Laravel 4中的JQuery没有返回数据.为什么?

我的项目是电子商务。。。。布局中的product.blade.php文件

{{ HTML::script('js/jquery.js') }}
<script type="text/javascript">
    $("#frm").submit(function(e){
        $('#returncart').html("loading ...");
        e.preventDefault();
        var token =  $("input[name=_token]").val();
        $.ajax({
            type: "POST",
            url : "{{ route('add-to-cart') }}",
            data : dataString,
            success : function(data){
               $('#returncart').html(data);
            }
        },"json");
    });
    </script>

和在cartControlle.php文件中

public function addToCart(){
    $maxQuantity = Product::where('id','=',Input::get('id'));
    $maxQuantity = $maxQuantity->first();
    $validator = Validator::make(Input::all(),array(
        'quantity' =>'required|integer|min:1|max:'.$maxQuantity->quantity.''
    ));
    return "added to cart";
}

和在路由文件中

Route::post('/cart',array('as'=>'add-to-cart','uses'=>'CartController@addToCart'));

和product.blade.php文件中

<form id='frm' action='{{ URL::route("add-to-cart") }}' method='post'>
        <div class="row">
            <div class="col-md-4 centeration">
                <img src="{{ URL::to($productDetails->image) }}">
            </div>
            <div class="col-md-7">
                <div class='row'>
                    <h3> {{ $productDetails->name }}<br>
                    <small> Category: <a href="{{ URL::route('category-details',$productDetails->category_id) }}">{{ $productDetails->category_name }}</a></small></h3>
                </div>
                <div class='row'>
                    <p> {{ $productDetails->details }}</p>
                </div>
                <div class='row'>
                    <p><label>Sale Price:</label> {{ $productDetails->price }} LE</p>
                    <div class="row">
                        <div  class="col-md-2"><label for='quantity'>Quantity: </label> </div>
                        <div  class="col-md-3">
                             <input type='number' value='1' min='1' max='{{ $productDetails->quantity }}'  name='quantity'  class="form-control col-md-4">
                        </div>
                    </div>
                    <div class="row top-2">
                        <div class="col-md-6">
                            <input type='hidden' name='id'  value="{{ $productDetails->id }}">
                            <input type='submit'  class="btn btn-success btn-block" value="Add to cart">
                        </div>
                    </div>
                </div>
            </div>
        </div>
        {{ Form::token() }}
    </form>
    <div id='returncart'></div>

为什么我的代码是错误的？这段代码没有在div#returncart中返回我的字符串"added to cart"，为什么？！