {              
        $this->load->view("header");
        $this->load->view("section-aside");
        $this->load->view("vista_nuevo_elemento_diccionario", $datos);
        $this->load->view("footer");
    }
    else
    {
        $last_id = $this->mod_diccionarios->nuevo_elemento_diccionario();
        echo json_encode(array('last_id' => $last_id));
        // i`m not sure that you need to use redirect
        redirect('/diccionarios/lista_diccionarios');
    }
}

In you js edit success function:

 success: function(data) {
     var result = JSON.parse(data);
     var insert_id = result.insert_id;