without select category not show subcategory

This is create.blade.php file. In this include css and js file too.. Html code and ajax code view file

@extends('layouts.app')

@section('content')
       <link rel="stylesheet" href="http://www.codermen.com/css/bootstrap.min.css">    
    <script src="http://www.codermen.com/js/jquery.js"></script>
<form enctype="multipart/form-data" method="post" action="" >
                @csrf

<div class="form-group col-md-8">
                    Category<select name="category" id="category" class="form-control">
                                    <option>select</option>
                        @foreach($categories as $category)
                <option value=""></option>
                @endforeach
                            </select>
                </div>
                <div class="form-group col-md-8">
                    Category<select name="subcategory" id="subcategory" class="form-control">
                                    <option>select</option>
                        @foreach($subcategories as $subcategory)
                <option value=""></option>
                @endforeach
                            </select>
                </div>
</form>

<script type="text/javascript">
    $('#category').change(function(){
    var categoryID = $(this).val();    
    if(categoryID){
        $.ajax({
           type:"GET",
           url:"?category_id="+categoryID,
           success:function(res){               
            if(res){
                $("#subcategory").empty();
                $("#subcategory").append('<option>Select</option>');
                $.each(res,function(key,value){
                    $("#subcategory").append('<option value="'+key+'">'+value+'</option>');
                });
           
            }else{
               $("#subcategory").empty();
            }
           }
        });
    }else{
        $("#subcategory").empty();
        
    }      
   });
</script>
@endsection


 

This is controller code which create function code of category and subcategory

public function create(Request $request){
    $categories = Category::all();
  $subcategories = DB::table('subcategories')
              ->where('category_id', $request->category_id)
              ->pluck('subcategory', 'id');
  return view('post.create', compact('categories', 'subcategories'));
}

This is route Route::get('/post/create', 'PostController@create')->name('post.create');

Problem is if i select category still no show related to subcategory

via Chebli Mohamed