Laravel 5.4 - Populate dropdown based on another dropdown selected value

you can easily do it with a little bit of ajax and get method. May be you are trying to load brand depend on category lets roll :

Your controller:

public function index()
{

 $category = Category::pluck('categoryName', 'id');
// no need to query brand here because we will load it depend on category 
  $brand = [];


    return view ( 'site.indexS',compact('brand','category') );


}

// here we are adding another method in your controller which will return brand object depend on category id

 public get_brand($categpry_id){
      // hope your brand table contain category_id or any name as you wish which act as foreign key
      $brands= Brand::where('category_id',$category_id)
      ->pluck('brandName','id');
      return json_encode($brands);
    }

Now in route we need to add this to hit this url :

Route::get('get-brand','YourControllerName@get_brand');

In view : {{-- i am adding id for both dropdown --}} Catégorie(s):
{!! Form::select('category', $category,null, array('id' => 'category_dropdown','class' => 'form- control')) !!}

  <label for="brand">Marque:</label> 
        {!! Form::select('brand_name', $brand_name,null, array('id' => 'brand_dropdown','class' => 'form-control')) !!}

now in our view file we need to use ajax, there is many other way i am preferring ajax here

<script type="text/javascript">
  var url = "{{url('/')}}";
</script>
 <script type="text/javascript">

      $('#category_dropdown').on('change', function() {
        $('#brand_dropdown').empty();
var id = $('#category_dropdown').val();
        $('#brand_dropdown').html('<option selected="selected" value="">Loading...</option>');
    var url = url + '/get-brand/'+id;
    $.ajax({
        url: url,
        type: "GET",
        dataType: "json",
        success:function(data) {
            //console.log(data);
            $('#brand_dropdown').html('<option selected="selected" value="">Select Brand</option>');
            $.each(data, function(key, value) {

                $('#brand_dropdown').append('<option value="'+key+'">'+value+'</option>');
            });

        }
    });
      });
    </script>